TACI and BCMA are receptors for a TNF homologue implicated in B-cell autoimmune disease

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.

The dynamics of the B follicle: understanding the normal counterpart of B-cell-derived malignancies

The repertoire of B cells secreting antibodies with unique antigen-binding specificities is produced at two stages: a primary B-cell repertoire is formed in the bone marrow through immunoglobulin gene rearrangements, whereas a secondary B-cell repertoire is generated in the peripheral lymphoid organs (spleen, lymph nodes and mucosa-associated lymphoid tissue) through somatic hypermutation and class-switch recombination upon antigen encounter. The latter events take place within highly specialized histological structures, designated B follicles, which are composed of distinct microanatomical compartments namely the follicle centre, lymphocytic corona and marginal zone. Each compartment comprises a particular subset of B cells, characterized by unique properties, thereby reflecting the complexity and variability in the spectrum of defence mechanisms against invading pathogens. The past years have spawned an avalanche of new data and information that encompasses both the structure and function of each compartment and its B cells. This review incorporates up-to-date information on peripheral B-cell differentiation into a challenging working model, thereby pointing to the structural and functional imprint of both the T-cell-dependent and T-cell-independent immune response on the B follicle. As such, this article aims to form an excellent base for a better understanding of the normal counterpart of B-cell-derived haematological malignancies (leukemias and lymphomas).

New biological therapies from the human genome

Over the past 20 years, the biotechnology industry has been extraordinarily successful in bringing a wide variety of new products to the market, including recombinant versions of natural proteins such as growth hormone, insulin and the gonadotropins. The availability of the human genome sequence has given us the chance to identify the entire catalogue of human secreted proteins, often called the secretome. One of the challenges of biotechnology research has been to identify the biological activities of these proteins and to identify if any of them could have a therapeutic or pharmacologic use. The paradigm has effectively been reversed, in that it used to be easy to know the biological activity, but difficult to clone, whereas now the contrary is true. Five years on, it is clear that finding new biological activities is a very difficult process. Much of the ground gained in this period has either been through the development of antibodies as therapies or by the use of protein engineering to produce better versions of the proteins that are already being produced.

Safety, pharmacokinetics and pharmacodynamics of atacicept in healthy volunteers

OBJECTIVE

Atacicept, a recombinant fusion protein, blocks the activity of BLyS (a B-lymphocyte stimulator) and APRIL (a proliferation-inducing ligand) and may be a potential treatment for B-cell-mediated diseases. This study assesses the safety, pharmacokinetics (PK) and pharmacodynamics (PD) of atacicept.

METHODS

In this Phase I study, healthy male volunteers received a single subcutaneous dose of atacicept (2.1, 70, 210 or 630 mg) or placebo and were monitored over 7 weeks for injection-site pain, local tolerability, vital signs, echocardiography, haematology, coagulation, blood chemistry, serum virology, urinalysis and PK/PD markers [lymphocyte cell counts, BLyS-atacicept complex, immunoglobulin G (IgG), IgM].

RESULTS

Atacicept was well tolerated at all doses (n = 23). There were no clinically significant changes in vital signs or laboratory parameters during the study. Treatment-emergent adverse events (AEs) were mainly mild or moderate in severity, and all were transient, resolving without any clinical sequelae. There was no evidence of any relationship between atacicept dose and the incidence of AEs. Local tolerability was good. Serum atacicept peaked 16 h after dosing, and the area under the concentration-time curve increased in an approximately dose-related manner. The 70-, 210- and 630-mg doses of atacicept demonstrated a dose-dependent biological effect on IgM levels, which was apparent up to 210 days post-dose. There were no treatment-related effects on IgG levels or lymphocyte subpopulations.

CONCLUSIONS

These results showed that single subcutaneous doses of atacicept were well tolerated in healthy volunteers, demonstrated non-linear PK and were biologically active, according to IgM levels.

IL-21 Has a Pathogenic Role in a Lupus-Prone Mouse Model and Its Blockade with IL-21R.Fc Reduces Disease Progression

Systemic lupus erythematosus is a complex autoimmune disease characterized by dysregulated interactions between autoreactive T and B lymphocytes and the development of anti-nuclear Abs. The recently described pleiotropic cytokine IL-21 has been shown to regulate B cell differentiation and function. IL-21 is produced by activated T lymphocytes and its interactions with IL-21R are required for isotype switching and differentiation of B cells into Ab-secreting cells. In this report, we studied the impact of blocking IL-21 on disease in the lupus-prone MRL-Fas(lpr) mouse model. Mice treated for 10 wk with IL-21R.Fc fusion protein had reduced proteinuria, fewer IgG glomerular deposits, no glomerular basement membrane thickening, reduced levels of circulating dsDNA autoantibodies and total sera IgG1 and IgG2a, and reduced skin lesions and lymphadenopathy, compared with control mice. Also, treatment with IL-21R.Fc resulted in a reduced number of splenic T lymphocytes and altered splenic B lymphocyte ex vivo function. Our data show for the first time that IL-21 has a pathogenic role in the MRL-Fas(lpr) lupus model by impacting B cell function and regulating the production of pathogenic autoantibodies. From a clinical standpoint, these results suggest that blocking IL-21 in systemic lupus erythematosus patients may represent a promising novel therapeutic approach.

B cell targeted therapy in autoimmunity

Autoimmunity results from a break in self-tolerance involving humoral and/or cell-mediated immune mechanisms. Part of the pathological consequence of a failure in central and/or peripheral tolerance, results from survival and activation of self-reactive B cells. Such B cells produce tissue-damaging pathogenic autoantibodies, and subsequent formation of complement-fixing immune complexes that contribute to tissue damage. Current pharmacological strategies for treating autoimmune diseases involve global use of broad-acting immunosuppressants that with long term use have associated toxicities. The present drive in drug development is towards therapies that target a specific biological pathway or pathogenic cell population. This review focuses on some of the emerging therapies based on co-stimulation blockers, and compounds which contribute to a specific B cells depletion, based on studies in animal models and human clinical studies.

B-lymphocyte activating factor in systemic lupus erythematosus and rheumatoid arthritis in relation to autoantibody levels, disease measures and time

Overexpression of B-lymphocyte activating factor (BAFF) results in arthritis, glomerulonephritis and autoantibody formation in mice, but its role in human autoimmune disease is less obvious. Serum BAFF levels in patients with systemic lupus erythematosus (SLE) (n=42) and rheumatoid arthritis (RA) (n=60) were related to levels of disease activity, anti-dsDNA Ab, anti-ENA Ab, rheumatoid factor (RF) and anti-CCP Ab. BAFF levels were also followed over time in 19 SLE patients. BAFF levels correlated inversely with age, were higher in SLE than RA (median 2.7 versus 1.4 ng/mL, P < 0.01) and more SLE than RA patients had increased BAFF levels (57% versus 10%, P < or = 0.01). In SLE, BAFF levels correlated with SLEDAI scores but not with anti-dsDNA Ab levels. SLE patients with increased BAFF levels had higher SLEDAI and CRP levels. In RA, BAFF levels correlated weakly with anti-CCP levels (Rs 0.27, P = 0.07), but not with joint counts, ESR, CRP or RF levels. Longitudinal BAFF levels remained unaltered in two thirds of SLE patients and changes in BAFF levels were unrelated to disease flares. These findings suggest that BAFF stimulation of B-cells may contribute to SLE by other mechanisms than autoantibody production.

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.

Elevated serum BAFF levels in patients with localized scleroderma in contrast to other organ-specific autoimmune diseases

Serum levels of B-cell activating factor belonging to the tumor necrosis factor family (BAFF), a potent B-cell survival factor, are elevated in patients with systemic autoimmune diseases, such as systemic lupus erythematosus (SLE), rheumatoid arthritis and systemic sclerosis (SSc). The objective of this study was to determine serum BAFF levels and relate the results to the clinical features in patients with organ-specific autoimmune diseases of the skin, such as localized scleroderma and autoimmune bullous diseases. Serum BAFF levels were examined by enzyme-linked immunosorbent assay in 44 patients with localized scleroderma, 20 with pemphigus vulgaris/pemphigus foliaceus, 20 with bullous pemphigoid and 30 healthy controls. Twenty patients with SSc and 20 with SLE were also examined as disease controls. Serum BAFF levels were elevated in localized scleroderma patients compared with healthy controls. Concerning localized scleroderma subgroups, patients with generalized morphea, the severest form of localized scleroderma, had higher serum BAFF levels than linear scleroderma or morphea patients. The BAFF levels of generalized morphea were comparable with those of SSc or SLE. Furthermore, serum BAFF levels correlated positively with antihistone antibody levels and the severity of skin lesion as well as the number of skin lesions. By contrast, serum BAFF levels were not significantly elevated in patients with pemphigus or pemphigoid. These results suggest that BAFF may be contributing to autoimmunity and disease development in localized scleroderma.

B lymphocyte stimulator (BLyS) isoforms in systemic lupus erythematosus: disease activity correlates better with blood leukocyte BLyS mRNA levels than with plasma BLyS protein levels

Considerable evidence points to a role for B lymphocyte stimulator (BLyS) overproduction in murine and human systemic lupus erythematosus (SLE). Nevertheless, the correlation between circulating levels of BLyS protein and disease activity in human SLE is modest at best. This may be due to an inadequacy of the former to reflect endogenous BLyS overproduction faithfully, in that steady-state protein levels are affected not just by production rates but also by rates of peripheral utilization and excretion. Increased levels of BLyS mRNA may better reflect increased in vivo BLyS production, and therefore they may correlate better with biologic and clinical sequelae of BLyS overexpression than do circulating levels of BLyS protein. Accordingly, we assessed peripheral blood leukocyte levels of BLyS mRNA isoforms (full-length BLyS and ΔBLyS) and plasma BLyS protein levels in patients with SLE, and correlated these levels with laboratory and clinical features. BLyS protein, full-length BLyS mRNA, and ΔBLyS mRNA levels were greater in SLE patients (n = 60) than in rheumatoid arthritis patients (n = 60) or normal control individuals (n = 30). Although full-length BLyS and ΔBLyS mRNA levels correlated significantly with BLyS protein levels in the SLE cohort, BLyS mRNA levels were more closely associated with serum immunoglobulin levels and SLE Disease Activity Index scores than were BLyS protein levels. Moreover, changes in SLE Disease Activity Index scores were more closely associated with changes in BLyS mRNA levels than with changes in BLyS protein levels among the 37 SLE patients from whom repeat blood samples were obtained. Thus, full-length BLyS and ΔBLyS mRNA levels are elevated in SLE and are more closely associated with disease activity than are BLyS protein levels. BLyS mRNA levels may be a helpful biomarker in the clinical monitoring of SLE patients.

Increased serum levels of a proliferation-inducing ligand in patients with bullous pemphigoid

BACKGROUND

B cells have been demonstrated to have critical roles in developing autoimmune bullous diseases. Recently identified tumor necrosis factor-like molecules, B cell-activating factor of the TNF family (BAFF) and a proliferation-inducing ligand (APRIL) are essential molecules for B cell development, survival, and proliferation. Although the functions of APRIL have not been fully evaluated, recent studies suggest that circulating levels of APRIL are increased in various autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis.

OBJECTIVES

To determine serum APRIL levels in patients with pemphigus vulgaris (PV) and bullous pemphigoid (BP), and compare those with clinical findings and laboratory findings.

PATIENTS/METHODS

Sera from 15 PV patients, 43 BP patients, and 15 normal controls were subjected to ELISA assays to measure serum APRIL, BAFF, Dsg3, and BP180 levels.

RESULTS AND CONCLUSIONS

Circulating APRIL levels were significantly elevated in BP patients but not in PV patients, and correlated with serum BAFF levels. Our study revealed that serum APRIL levels tended to be increased in the quite early stage of disease. In conclusion, circulating APRIL levels may be a useful marker for early activation of autoimmune diathesis, and furthermore, an effective therapeutic target molecule in patients with BP.

Fate decisions regulating bone marrow and peripheral B lymphocyte development

In adult mammals, bone marrow pluripotent hematopoietic stem cells generate B lymphoid-specified progeny that progress through a series of well-characterized stages before generating B-cell receptor expressing B lymphocytes. These functionally immature B lymphocytes then migrate to the spleen wherein they differentiate through transitional stages into follicular or marginal zone B lymphocytes capable of responding to T-dependent and -independent antigens, respectively. During the terminal stages of B lymphocyte development in the bone marrow, as well as immediately following egress into the peripheral compartments, B lymphocytes are counterselected to eliminate B lymphocytes with potentially dangerous self-reactivity. These developmental and selection events in the bone marrow and periphery are dependent on the integration of intrinsic genetic programs with extrinsic microenvironmental signals that drive progenitors toward increasing B lineage commitment and maturation. This chapter provides a comprehensive overview of the various stages of primary and secondary B lymphocyte development with an emphasis on the selection processes that affect decisions at critical checkpoints. Our intent is to stress the concept that at many steps in the developmental process leading to a mature immunocompetent B lymphocyte, B lineage cells are integrating multiple and different signaling inputs that are translated into specific and appropriate cell fate decisions.

Systemic lupus erythematosus: multiple immunological phenotypes in a complex genetic disease

Systemic lupus erythematosus (SLE) is a complex polygenic autoimmune disease characterized by the presence of anti-nuclear autoantibodies (ANAs) that are often detectable years prior to the onset of clinical disease. The disease is associated with a chronic activation of the immune system, with the most severe forms progressing to inflammatory damage that can impact multiple organ systems in afflicted individuals. Current therapeutic strategies poorly control disease manifestations and are generally immunosuppressive. Recent studies in human patient populations and animal models have associated elements of the innate immune system and abnormalities in the immature B lymphocyte receptor repertoires with disease initiation. A variety of cytokines, most notably type I interferons, play important roles in disease pathogenesis and effector mechanisms. The genetic basis for disease susceptibility is complex, and analyses in humans and mice have identified multiple susceptibility loci, several of which are located in genomic regions that are syntenic between humans and mice. The complexities of the genetic interactions that mediate lupus have been investigated in murine model systems by characterizing the progressive development of disease in strains expressing various combinations of susceptibility alleles. These analyses indicate that genetic epistasis dramatically impact disease development and support the feasibility of identifying molecular pathways that can suppress disease progression without completely impairing normal immune function.

Analysis of an anti-B lymphocyte stimulator monoclonal antibody B7 and its binding activity to myeloma and lymphoma cell lines

Although there is convincing evidence of a link between B lymphocyte stimulator (BLyS) and the proliferation and survival of malignant B cells, previous observations about BLyS expression on B lymphoma cells were contradictory. In this study, BLyS expression on human lymphoma and myeloma cell lines was evaluated by Fluorescence-Activated Cell Sorter (FACS). First, specificity of a monoclonal antibody (MAb) against BLyS, was analyzed. The results showed that MAb B7 was immunoglobulin G(3) (IgG(3)) and recognized recombinant human BLyS specifically. In addition, MAb B7 bound to the histiocyte lymphoma cell line U937 in dosage-dependent manner, but not the T lymphoma cell line Jurkat, suggesting that the cellular binding of MAb B7 was specific. Using this MAb, BLyS expression on two multiple myeloma cell lines (XG-7 and SKO-007) and two Burkitt's lymphoma cell lines (Daudi and Raji) was evaluated. MAb B7 did bind to XG- 7 and SKO-007 (66.84% and 79.38% positive cells, respectively). But MAb B7 did not bind Burkitt's lymphoma cell lines, Daudi and Raji (4.09% and 3.02% positive cells, respectively). It will be interesting to further analyze the expression of BLyS on B malignant cells of multiple myeloma and lymphoma patients, and to evaluate the correlation of its expression and patient prognosis.

Targeting BAFF: Immunomodulation for autoimmune diseases and lymphomas

In an effort to develop more effective treatments for inflammatory diseases, immunologists have targeted numerous molecular pathways, but with limited success. Notable exceptions are anti-TNF agents, which have proved efficacious in a proportion of rheumatoid arthritis (RA) patients. Another TNF family member, termed BAFF ("B cell-activating factor belonging to the TNF family"), plays a central role in autoimmune diseases, as well as in B cell maturation, survival, and T cell activation. Agents that block BAFF have proven to be highly effective in the treatment of certain autoimmune conditions in mice. In addition, phase II data in human clinical trials for RA appear very promising. BAFF is also a survival factor for certain B cell lymphomas. Despite the relatively recent identification of BAFF, this molecule has provided considerable new insight into B cell homeostasis and immune function, and represents an important new molecular target for treatment of autoimmune diseases and lymphomas.

Correlation of the expression levels of BLyS and its receptors mRNA in patients with systemic lupus erythematosus

OBJECTIVES

To measure the levels of B-lymphocyte stimulator (BLyS) and its receptors mRNA expression in peripheral blood mononuclear cells (PBMCs) using quantitative real-time polymerase chain reaction (PCR) method and to investigate the relationship between BLyS and its receptors mRNA expression and systemic lupus erythematosus (SLE).

DESIGN AND METHODS

Specific primers and TaqMan probe were designed, and real-time PCR was performed. According to the standard curve of plasmids DNA, the levels of BLyS and its receptors mRNA expression in 37 patients with SLE and 30 healthy subjects were determined. The ratio of the expression levels of BLyS mRNA to that of beta2-microglobulin (beta2M) mRNA and the ratio of the expression levels of BLyS receptors mRNA to that of beta2M mRNA were regarded as indicator for the levels of BLyS and its receptors mRNA expression.

RESULTS

The expression of BLyS, TACI and BAFF-R mRNA in PBMCs from patients with SLE was significantly elevated compared to healthy controls (P<0.001 for each), and active patients with SLE group had higher mRNA expression than patients with SLE inactive group (P<0.001 for each). The patients with elevated anti-double-stranded DNA (anti-dsDNA) antibody titers had enhanced BLyS, TACI and BAFF-R mRNA expression (P<0.05 for BLyS; and P<0.01 for TACI and BAFF-R).

CONCLUSION

The BLyS, TACI and BAFF-R mRNA expression levels were significantly elevated in patients with SLE, which suggests that BLyS, TACI and BAFF-R might be involved in the pathogenesis, and that mRNA expression levels might serve as a biomarker of disease activity.

High-level serum B-cell activating factor and promoter polymorphisms in patients with idiopathic thrombocytopenic purpura

Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder in which platelets are opsonised by autoantibodies and destroyed by macrophages. Therefore, ITP represents a prototype of a B-cell-mediated autoimmune disorder. B-cell activating factor (BAFF) is a member of the tumour necrosis factor family and an important regulator of B-cell development. BAFF levels were determined in serum samples from 53 patients with ITP. Serum BAFF levels in patients with an active ITP were increased when compared with the healthy control group (median 1620 pg/ml vs. 977 pg/ml; P < 0.001). Moreover, immunosuppressive treatment was associated with strongly suppressed BAFF levels (median 629 pg/ml; P < 0.01). In addition, a polymorphic site was detected in the BAFF promoter region (-871) that appeared to occur more frequently in ITP patients than in healthy persons. This promoter variant was associated with very high BAFF levels in ITP patients. Our data suggest that BAFF is an important pathogenetic factor in the development of ITP.

The growth factor fusion construct containing B-lymphocyte stimulator (BLyS) and the toxin rGel induces apoptosis specifically in BAFF-R-positive CLL cells

The cytokine B lymphocyte stimulator (BLyS) mediates its effect through cell-surface receptors BAFF-R, TACI, and BCMA. BLyS receptors are expressed only on B cells and not present in other normal cells including normal T lymphocytes. Chronic lymphocytic leukemia (CLL) is a B-cell disease and CLL lymphocytes express BLyS receptors. Gelonin, a type 1 ribosome-inactivating toxin, lacks cell membrane binding domain and hence is nontoxic to intact cells. We generated a construct of recombinant gelonin (rGel) fused to BLyS to specifically target quiescent B-CLL lymphocytes. The construct rGel/BLyS specifically binds and internalizes through BAFF-R into CD19(+) B-CLL lymphocytes and induces apoptosis at nanomolar concentrations. In contrast, rGel alone was not able to internalize into these leukemic lymphocytes. Mechanistically, the rGel/BLyS construct inhibits protein synthesis with an IC(50) of less than 3 nM compared with more than 5000 nM for rGel toxin alone. This rGel/BLyS-mediated decrease in protein synthesis was associated with a decline in short-lived proteins such as MCL-1 and XIAP, the 2 survival proteins in B-CLL. There was a strong relationship between a decrease in these proteins and the cleavage of PARP, a hallmark feature of apoptosis. Taken together, these data suggest that the rGel/BLyS fusion toxin may have potential therapeutic efficacy for B-CLL patients.

B lymphocytes as therapeutic targets in systemic lupus erythematosus

In recent years, experimental evidence supporting a major role of B cells in the pathogenesis of autoimmune diseases has grown. This includes the discovery of novel mechanisms of autoantibody pathogenicity and the potential of B cells to mediate inflammation and tissue injury. In some instances, engagement of the B cell receptor and other surface receptors is sufficient to stimulate B cells to produce antibody. As a result, B cells have become targets for immunointervention. In lupus, targeting B cell activation factor (BAFF, BLys) indicates that specific blockade of this longevity factor might be sufficient to suppress systemic autoimmunity. Targeting CD20 represents another promising avenue for the treatment of refractory lupus in both adults and children. Although the clinical data add weight to the importance of B cells in the pathogenesis of lupus, new targets for B cell depletion therapy are being investigated. In experimental models, combining CD19 and CD20 antibodies was more effective than either treatment alone.

Unexpected development of autoimmunity in BAFF-R-mutant MRL-lpr mice

BAFF-R is the predominant receptor that mediates B-cell activating factor (BAFF)-dependent B-cell signalling and plays a critical role in late-stage B-cell maturation and survival. BAFF has been implicated in the development of autoimmunity and systemic lupus erythematosus (SLE). To define the role of BAFF-R in autoimmunity and SLE, we crossed A/WySnJ mice with MRL-lpr mice and generated BAFF-R-mutant MRL-lpr mice. The BAFF-R mutation markedly impaired the development of immature, mature and marginal zone B cells in the spleens of MRL-lpr mice. Unexpectedly, the BAFF-R mutation in MRL-lpr mice did not result in decreased autoantibody production, hypergammaglobulinaemia or immune complex-mediated glomerulonephritis. Rather, the ability of BAFF-R-mutant lpr splenic B cells to produce immunoglobulins in vitro was not decreased, although germinal centre formation, antibody response and B-cell proliferation were impaired. Further studies found increased numbers of B cells in the bone marrow of BAFF-R-mutant MRL-lpr mice compared to the BAFF-R-intact lupus mice. ELISPOT analysis revealed that BAFF-R-mutant MRL-lpr mice had more antibody-secreting cells in their bone marrow than the control mice. Thus, these findings could explain the development of autoimmunity and hypergammaglobulinaemia observed in BAFF-R-mutant MRL-lpr mice.

BAFF controls B cell metabolic fitness through a PKC beta- and Akt-dependent mechanism

B cell life depends critically on the cytokine B cell–activating factor of the tumor necrosis factor family (BAFF). Lack of BAFF signaling leads to B cell death and immunodeficiency. Excessive BAFF signaling promotes lupus-like autoimmunity. Despite the great importance of BAFF to B cell biology, its signaling mechanism is not well characterized. We show that BAFF initiates signaling and transcriptional programs, which support B cell survival, metabolic fitness, and readiness for antigen-induced proliferation. We further identify a BAFF-specific protein kinase C β–Akt signaling axis, which provides a connection between BAFF and generic growth factor–induced cellular responses.

Elevated serum B-Lymphocyte activating factor (BAFF) in chronic hepatitis C virus infection: association with autoimmunity

In this study we aimed to determine whether serum B-lymphocyte activating factor (BAFF) level is increased in patients with chronic hepatitis C virus (HCV) infection, and to assess its association with HCV-related autoimmunity. Sixty-five patients with chronic HCV infection were compared with two disease control groups [57 patients with systemic lupus erythematosus (SLE) and 15 with chronic hepatitis B virus (HBV) infection] and a healthy control group of 35 individuals. A special attention was given to HCV-related arthralgia and or vasculitis. Serum BAFF was assessed in all studied individuals, whereas rheumatoid factor (RF), anti-cardiolipin antibodies (aCL), and cryoglobulins were determined in HCV and HBV infected patients, and anti-dsDNA antibodies and aCL were assessed in patients with SLE. Mean serum BAFF was increased in patients with HCV infection and SLE (2.4+/-0.8 ng/ml and 3.1+/-1.34 ng/ml respectively) compared to 1.1+/-0.14 ng/ml in patients with HBV; and to 1.1+/-0.27 in healthy controls (all, p<0.0001). The elevation in serum BAFF was associated with HCV-related arthralgia and or vasculitis (p<0.0001), and with the presence of aCL and of cryoglobulins. HBV patients lacked features suggestive of autoimmunity. In SLE patients, elevated serum BAFF was in association with the presence of anti-dsDNA (p=0.002). As in other autoimmune diseases, increased serum BAFF was also found in patients with chronic HCV infection. Elevated serum BAFF levels were associated with clinical and laboratory features of autoimmunity, suggesting that BAFF may play a role in HCV-related autoimmunity.

Treatment of autoimmune hyperthyroidism in a murine model of Graves' disease with tumor necrosis factor-family ligand inhibitors suggests a key role for B cell activating factor in disease pathology

Hyperthyroid Graves' disease is a common autoimmune disorder mediated by agonistic antibodies to the TSH receptor, termed thyroid stimulating antibodies (TSAbs). Recently members of the TNF superfamily, B cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL), have been identified along with their receptors, B cell maturation antigen and transmembrane activator and calcium-modulator and cyclophilin ligand interactor, and the BAFF-specific receptor. BAFF is a fundamental B cell survival/maturation factor, and both BAFF and APRIL have been implicated in antibody production. We investigated the effect of interfering with BAFF- and APRIL-mediated signals in an induced model of Graves' disease by blockade of these factors using soluble decoy receptors. In a therapeutic setting in mice with established hyperthyroidism, we show that blockade of BAFF or BAFF+APRIL with BAFF-specific receptor-Fc and B cell maturation antigen-Fc, respectively, leads to significant reductions in the induced hyperthyroidism. This was supported by a parallel pattern of declining TSAbs in the responding animals. Histopathological analysis of splenic sections from treated animals revealed marked reductions in the B cell follicle regions, but staining with anti-CD138 revealed the persistence of plasma cells. Thus, the reductions in TSAbs in the treated animals were not related to overall plasma cell numbers in the secondary lymphoid organs. Our results are the first to demonstrate attenuation of established hyperthyroidism by therapeutic intervention aimed at autoreactive B cells and indicate that both BAFF and APRIL appear to play important roles in the development and survival of the autoantibody producing cells in this model.

Molecules involved in T-B co-stimulation and B cell homeostasis: possible targets for an immunological intervention in autoimmunity

The deepened knowledge of co-stimulatory mechanisms within the immunological synapse and the emerging biological principles governing B cell homeostasis provide a plethora of new possibilities to selectively block or enhance immune responses. These mechanisms are highly relevant to the development of new treatment modalities for autoimmune diseases. Here we review approaches to antagonise members of the CD28-B7 superfamily as well as the TNF receptor ligand superfamily members, BAFF and APRIL, and their corresponding receptors on B cells (BAFF-R, TACI and BCMA). The proof of principle that such manipulations have indeed profound consequences for the human immune response comes from genetically manipulated mouse models, and, more importantly, from human immunodeficiency syndromes. Thus, the recent discovery of deletions in the ICOS, BAFF-R and TACI genes leading to disturbances in late B cell differentiation and hypogammaglobulinaemia underline the potential impact of targeting these molecules for therapeutic strategies in autoimmune disorders.

BAFF induces a hyper-IgA syndrome in the intestinal lamina propria concomitant with IgA deposition in the kidney independent of LIGHT

BAFF is a peripheral B cell survival factor and can mediate antibody (Ab) class switching. Over-expression of BAFF in mice results in B cell hyperplasia, elevated serum immunoglobulin (Ig), spontaneous germinal centre (GC) reactions and mild glomerulonephritis (GN). Here we show that, in addition to driving excessive levels of serum IgA, BAFF over-expression results in increased IgA levels within the intestinal lamina propria (LP) and deposition of IgA immune complexes in the renal glomerular mesangium. LIGHT has been previously shown to mediate a similar phenotype via signaling through the lymphotoxin-beta receptor (LTbetaR). We evaluated if LIGHT and BAFF cooperate in the etiology of a hyper-IgA syndrome in BAFF-overexpressing transgenic (BAFF-Tg) mice. We find that LIGHT-deficient BAFF-Tg mice exhibit similar levels of IgA in the serum, gut and kidney and develop nephritis to the same degree as LIGHT-sufficient BAFF-Tg mice. Therefore, in the context of BAFF over-expression, LIGHT is dispensable for the generation of a hyper-IgA syndrome accompanied by nephritis.

BAFF and APRIL support chronic lymphocytic leukemia B-cell survival through activation of the canonical NF-kappaB pathway

Chronic lymphocytic leukemia (CLL) B cells express BR3, the specific receptor for the B cell-activating factor of tumor necrosis factor family (BAFF). CLL cells also express 2 other receptors for BAFF, namely B-cell maturation antigen (BCMA) and the transmembrane activator and calcium modulator and cyclophilin ligand-interactor (TACI), which also bind a proliferation-inducing ligand (APRIL). We found that signaling through BR3, but not BCMA or TACI, activated the alternative nuclear factor of kappa B (NF-kappaB) pathway in CLL cells, whereas signaling through BCMA/TACI induced activation of the canonical NF-kappaB pathway. Blocking BR3 did not inhibit the capacity of BAFF to support CLL cell survival in vitro. On the other hand, specifically blocking the canonical NF-kappaB pathway with UTC, an inhibitor of IkappaB kinase beta (IKKbeta), or transfection of CLL cells with the IkappaBalpha super-repressor, blocked the capacity of BAFF and APRIL to promote CLL cell survival in vitro. This contrasts what is found with normal blood B cells, which apparently depend on activation of the alternative NF-kappaB pathway for BAFF-enhanced survival. These findings suggest that inhibitors of protein kinase IKKbeta, which is required for activation of the canonical NF-kappaB pathway, might have a therapeutic role in this disease.

Persistent expression of autoantibodies in SLE patients in remission

A majority of the antibodies expressed by nascent B cells in healthy humans are self-reactive, but most of these antibodies are removed from the repertoire during B cell development. In contrast, untreated systemic lupus erythematosus (SLE) patients fail to remove many of the self-reactive and polyreactive antibodies from the naive repertoire. Here, we report that SLE patients in clinical remission continue to produce elevated numbers of self-reactive and polyreactive antibodies in the mature naive B cell compartment, but the number of B cells expressing these antibodies is lower than in patients with active disease. Our finding that abnormal levels of self-reactive mature naive B cells persist in the majority of patients in clinical remission suggests that early checkpoint abnormalities are an integral feature of SLE.

Role of B-cell-activating factor in adhesion and growth of human multiple myeloma cells in the bone marrow microenvironment

Recent studies have underscored the role of B-cell-activating factor (BAFF), a member of the tumor necrosis factor superfamily, in promoting the survival of malignant B cells, including human multiple myeloma. We here characterized the functional significance of BAFF in the interaction between multiple myeloma and bone marrow stromal cells (BMSC) and further defined the molecular mechanisms regulating these processes. BAFF is detected on BMSCs derived from multiple myeloma patients as evidenced by flow cytometry. BAFF secretion is 3- to 10-fold higher in BMSCs than in multiple myeloma cells, and tumor cell adhesion to BMSCs augments BAFF secretion by 2- to 5-fold, confirmed by both ELISA and immunoblotting. Adhesion of MM1S and MCCAR multiple myeloma cell lines to KM104 BMSC line transfected with a luciferase reporter vector carrying the BAFF gene promoter (BAFF-LUC) significantly enhanced luciferase activity, suggesting that nuclear factor-kappaB (NF-kappaB) activation induced by multiple myeloma adhesion to BMSCs mediates BAFF up-regulation. Moreover, BAFF (0-100 ng/mL) increases adhesion of multiple myeloma lines to BMSCs in a dose-dependent manner; conversely, transmembrane activator and calcium modulator and cyclophylin ligand interactor-Ig or B-cell maturation antigen/Fc blocked BAFF stimulation. Using adenoviruses expressing dominant-negative and constitutively expressed AKT as well as NF-kappaB inhibitors, we further showed that BAFF-induced multiple myeloma cell adhesion is primarily mediated via activation of AKT and NF-kappaB signaling. Importantly, BAFF similarly increased adhesion of CD138-expressing patient multiple myeloma cells to BMSCs. These studies establish a role for BAFF in localization and survival of multiple myeloma cells in the bone marrow microenvironment and strongly support novel therapeutics, targeting the interaction between BAFF and its receptors in human multiple myeloma.

Development of nephritis but not sialadenitis in autoimmune-prone BAFF transgenic mice lacking marginal zone B cells

B cell-activating factor belonging to the TNF family (BAFF) is a B cell survival factor required for B cell maturation. BAFF transgenic (Tg) mice develop autoimmune disorders characterized by autoantibody production, which leads to nephritis and salivary gland destruction (sialadenitis), features reminiscent of systemic lupus erythematosus and Sjögren's syndrome (SS), respectively. Disease in BAFF Tg mice correlates with the expansion of the marginal zone (MZ) B cell compartment and the abnormal presence of MZ-like B cells in the blood, LN and inflamed salivary glands, suggesting a role for these cells in BAFF-induced autoimmunity. Lymphotoxin-beta (LTbeta)-deficient mice show disrupted splenic architecture, lack MZ B cells and some peripheral LN, and are unable to mount T cell-dependent immune responses. BAFF Tg mice lacking LTbeta (LTbetaDelta-BTg) retained these defects, yet still developed nephritis associated with the presence of B-1 B cells in the kidneys. However, in contrast to old BAFF Tg mice, aging LTbetaDelta-BTg mice no longer developed sialadenitis. Thus, autoimmune disorders in BAFF Tg mice are possibly events coordinated by MZ and B-1 B cells at separate anatomical sites.

Impact of the BAFF/BR3 axis on B cell survival, germinal center maintenance and antibody production

The development of the B cell lineage has been extensively studied along with the soluble and cellular components involved in the maturation and selection process. It was not always clear, however, what factors were involved in supporting mature B cell survival. Identification of the B cell survival factor, BAFF, was a key discovery in understanding the survival mechanism for mature B cells in the periphery. More recent investigations have illuminated roles for BAFF in B cell biology outside of a survival mechanism. These include germinal center maintenance, isotype switching, and regulation of specific B cell surface markers. More importantly, a role for BAFF in B cell biology has been validated in vivo in humans.

The role of the BAFF/APRIL system on T cell function

BAFF is a key factor controlling B cell survival and maturation and its over-expression promotes B cell-mediated autoimmune disorders and participates in the progression of B cell lymphomas. Yet, BAFF and a related ligand APRIL are expressed by T lymphocytes and modulate their functions. BAFF and APRIL promote T cell activation and survival. BAFF over-expression in transgenic (Tg) mice enhances T helper 1 (Thl)-driven delayed-type hypersensitivity (DTH), but inhibits T helper 2 (Th2) cell-mediated allergic airway inflammation in mice. Some of these effects are also dependent on BAFF-induced modification of the B cell compartment. Therefore, direct BAFF/APRIL signalling in T cells and/or T cell modulation in response to a BAFF-modified B cell compartment may play an important role in inflammation and immunomodulation.

BLyS receptor signatures resolve homeostatically independent compartments among naïve and antigen-experienced B cells

The BLyS family of receptors includes two cytokines, BLyS and APRIL; and three receptors, BR3, BCMA and TACI. Together, these regulate the size and composition of peripheral B cell pools. The multiplicity of ligand-receptor sets, in conjunction with differential receptor expression, alternative binding partners and disparate downstream signaling characteristics, affords the potential to establish independently regulated homeostatic niches among primary and antigen-experienced B cell subsets. Thus, BLyS signaling via BR3 is the dominant homeostatic regulator of primary B cell pools, whereas APRIL interactions with BCMA likely govern memory B cell populations. Short-lived antibody forming cell populations and their proliferating progenitors express a TACI-predominant signature. Further, within each niche, relative fitness to compete for available cytokine is determined by exogenous inputs via adaptive and innate receptor systems, affording intramural hierarchies that determine clonotype composition.

BAFF, APRIL and human B cell disorders

B cells require signals from multiple sources for their development from precursor cells, and differentiation into effector cells. BAFF has been identified as a critical regulator of B cell development and differentiation. Defects in the production of BAFF and/or expression of its receptors have been associated with a diverse array of human immunopathologies characterised by perturbed B cell function and behaviour, including autoimmunity, malignancy, and immunodeficiency. This review will discuss the role of BAFF in the pathogenesis of these human immune disorders. It will also highlight relevant differences between the function of BAFF in humans and mice and the impact of this on the therapeutic utility of BAFF antagonists in the treatment of different human diseases.

Paucity of clinical disease despite serological autoimmunity and kidney pathology in lupus-prone New Zealand mixed 2328 mice deficient in BAFF

Constitutive overexpression of B cell-activating factor belonging to the TNF family (BAFF) promotes development of systemic lupus erythematosus (SLE), and treatment of SLE mice with BAFF antagonists ameliorates disease. To determine whether SLE can develop de novo in BAFF-deficient hosts, BAFF-deficient New Zealand Mixed (NZM) 2328 (NZM.Baff(-/-)) mice were generated. In NZM.Baff(-/-) mice, spleen B cells (including CD5(+) B1a and CD5(-) B1b B cells), germinal centers, Ig-secreting cells, and T cells were reduced in comparison to NZM.Baff(+/+) mice. Serum total Ig and autoantibody levels were reduced at 4-6 mo but approached wild-type levels with increasing age, indicating that autoreactive B cells can survive and secrete autoantibodies despite the complete absence of BAFF. At least some of these autoantibodies are nephrophilic in that glomerular deposition of total IgG and IgG1 (but not of IgG2a, IgG2b, or C3) was substantial in NZM.Baff(-/-) mice by 12-13 mo of age. Despite proliferative glomerulonephritis, highlighted by widespread glomerular hyaline thrombi, being common among NZM.Baff(-/-) mice by 6-7 mo of age, severe proteinuria and mortality were greatly attenuated. These results demonstrate that the lifelong absence of BAFF does not protect NZM 2328 mice from serological autoimmunity and renal pathology. Nevertheless, the character of the renal pathology is altered, and the mice are largely spared from clinically overt disease (severe proteinuria and premature death). These observations may have profound ramifications for the use of BAFF antagonists in human SLE and related diseases.

Role of BAFF and APRIL in human B-cell chronic lymphocytic leukaemia

B-cell chronic lymphocytic leukaemia (B-CLL) is the most prevalent leukaemia in Western countries and is characterized by the gradual accumulation in patients of small mature B cells. Since the vast majority of tumoral cells are quiescent, the accumulation mostly results from deficient apoptosis rather than from acute proliferation. Although the phenomenon is relevant in vivo, B-CLL cells die rapidly in vitro as a consequence of apoptosis, suggesting a lack of essential growth factors in the culture medium. Indeed, the rate of B-CLL cell death in vitro is modulated by different cytokines, some favouring the apoptotic process, others counteracting it. Two related members of the tumour necrosis factor family, BAFF (B-cell activating factor of the TNF family) and APRIL (a proliferation-inducing ligand), already known for their crucial role in normal B-cell survival, differentiation and apoptosis, were recently shown to be expressed by B-CLL cells. These molecules are able to protect the leukaemic cells against spontaneous and drug-induced apoptosis via autocrine and/or paracrine pathways. This review will focus on the role of BAFF and APRIL in the survival of tumoral cells. It will discuss the expression of these molecules by B-CLL cells, their regulation, transduction pathways and their effects on leukaemic cells. The design of reagents able to counteract the effects of these molecules seems to be a new promising therapeutic approach for B-CLL and is already currently developed in the treatment of autoimmune diseases.

A novel BLyS antagonist peptide designed based on the 3-D complex structure of BCMA and BLyS

B lymphocyte stimulator (BLyS) is a member of tumor necrosis factor (TNF) family. Because of its roles in autoimmune diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and Sjogren syndrome (SS), BLyS antagonists have been tested to treat SLE- and RA-like symptoms in mice and obtained optimistic results. So far, reported BLyS antagonists were mostly decoyed BLyS receptors or anti-BLyS antibodies. In this study, a novel BLyS antagonist peptide, PT, was designed based on the modeling 3-D complex structure of BCMA and BLyS. The interaction mode of PT with BLyS was analyzed theoretically. The results of competitive ELISA demonstrated that PT could inhibit the binding of BCMA-Fc and anti-BLyS antibody to BLyS in vitro. In addition, PT could partly block the proliferating activity of BLyS on mice splenocytes. The BLyS antagonizing activity of PT was significant (p<0.05). This study highlights the possibility of using BLyS antagonist peptide to neutralize BLyS activity. Further optimization of PT with computer-guided molecular design method to enhance its biopotency may be useful in developing new BLyS antagonists to treat BLyS-related autoimmune diseases.

Serum levels of BAFF are increased in bullous pemphigoid but not in pemphigus vulgaris

BACKGROUND

BAFF [B-cell activating factor belonging to the tumour necrosis factor (TNF) family] is a member of the TNF superfamily that regulates B-lymphocyte proliferation and survival. It has been demonstrated that increased levels of soluble BAFF are associated with systemic autoimmunity in patients with systemic lupus erythematosus, rheumatoid arthritis and Sjögren's syndrome, and in animal models of spontaneous autoimmune diseases. However, the significance of circulating BAFF in autoimmune bullous diseases is unknown.

OBJECTIVES

To examine whether BAFF levels are elevated in the autoimmune blistering diseases pemphigus vulgaris (PV) and bullous pemphigoid (BP).

METHODS

We examined sera obtained from 21 patients with PV, 39 patients with BP and 22 healthy donors. We performed enzyme-linked immunosorbent assays for soluble BAFF and each disease-specific antibody: antidesmoglein-3 antibody for PV and anti-BP180 antibody for BP.

RESULTS

Significant elevations of serum BAFF levels were found in the patients with BP, but not with PV. There was apparently no significant association between the serum BAFF levels and titres of anti-BP180 antibodies in the patients with BP. However, serum BAFF levels tended to be more elevated in patients with a shorter disease duration. There was a tendency that BAFF levels increased before the anti-BP180 antibody levels increased at the onset of BP and quickly decreased in response to treatment.

CONCLUSIONS

BAFF may be a useful marker for early activation of an autoimmune diathesis and may play a critical role in triggering activation of self-antigen-driven autoreactive B cells in BP.

Influenza virus host response of C57Bl/6 mice treated with TACI-Ig

TACI-Ig is a soluble glycoprotein comprised of a human IgG1-Fc fused with the extracellular domain of the human TACI receptor. Chronic exposure to TACI-Ig is associated with reduced circulating B cells in mouse and non-human primates, and a concomitant decrease in circulating immunoglobulin. Because of these activities, TACI-Ig is in clinical evaluation for treatment of various autoimmune diseases and B cell malignancies. In this study, the effect of TACI-Ig treatment on the ability of C57Bl/6 mice to clear influenza virus was evaluated. C57Bl/6 mice were exposed to vehicle (negative control), dexamethasone (positive control), or TACI-Ig (0.05, 0.50, or 5.0 mg/kg, SC, thrice weekly) from within one week prior to viral exposure through 21 days thereafter. Dexamethasone treatment of influenza-infected mice prolonged the infection, and decreased survival, body weight, lymphoid organ weight, influenza-specific IgM and IgG, and viral clearance relative to control animals, consistent with its expected immunosuppressive activity. Animals treated with TACI-Ig (0.05, 0.50, and 5.0 mg/kg) demonstrated a dose-dependent decrease in spleen weight and influenza-specific IgG and IgM in both lung and serum relative to control animals. In addition, flow cytometric analyses showed a decrease in B cells, but not T cells, in peripheral blood in animals treated with TACI-Ig. However, neither viral clearance nor survival was affected by TACI-Ig treatment. These data demonstrate the expected B cell-specific pharmacological effects of TACI-Ig in influenza-challenged C57Bl/6 mice without apparent effect on influenza virus clearance. It is concluded that non-B cell related antiviral competence remains intact during TACI-Ig treatment.

Space, selection, and surveillance: setting boundaries with BLyS

The BLyS family of ligands and receptors governs B cell homeostasis by controlling survival, differentiation, and lifespan. This family consists of multiple receptors and ligands, allowing independent regulation of different B cell subsets by varying the combination and levels of receptors expressed. Multiple downstream signaling pathways are implicated in these activities, reflecting this receptor complexity as well as cross-talk with other B cell signaling systems. BLyS levels are associated with multiple forms of humoral autoimmunity and can modulate tolerogenic elimination at the transitional checkpoint. BLyS responsiveness thus balances peripheral selection against cell numbers, providing an elastic system that varies selective stringency based on homeostatic demands.

A role for BLyS in the activation of innate immune cells

B-lymphocyte stimulator (BLyS) is a member of the tumor necrosis factor (TNF) ligand superfamily. Although BLyS costimulates adaptive immune cells, the ability of BLyS to stimulate innate immune cells has not been described. Here, we show that BLyS strongly induces human monocyte survival, and activation as measured by proinflammatory cytokine secretion and up-regulation of costimulatory molecule expression. In addition, monocytes cultured with BLyS differentiated into macrophage-like cells. Regarding BLyS receptor(s) expression, freshly isolated monocytes bound low levels of exogenous BLyS and expressed primarily intracellular TACI, and cell surface TACI levels increased following monocyte activation. Of interest, bone marrow monocytes from some multiple myeloma patients expressed significant levels of cell surface TACI at isolation. Our findings indicate that BLyS plays a role in activating innate immune cells. Moreover, this study may explain more clearly why high BLyS production is often correlated with certain inflammatory autoimmune diseases and B-lymphocyte malignancies.

Signalling pathways in B cells: implications for autoimmunity

Following investigations of the pathogenic role of autoantibodies in rheumatic diseases, preclinical and clinical studies suggest a more central role of B cells in the maintenance of the disease process beyond just being precursors of (auto)antibody-producing plasma cells. Detailed analyses have implicated a number of surface molecules and subsequent downstream signalling pathways in the regulation of the events induced by BCR engagement. In this review, we discuss the potential role of molecules involved in altered B cell longevity, especially molecules involved in apoptosis (bcl-2, bcl-x, mutations in the Fas/Fas-L pathway), as well as molecules that might alter activation thresholds of B cells (CD19, CD21, CD22, lyn, SHP, SHIP-1) in the development of autoimmunity. Although focused on intrinsic B cell abnormalities, the complexity of interactions of B cells with other immune cells also makes it possible that increased B cell activation can be induced by distortions in the interaction with other cells. Further delineation of these alterations of B cell function in autoimmune conditions will allow development of more precise B cell-directed therapies beyond drastic B cell depletion, with the potential to improve the risk-benefit ratio of the treatments of autoimmune diseases.

Construction and characterization of a bi-functional EGFP/sBAFF fusion protein

A fusion between gene encoding fluoresce-enhanced green fluorescent protein variant (EGFP) and soluble domain of human B-cell-activating factor of the TNF family (sBAFF) was constructed and expressed in Escherichia coli. The EGFP/sBAFF had an apparent molecular weight of 45 kDa and was detected with anti-hsBAFF and anti-His(6) monoclonal antibodies. After being purified by immobilized metal affinity chromatography (IMAC), the fusion protein retained similar fluorescence spectra to those of EGFP. Biological activity assays showed the EGFP/sBAFF as well as sBAFF could co-stimulated human B lymphocyte proliferation in vitro. In addition, EGFP/sBAFF has shown specific binding to BAFF receptors positive-cells and the stained cells could be analyzed with flow cytometry. Thus, the fusion protein represents a readily obtainable source of biologically active sBAFF that may prove useful in further studies on BAFF and its receptors.

Aberrant expression of BAFF in T cells of systemic lupus erythematosus, which is recapitulated by a human T cell line, Loucy

B cell-activating factor of the tumor necrosis factor (TNF) family, or BAFF, is mainly produced in monocytes and dendritic cells, and indispensable for proliferation, differentiation and survival of B cells. BAFF is a type II membrane-bound protein and the extracellular C-terminal fragment is released from the cells as soluble BAFF (sBAFF), which binds to specific receptors on B cells. Accumulating evidence suggests that BAFF plays an important role in the pathogenesis of autoimmune diseases, such as systemic lupus erythematosus (SLE). In this study, we developed a sensitive sandwich ELISA system to quantify the amount of sBAFF using our own mAb. Treatment of peripheral T cells of SLE patients with an anti-CD3 antibody triggered robust expression of BAFF and subsequent release of sBAFF from the cells. On the other hand, the stimulus induced only marginal elevation of sBAFF from normal T cells. These data indicate that BAFF is expressed in T cells upon stimulation at least under pathological conditions. Expression of BAFF was also largely induced in a human T cell line, Loucy (American Type Tissue Collection CRL-2629), in response to several stimuli, while other T cell lines so far examined produced the cytokine almost constitutively. These data suggest that Loucy recapitulates some of the characteristics of SLE T cells. Investigation of molecular and cellular mechanisms of production of BAFF in Loucy demonstrated that expression of BAFF was regulated through a signal transduction pathway which involves c-jun NH2-terminal kinase and p38, and that shedding of BAFF was catalyzed by a membrane-bound protease, furin.

New therapies for systemic lupus erythematosus: cellular targets

Antilymphocyte antibodies have been widely used in oncology and transplantation and are now being tested in autoimmune diseases. For systemic lupus erythematosus, anti-B-cell antibodies are furthest along in development. This article discusses the B-cell abnormalities found in systemic lupus erythematosus and the clinical and immunologic effects of anti-B-cell therapies.

Effect of cell:cell competition and BAFF expression on peripheral B cell tolerance and B-1 cell survival in transgenic mice expressing a low level of Igkappa-reactive macroself antigen

In mice carrying a synthetic Igkappa-reactive superantigen ("kappa macroself antigen"), low level expression induced split peripheral B cell tolerance in the sIgkappa+ compartment, with striking reductions in follicular and marginal zone (MZ) B cells and the retention of significant numbers of sIgkappa+ B-1a but not B-1b cells in the peritoneum. Here, we characterize the transgenic line pKkappa with this split tolerance phenotype and assess the effects of B cell competition and the survival cytokine BAFF (B cell activating factor belonging to the TNF family) on peripheral tolerance. In pKkappa mice the surviving peritoneal and splenic kappa+ B cells were largely lost in mice carrying one copy of the human Ckappa exon in place of the mouse version, a maneuver that generates additional antigen non-reactive competitor B cells in this model. Furthermore, overexpression of BAFF suppressed kappa-macroself antigen-induced deletion and promoted production of both IgM,kappa and IgA,kappa antibodies in mice with normal Igkappa alleles but not in mice carrying one copy of the human Ckappa allele. These findings suggest that BAFF overexpression has minimal effects on the survival of autoreactive B cells in a polyclonal immune system and that B cell:B cell competition plays a potent role in suppressing the survival of B-1 and splenic B cells with excessive autoreactivity.

B cell depletion therapy in systemic rheumatic diseases: different strokes for different folks?

Autoantibodies have, until recently, been the overriding focus of investigators of autoantibody-associated diseases. Increasing attention is now being paid to B cells, which not only are the producers of autoantibodies but also contribute to autoimmune disease via autoantibody-independent mechanisms. Therapeutic measures that target B cells for depletion are gaining in popularity. In this review, we will focus on two distinct approaches of depleting B cells; one employing a direct-kill approach by engagement of B cell surface CD20 with an anti-CD20 monoclonal antibody (rituximab), and the other employing an indirect starvation approach by neutralization of B lymphocyte stimulator (BLyS), a potent B cell survival factor. Among the systemic immune-based rheumatic disorders, we will focus on rheumatoid arthritis and systemic lupus erythematosus, two disorders for which therapeutic B cell targeting is being intensely investigated.

Circulating levels of B lymphocyte stimulator in patients with rheumatoid arthritis following rituximab treatment: relationships with B cell depletion, circulating antibodies, and clinical relapse

OBJECTIVE

To assess the effects of B lymphocyte depletion on serum B lymphocyte stimulator (BLyS; trademark of Human Genome Sciences, Rockville, MD) levels in patients with rheumatoid arthritis (RA), and to assess the relationship of serum BLyS levels with peripheral blood B cell depletion, levels of autoantibodies and antimicrobial antibodies, the return of peripheral blood B cells, and clinical relapse.

METHODS

Fifteen patients with active RA underwent rituximab-based B cell depletion therapy (BCDT). Disease activity was assessed clinically, peripheral blood CD19+ B cell counts were determined by flow cytometry, and serum levels of BLyS, IgM, IgA, and IgG rheumatoid factors (RFs), anti-cyclic citrullinated peptide, and antimicrobial antibodies were assessed using enzyme-linked immunosorbent assays.

RESULTS

Peripheral blood B cell depletion was achieved in all 15 patients, and an American College of Rheumatology 20% response was achieved in 13 patients. Following clinical relapse, 7 patients underwent at least 1 additional cycle of BCDT. In every case, serum BLyS levels markedly rose post-BCDT and remained elevated for at least 1-2 months. Serum levels of RF, but not those of anti-tetanus toxoid or anti-pneumococcal polysaccharide antibodies, fell significantly. A decline in serum BLyS levels was associated with the reemergence of B cells in peripheral blood, which, in turn, antedated clinical relapse by variable periods of time. The patterns of B cell depletion, serum BLyS and antibody levels, and clinical relapse for each BCDT cycle were remarkably similar in re-treated patients.

CONCLUSION

Rituximab-based BCDT leads to marked increases in serum BLyS levels. This may contribute significantly to the survival and/or regeneration of B cell populations capable of triggering clinical relapse.

An APRIL to remember: novel TNF ligands as therapeutic targets

Since their discovery in 1998, the two TNF family members APRIL and BLyS/BAFF have received increasing attention. In addition to regulating normal B-cell development and immune responses, these molecules might be crucial in a diverse set of diseases, including autoimmunity and cancer. Although more has been published about the general biology of BLyS/BAFF than that of APRIL, many recent articles have described novel APRIL biology. Here we focus on APRIL, exploring its normal and pathological functions, and comparing the therapeutic molecules currently under development that target BLyS/BAFF alone, or APRIL and BLyS/BAFF together.