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

Blockade of B-cell-activating factor suppresses lupus-like syndrome in autoimmune BXSB mice

B-cell-activating factor (BAFF), a member of the tumour necrosis factor superfamily, plays a critical role in the maturation, homeostasis and function of B cells. In this study, we demonstrated the biological outcome of BAFF blockade in BXSB murine lupus model, using a soluble fusion protein consisting of human BAFF-R and human mutant IgG4 Fc. Mutation of Leu(235) to Glu in IgG4 Fc eliminated antibody-dependent cell cytotoxicity (ADCC) and complement lysis activity, and generated a protein devoid of immune effector functions. Treatment of BXSB mice with BAFF-R-IgG4mut fusion protein for 5 weeks resulted in significant B-cell reduction in both the peripheral blood and spleen. Treated mice developed lower proteinuria, reduced glomerulonephritis and much delayed host death than untreated animals. Thus, BAFF blockade with BAFF-R-IgG4mut protein is an effective strategy to treat B-cell-mediated lupus-like pathology. Moreover, compared with other IgG isotypes with undesired effector functions, mutant IgG4 Fc should prove useful in constructing novel therapeutic reagents to block immune molecule signalling in various diseases.

B cell abnormality and autoimmune disorders

Various abnormalities have been described in B cells from patients with systemic autoimmune diseases such as systemic lupus erythematosus (SLE) and lupus-prone mice. Many of the abnormalities do not appear to be connected with the pathogenesis of the disease. However, various animal models developing lupus-like disease including both spontaneous mutans such as (NZB x NZW)F1 and MRL/lpr and mice generated by transgenic or knockout technology such as Bim-deficient and CD40L-transgenic mice show defect in apoptosis of mature B cells induced by ligation of the B cell antigen receptor (BCR). BCR-mediated apoptosis appears to be involved in deletion of self-reactive B cells. Thus, defect in BCR-mediated apoptosis is a widely observed B cell abnormality in lupus-prone mice and may play a role in the pathogenesis of systemic autoimmune diseases by abrogating deletion of self-reactive B cells.

CD4(+)CD25(+) T-cells control autoimmunity in the absence of B-cells

OBJECTIVE

Tumor necrosis factor ligand family members B-cell-activating factor (BAFF) and a proliferation-inducing ligand (APRIL) can exert powerful effects on B-cell activation and development, type 1 T-helper cell (Th1) immune responses, and autoimmunity. We examined the effect of blocking BAFF and APRIL on the development of autoimmune diabetes.

RESEARCH DESIGN AND METHODS

Female NOD mice were administered B-cell maturation antigen (BCMA)-Fc from 9 to 15 weeks of age. Diabetes incidence, islet pathology, and T- and B-cell populations were examined.

RESULTS

BCMA-Fc treatment reduced the severity of insulitis and prevented diabetes development in NOD mice. BCMA-Fc-treated mice showed reduced follicular, marginal-zone, and T2MZ B-cells. B-cell reduction was accompanied by decreased frequencies of pathogenic CD4(+)CD40(+) T-cells and reduced Th1 cytokines IL-7, IL-15, and IL-17. Thus, T-cell activation was blunted with reduced B-cells. However, BCMA-Fc-treated mice still harbored detectable diabetogenic T-cells, suggesting that regulatory mechanisms contributed to diabetes prevention. Indeed, BCMA-Fc-treated mice accumulated increased CD4(+)CD25(+) regulatory T-cells (Tregs) with age. CD4(+)CD25(+) cells were essential for maintaining euglycemia because their depletion abrogated BCMA-Fc-mediated protection. BCMA-Fc did not directly affect Treg homeostasis given that CD4(+)CD25(+)Foxp3(+) T-cells did not express TACI or BR3 receptors and that CD4(+)CD25(+)Foxp3(+) T-cell frequencies were equivalent in wild-type, BAFF(-/-), TACI(-/-), BCMA(-/-), and BR3(-/-) mice. Rather, B-cell depletion resulted in CD4(+)CD25(+) T-cell-mediated protection from diabetes because anti-CD25 monoclonal antibody treatment precipitated diabetes in both diabetes-resistant NOD.microMT(-/-) and BCMA-Fc-treated mice.

CONCLUSIONS

BAFF/APRIL blockade prevents diabetes. BCMA-Fc reduces B-cells, subsequently blunting autoimmune activity and allowing endogenous regulatory mechanisms to preserve a prehyperglycemic state.

Atacicept, a novel B cell-targeting biological therapy for the treatment of rheumatoid arthritis

BACKGROUND

Recent data suggest a key role for B cells in the pathogenesis of many autoimmune diseases including rheumatoid arthritis (RA), and biological therapies targeting B cells are promising treatments for patients with RA. Atacicept inhibits B cell maturation, differentiation and survival, and immunoglobulin production by depriving B cells of growth and development signals. Therefore, atacicept may represent an effective strategy in RA treatment.

OBJECTIVE

To evaluate the potential value of atacicept in RA treatment based on preclinical and clinical studies.

METHODS

Preclinical and clinical data on atacicept were identified using PubMed and systematically reviewed.

RESULTS/CONCLUSION

Preclinical and clinical studies show that atacicept is well tolerated, with no increased incidence of infections. Atacicept displays non-linear pharmacokinetics, with a more than dose-proportional increase in free drug and less than dose-proportional, saturated increase in atacicept-ligand complex. Overall, the pharmacokinetic profiles of atacicept were consistent, dose-related and predictable. Dose-dependent reductions in immunoglobulins and other biomarkers, including rheumatoid factor, occurred rapidly but returned to baseline after discontinuation. There was a biphasic response in B cell number, but no effect on other leucocytes. Atacicept improved the signs and symptoms of RA, although larger studies are needed to confirm its efficacy and its optimal use.

Serum levels of tumour necrosis factor family members a proliferation-inducing ligand (APRIL) and B lymphocyte stimulator (BLyS) are inversely correlated in systemic lupus erythematosus

OBJECTIVE

To determine whether serum levels of a proliferation-inducing ligand (APRIL) are altered in patients with systemic lupus erythematosus (SLE), and correlate with disease parameters.

METHODS

Clinical and biological parameters were analysed for 43 patients that fulfilled American College of Rheumatology (ACR) criteria for SLE classification and were positive for anti-double-stranded DNA (dsDNA) antibodies at least once in their medical records. Tests included measurement of serum levels of the tumour necrosis factor (TNF) family members APRIL and B lymphocyte stimulator (BLyS; a cytokine shown to promote SLE disease).

RESULTS

Median APRIL levels were elevated in patients with SLE compared to patients with osteoarthritis and healthy controls, but did not correlate with the SLE Disease Activity Index (SLEDAI). APRIL serum levels showed an inverse correlation with BLyS serum levels (r = -0.339; p = 0.03). For patients with SLE with positive anti-dsDNA titres (>40 arbitrary units (AU)/ml) at inclusion (n = 25), circulating APRIL was inversely correlated with BLyS levels (r = -0.465; p = 0.022) and anti-dsDNA antibody titres (r = -0.411; p = 0.046). In a follow-up study at their second visit, 27 patients showed an inverse correlation of APRIL serum levels with BLyS (r = -0.398; p = 0.03) as well as with anti-dsDNA (r = -0.408; p = 0.03) titres and SLEDAI (r = -0.408; p = 0.01).

CONCLUSION

The inverse correlation observed between APRIL and BLyS suggests that APRIL acts as a protective factor. APRIL and BLyS may thus have opposite roles in SLE, which must be considered when defining therapeutic applications of these cytokines.

B cell-targeted therapies in autoimmunity: rationale and progress

B cells are recognized as main actors in the autoimmune process. Autoreactive B cells can arise in the bone marrow or in the periphery and, if not properly inhibited or eliminated, can lead to autoimmune diseases through several mechanisms: autoantibody production and immune complex formation, cytokine and chemokine synthesis, antigen presentation, T cell activation, and ectopic lymphogenesis. The availability of agents capable of depleting B cells (that is, anti-CD20 and anti-CD22 monoclonal antibodies) or targeting B cell survival factors (atacicept and belimumab) opens new perspectives in the treatment of diseases such as systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes, and multiple sclerosis.

Regulators of B-cell activity in SLE: a better target for treatment than B-cell depletion?

B cells, being a source of characteristic antinuclear autoantibodies, play a crucial role in the pathogenesis of systemic lupus erythematosus (SLE). Evidences indicate that alterations in B-cell regulation are responsible for B-cell hyperactivity as seen in SLE. T cells, soluble factors, and even B cells themselves regulate effector B-cell functions. The latter, so-called regulatory B cells possess regulatory function through production of the cytokine interleukin-10 (IL-10) that can damp down the humoral immune responses. This review will focus on B-cell regulation in the pathogenesis of SLE as a target for intervention. In particular, the regulatory impact of T cells through costimulation, soluble factors such as B lymphocyte stimulator, and the characteristics of IL 10–producing regulatory B cells will be discussed. Therapies targeting B cells as well as B-cell regulation seem promising, but the precise mechanisms involved in these interventions are not completely understood. More insight into B-cell regulation in SLE, and particularly in regulatory B cells, could lead to novel therapeutic strategies.

Two negative randomized controlled trials in lupus: now what?

Recently, two large randomized controlled trials of distinct biologic therapies in systemic lupus erythematosus, B-cell depletion with rituximab and co-stimulatory blockade with CTLA4Ig (abatacept), failed to meet primary endpoints. Given the great need for new treatments in lupus, these results were met with disappointment and have left the rheumatology and immunology community searching for an explanation. Are these experimental agents ineffective in lupus or are there trial design issues or other considerations? In this commentary, we discuss our perspective on these results within the context of current understanding of the pathophysiology of lupus and the mechanism of action of biologic therapies.

Crucial role for BAFF-BAFF-R signaling in the survival and maintenance of mature B cells

Defects in the expression of either BAFF (B cell activating factor) or BAFF-R impairs B cell development beyond the immature, transitional type-1 stage and thus, prevents the formation of follicular and marginal zone B cells, whereas B-1 B cells remain unaffected. The expression of BAFF-R on all mature B cells might suggest a role for BAFF-R signaling also for their in vivo maintenance. Here, we show that, 14 days following a single injection of an anti-BAFF-R mAb that prevents BAFF binding, both follicular and marginal zone B cell numbers are drastically reduced, whereas B-1 cells are not affected. Injection of control, isotype-matched but non-blocking anti-BAFF-R mAbs does not result in B cell depletion. We also show that this depletion is neither due to antibody-dependent cellular cytotoxicity nor to complement-mediated lysis. Moreover, prevention of BAFF binding leads to a decrease in the size of the B cell follicles, an impairment of a T cell dependent humoral immune response and a reduction in the formation of memory B cells. Collectively, these results establish a central role for BAFF-BAFF-R signaling in the in vivo survival and maintenance of both follicular and marginal zone B cell pools.

Expression and localization of rabbit B-cell activating factor (BAFF) and its specific receptor BR3 in cells and tissues of the rabbit immune system

Rabbits are widely used for vaccine development, and investigations of human infectious and autoimmune diseases such as Systemic Lupus Erythematosus (SLE). For these applications, we cloned, sequenced and expressed rabbit B-cell Activating Factor (BAFF), and localized BAFF in cells and tissues of the rabbit immune system. The rabbit homolog of the human BAFF binding site (miniBR3 peptide) within the BAFF-specific receptor BR3 was synthesized. This 26-residue core domain binds to recombinant rabbit BAFF protein. Flow cytometric analyses using purified recombinant rabbit BAFF combined with real-time PCR findings revealed that BAFF detected on peripheral blood B-cells from normal rabbits is probably complexed to BAFF receptors rather than produced by the B-cells. BAFF was detected in developing appendix of young rabbits by immunohistochemical staining suggesting that BAFF plays a role during the period following birth when rabbit B-cell development and pre-immune antibody repertoire diversification and selection is occurring.

The role of B lymphocyte stimulator (BLyS) in systemic lupus erythematosus

SLE, a chronic, multisystem autoimmune disorder with a broad range of symptoms, involves defective B cell selection and elimination of self-reactive B cells. B lymphocyte stimulator (BLyS), a soluble ligand of the TNF cytokine family, is a prominent factor in B cell differentiation, homeostasis, and selection. BLyS levels affect survival signals and selective apoptosis of autoantibody-producing B cells. High levels of BLyS may relax B cell selection and contribute to autoantibody production, exacerbating the SLE disease state. This review discusses the mechanism of BLyS action on B cells, its role in SLE, and specific targeting of BLyS in the treatment of SLE.

Functional implication of BAFF synthesis and release in gangliosides-stimulated microglia

BAFF is a recently identified member of the TNF ligand superfamily that plays a critical role in B cell differentiation, survival, and regulation of Ig production. In the present study, we examined whether BAFF is expressed in microglia, and the expression and release of BAFF are regulated by gangliosides. The results showed that BAFF was expressed and released in rat primary microglia as well as in BV-2 cells. Furthermore, its expression and release were increased by gangliosides stimulation and regulated by JAK-STAT, especially the STAT1- and STAT3-dependent signaling pathways. It was of particular interest to observe that SP600125 and SB203580, specific inhibitors of JNK and p38, did not inhibit BAFF synthesis but inhibited the release of sBAFF in gangliosides-treated cells by regulating furin expression, suggesting that the JNK and p38 signaling pathways regulate the release but not the synthesis of BAFF. Moreover, BV-2 cells expressed BAFF-R on their cell surface, and rat primary microglia expressed BAFF-R and TACI on their cell surface. rBAFF increased the release of cytokines, especially IL-6, TNF-alpha, and IL-10, in rat primary microglia as well as in BV-2 cells. These findings imply that BAFF secreted by microglia may play important roles in CNS inflammation by regulating microglia as well as infiltrated B cells.

Interleukin-10 and interferon-gamma up-regulate the expression of B-cell activating factor in cultured human promyelocytic leukemia cells

B-cell activating factor (BAFF) is a potent cell-survival factor, expressed in many hematopoietic cells, for B-cell maturation and activation. It is involved in pathogenesis of autoimmune disorders and B-cell malignancies. Although BAFF is produced by interferon-gamma (IFN-gamma) and interleukin-10 (IL-10) in monocytes, the mechanisms of the modulation of BAFF production and expression under normal and pathologic conditions have not been completely elucidated. In this study, we examined the effects of several inflammatory cytokines on BAFF expression in cultured human promyelocytic leukemia (HL-60) cells both at the transcriptional and posttranscriptional level. Incubation of the cells with IL-10 and IFN-gamma elevated the expression of membrane-bound and soluble forms of BAFF. A similar increase in BAFF-specific mRNA was noted in cultured cells. Unexpectedly, interleukin-4 (IL-4) treatment hardly affected BAFF expression at the mRNA and protein levels. Transcriptional regulation was examined in cultures transfected with a human BAFF promoter/reporter gene (chloramphenicol acetyltransferase) construct. IL-10 and IFN-gamma elicited marked enhancement of the human BAFF promoter activity. Collectively, these results demonstrated that IL-10 and IFN-gamma both regulate BAFF expression and synthesis in human promyelocytic leukemia cell cultures, and the activation occurs at the transcriptional level.

Expression of B-cell activating factor enhances protective immunity of a vaccine against Pseudomonas aeruginosa

B-cell activating factor (BAFF), a member of the TNF family, is a potent cytokine with stimulatory effects on B and T cells. To evaluate the potential of transient overexpression of BAFF to enhance vaccine immunogenicity, a replication-deficient adenovirus expressing full-length murine BAFF (AdBAFF) was tested in a mouse vaccine model against Pseudomonas aeruginosa. When coadministered with heat-killed P. aeruginosa, AdBAFF mediated a significant increase in anti-P. aeruginosa-specific serum and lung mucosal antibodies and resulted in improved protection against a lethal respiratory challenge with P. aeruginosa. This effect was independent of the site of administration of AdBAFF and was observed both when AdBAFF was given simultaneously with heat-killed P. aeruginosa as well as when AdBAFF was administered 4 weeks after immunization with heat-killed P. aeruginosa. These data demonstrate that a temporal increase in systemic BAFF levels is able to augment a P. aeruginosa-specific immune response upon immunization with heat-killed P. aeruginosa, suggesting that the immune-stimulatory effects of BAFF may be exploited as a molecular adjuvant for genetic vaccines.

Treatment of lupus patients with a tolerogenic peptide, hCDR1 (Edratide): immunomodulation of gene expression

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by dysregulation of cytokines, apoptosis, and B- and T-cell functions. The tolerogenic peptide, hCDR1 (Edratide), ameliorated the clinical manifestations of murine lupus via down-regulation of pro-inflammatory cytokines and apoptosis, up-regulation of the immunosuppressive cytokine TGF-beta, and the induction of regulatory T-cells. In the present study, gene expression was determined in peripheral blood mononuclear cells of 9 lupus patients that were treated for 26 weeks with either hCDR1 (five patients), or placebo (four patients). Disease activity was assessed by SLEDAI-2K and the BILAG scores. Treatment with hCDR1 significantly down-regulated the mRNA expression of the pathogenic cytokines IL-1beta, TNF-alpha, IFN-gamma, and IL-10, of BLyS (B-lymphocyte stimulator) and of the pro-apoptotic molecules caspase-3 and caspase-8. In contrast, the treatment up-regulated in vivo gene expression of both TGF-beta and FoxP3. Furthermore, hCDR1 treatment resulted in a significant decrease in SLEDAI-2K (from 8.0+/-2.45 to 4.4+/-1.67; P=0.02) and BILAG (from 8.2+/-2.7 to 3.6+/-2.9; P=0.03) scores. Thus, the tolerogenic peptide hCDR1, immunomodulates, in vivo, the expression of genes that play a role in SLE, consequently restoring the global immune dysregulation of lupus patients. Hence, hCDR1 has a potential role as a novel disease-specific treatment for lupus patients.

A proliferation-inducing ligand mediates follicular lymphoma B-cell proliferation and cyclin D1 expression through phosphatidylinositol 3-kinase-regulated mammalian target of rapamycin activation

A proliferation-inducing ligand (APRIL), as well as its receptors transmembrane activator and calcium-modulating cyclophilin ligand (CAML) interactor (TACI) and B-cell maturation antigen (BCMA), has been shown to be important in B-cell biology, and overexpression of APRIL in mice results in development of lymphoma. Limited data are available on APRIL-specific signaling responses, but knockout models suggest that signaling through TACI is critical to B-cell homeostasis. To better understand the mechanism by which APRIL exerts its effects and how it may contribute to lymphomagenesis, we sought to characterize the outcome of APRIL-TACI interactions. In support of murine studies, we find that APRIL induces proliferation of human patient follicular lymphoma (FL) B cells in a TACI-dependent manner. This study also shows that APRIL is expressed within the tumor microenvironment and that, upon engagement with TACI, APRIL mediates activation of the phosphatidylinositol 3-kinase (PI3K) pathway. Activation of PI3K via APRIL results in phosphorylation of Akt and mammalian target of rapamycin (mTOR) and the mTOR-specific substrates p70S6 kinase and 4E-binding protein 1 in a TACI-dependent manner. APRIL-mediated signaling also results in phosphorylation of Rb and up-regulation of cyclin D1. These studies are the first to characterize APRIL-TACI-specific signaling and suggest a role for this ligand-receptor pair in FL B-cell growth.

The receptor tyrosine kinase MerTK regulates dendritic cell production of BAFF

The MerTK receptor tyrosine kinase is an important negative regulator of dendritic cell function and is required to prevent B cell autoimmunity in vivo. It is not currently known however, if any causal relationship exists between these two aspects of MerTK function. We sought to determine if dendritic cells (DC) from mice lacking MerTK (mertk(- / - ) mice) have characteristics that may aid in the development of B cell autoimmunity. Specifically, we found that mertk(- / - ) mice contain an elevated number of splenic DC, and this population contains an elevated proportion of cells secreting the critical B cell pro-survival factor, B cell activating factor (BAFF). Elevated numbers of BAFF-secreting cells were also detected among mertk(- / - ) bone marrow-derived dendritic cell (BMDC) populations. This was observed in both resting BMDC, and BMDC stimulated with lipopolysaccharide (LPS) or treated with exogenous apoptotic cells. We also found that DC in general have a pro-survival effect on resting B cells in co-culture. However, despite containing more BAFF-secreting cells, mertk(- / - ) BMDC were not superior to C57BL/6 or baff-deficient BMDC at promoting B cell survival. Furthermore, using decoy receptors, we show that DC may promote B cell survival and autoimmunity through a BAFF-and a proliferation-inducing ligand-independent mechanism.

Increased CD4+Foxp3+ T cells in BAFF-transgenic mice suppress T cell effector responses

The cytokine B cell activation factor of the TNF family (BAFF) is considered to perform a proinflammatory function. This paradigm is particularly true for B cell-dependent immune responses; however the exact role for BAFF in regulating T cell immunity is ill-defined. To directly assess the effect of BAFF upon T cells, we analyzed T cell-dependent immune responses in BAFF-transgenic (Tg) mice. We found that T cell responses in BAFF-Tg mice are profoundly compromised, as indicated by their acceptance of islet allografts and delayed skin graft rejection. However, purified BAFF-Tg effector T cells could reject islet allografts with a normal kinetic, suggesting that the altered response did not relate to a defect in T cell function per se. Rather, we found that BAFF-Tg mice harbored an increased number of peripheral CD4+Foxp3+ T cells. A large proportion of the BAFF-expanded CD4+CD25+Foxp3+ regulatory T cells (Tregs) were CD62LlowCD103high and ICAM-1high, a phenotype consistent with an ability to home to inflammatory sites and prevent T cell effector responses. Indeed, depletion of the endogenous BAFF-Tg Tregs allowed allograft rejection to proceed, demonstrating that the increased Tregs were responsible for preventing alloimmunity. The ability of BAFF to promote Treg expansion was not T cell intrinsic, as Tregs did not express high levels of BAFF receptor 3, nor did excessive BAFF trigger NF-kappaB2 processing in Tregs. In contrast, we found that BAFF engendered Treg expansion through an indirect, B cell-dependent mechanism. Thus, under certain conditions, BAFF can play a surprising anti-inflammatory role in T cell biology by promoting the expansion of Treg cells.

Immunology of mercury

The heavy metal mercury is ubiquitously distributed in the environment resulting in permanent low-level exposure in human populations. Mercury can be encountered in three main chemical forms (elemental, inorganic, and organic) which can affect the immune system in different ways. In this review, we describe the effects of these various forms of mercury exposure on immune cells in humans and animals. In genetically susceptible mice or rats, subtoxic doses of mercury induce the production of highly specific autoantibodies as well as a generalized activation of the immune system. We review studies performed in this model and discuss their implications for the role of environmental chemicals in human autoimmunity.

Serum BLyS levels increase after rituximab as initial therapy in patients with follicular Grade 1 non-Hodgkin lymphoma

Serum B-lymphocyte stimulator (BLyS) levels are elevated in a subset of non-Hodgkin lymphoma (NHL) patients, particularly those with a family history of B-cell malignancies or a polymorphism in the BLyS gene. BLyS promotes growth of malignant B-cells and increased serum BLyS levels are associated with a poor clinical outcome. In this study, BLyS levels were measured before and after 4 weekly doses of rituximab in 30 patients with previously untreated follicular Grade 1 NHL. A significant increase was seen in the serum levels of BLyS (P = 0.0001) after rituximab therapy. The increase was independent of genetic variability in the BLyS gene.

B-Lymphocyte stimulator: a new biomarker for multiple myeloma

Multiple myeloma (MM) is a common malignant tumor, characterized by unlimited proliferation of abnormal plasmocytes in bone marrow. Considering the biological function of B-Lymphocyte stimulator (BLyS) and its receptors in B cell, we examined BLyS and its receptors expression in MM cells. Our studies confirmed that BLyS and its receptors are expressed in MM cells, including KM3, CZ-1, and primary MM cells, playing an important role in the survival and proliferation of MM cells. Additionally, we provide evidence that BLyS protein is located in the MM cell plasma membrane. We also found that IFN-gamma and IL-6 can induce BLyS expression on MM cells, while after the treatment of BAY11-7082, an IkB-alpha phosphorylation inhibitor, IFN-gamma induced up regulation of BLyS was completely inhibited, suggesting that nuclear factor kappaB (NF-kappaB) might be involved in the mechanism of the regulation of BLyS levels in response to cytokines. Finally, linear correlation analysis of the Lactate Dehydrogenase concentration and beta 2-microglobulin level with BLyS, and expressions of BLyS mRNA in MM patients revealed a significant correlation between them (P < 0.01 in all case), showing that BLyS could be a biomarker for the diagnosis and treatment of MM.

APRIL in B-cell malignancies and autoimmunity

A Proliferation Inducing Ligand (APRIL) was first identified as a cytokine expressed predominantly by tumour tissues and was not found in most normal tissues. The activity of this new cytokine, in terms of its ability to stimulate tumour cell proliferation in vivo, determined the catchy acronym of yet another TNF family cytokine: APRIL. Reports showing an association between APRIL and cancer have since been prolific, in particular, those showing a link with B cell malignancies. Evidence is accumulating that APRIL is also a player in several autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis, and Sjoegren's syndrome. However, we now know that APRIL also plays an important role in the immune system and in lymphocyte biology. In this chapter we outline the physiological role of APRIL in immunity and describe what is known regarding the role of APRIL in B cell malignancies and autoimmune disease.

Preclinical Support for Combination Therapy in the Treatment of Autoimmunity with Atacicept

Atacicept, a fully human recombinant fusion protein that blocks the activity of BLyS (B-Lymphocyte Stimulator) and APRIL (a proliferation-inducing ligand), is undergoing clinical evaluation in B-cell-mediated diseases, including autoimmune disorders. Nonclinical studies in mice and cynomolgus monkeys demonstrate dose-dependent, reversible decreases in circulating Ig concentrations and reductions in mature B cells in the peripheral blood and lymphoid tissues. However, the combination of atacicept with purine synthesis inhibitors (e.g., mycophenolate mofetil [MMF]) and anti-B-cell monoclonal therapy (e.g., rituximab) has not been evaluated. Atacicept does not augment hematological toxicities associated with MMF, including anemia or thrombocytopenia. Combination of atacicept with MMF or rituximab reduced B cells in the periphery (MMF) or tissues (MMF and rituximab) further than did monotherapy, as was the case with atacicept—MMF combination therapy and serum Ig concentrations. Overall, atacicept appears to augment the pharmacologic activity toward B cells of current immunosuppressive therapies without increasing the hematological toxicities associated with MMF. Enhanced reduction in B cells and Ig concentrations associated with atacicept combination therapy may increase therapeutic activity or allow dose reduction in autoimmune patients. Findings from nonclinical safety studies support clinical evaluation of atacicept combination therapies.

B-cell-activating factor inhibits CD20-mediated and B-cell receptor-mediated apoptosis in human B cells

B-cell-activating factor (BAFF) is a survival and maturation factor for B cells belonging to the tumour necrosis factor superfamily. Among three identified functional receptors, the BAFF receptor (BAFF-R) is thought to be responsible for the effect of BAFF on B cells though details of how remain unclear. We determined that a hairy-cell leukaemia line, MLMA, expressed a relatively high level of BAFF-R and was susceptible to apoptosis mediated by either CD20 or B-cell antigen receptor (BCR). Using MLMA cells as an in vitro model of mature B cells, we found that treatment with BAFF could inhibit apoptosis mediated by both CD20 and BCR. We also observed, using immunoblot analysis and microarray analysis, that BAFF treatment induced activation of nuclear factor-kappaB2 following elevation of the expression level of Bcl-2, which may be involved in the molecular mechanism of BAFF-mediated inhibition of apoptosis. Interestingly, BAFF treatment was also found to induce the expression of a series of genes, such as that for CD40, related to cell survival, suggesting the involvement of a multiple mechanism in the BAFF-mediated anti-apoptotic effect. MLMA cells should provide a model for investigating the molecular basis of the effect of BAFF on B cells in vitro and will help to elucidate how B cells survive in the immune system in which BAFF-mediated signalling is involved.

Apoptosis-related genes change their expression with age and hearing loss in the mouse cochlea

To understand possible causative roles of apoptosis gene regulation in age-related hearing loss (presbycusis), apoptotic gene expression patterns in the CBA mouse cochlea of four different age and hearing loss groups were compared, using GeneChip and real-time (qPCR) microarrays. GeneChip transcriptional expression patterns of 318 apoptosis-related genes were analyzed. Thirty eight probes (35 genes) showed significant differences in expression. The significant gene families include Caspases, B-cell leukemia/lymphoma2 family, P53, Calpains, Mitogen activated protein kinase family, Jun oncogene, Nuclear factor of kappa light chain gene enhancer in B-cells inhibitor-related and tumor necrosis factor-related genes. The GeneChip results of 31 genes were validated using the new TaqMan Low Density Array (TLDA). Eight genes showed highly correlated results with the GeneChip data. These genes are: activating transcription factor3, B-cell leukemia/lymphoma2, Bcl2-like1, caspase4 apoptosis-related cysteine protease 4, Calpain2, dual specificity phosphatase9, tumor necrosis factor receptor superfamily member12a, and Tumor necrosis factor superfamily member13b, suggesting they may play critical roles in inner ear aging.

Lymphoma cell adhesion-induced expression of B cell-activating factor of the TNF family in bone marrow stromal cells protects non-Hodgkin's B lymphoma cells from apoptosis

This study explores whether lymphoma cell adhesion-induced B cell-activating factor (BAFF) expression in bone marrow stromal cells (BMSCs) protects B lymphoma cells from apoptosis. We first showed protection of lymphoma cells from apoptosis by conditioned medium of a stromal cell-lymphoma cell coculture, either spontaneous or induced by mitoxantrone, implying a role for soluble factor(s) in lymphoma cell survival. Addition of BAFF counteracted mitoxantrone-induced apoptosis and elicited a reduction in spontaneous apoptosis in primary lymphomas, suggesting a role of BAFF in sustaining B-cell survival. Abundant BAFF was detected in the BMSC cell line (HS-5) and primary BMSCs by flow cytometry, RT-PCR and immunoblotting. BAFF levels were 20- to 200-fold higher in BMSCs than in lymphoma cells, and lymphoma cell adhesion to BMSCs augmented BAFF secretion twofold through upregulation of BAFF gene expression. Finally, neutralization of BAFF by TACI-Ig or depletion of BAFF by small hairpin RNA (shRNA) in BMSCs significantly enhanced lymphoma cell response to chemotherapy and overcame stroma-mediated drug resistance, suggesting that lymphoma cells use BMSC-derived BAFF as a survival factor. These findings support the hypothesis that lymphoma cells interact with BMSCs, resulting in stromal niches with high BAFF concentration, and identify BMSC-derived BAFF as a functional determinant for B lymphoma cell survival in the bone marrow environment.

The B cell in systemic lupus erythaematosus: a rational target for more effective therapy

Current treatment options for systemic lupus erythaematosus (SLE) are diverse and poorly defined, and aggressive therapy can be associated with serious toxicity and tolerability issues. There is, therefore, a need for new and improved treatments to be studied thoroughly in well-designed controlled trials. B Cell dysfunction has emerged as a key pathophysiological component of SLE and is a prime target for the development of new agents for a wide range of lupus severity, including advanced disease. Although many current drugs appear to modify B cell function, the advent of new, targeted therapies offers the hope of improved efficacy and a better long-term tolerability profile.

B cells flying solo

Systemic autoimmunity such as systemic lupus erythematosus (SLE) is associated with the loss of B-cell tolerance, B-cell dysregulation and autoantibody production. While some autoantibodies may contribute to the pathology seen with SLE, numerous studies have shown that dysregulation of T-cell function is another critical aspect driving disease. The positive results obtained in clinical trials using T-cell- or B-cell-specific treatments have suggested that cooperation between T and B cells probably underlies disease progression in many patients. A similar cooperative mechanism seemed to explain SLE developing in mice overexpressing the B-cell-activating factor from the tumor necrosis factor family (BAFF). However, surprisingly, T-cell-deficient BAFF transgenic (Tg) mice develop SLE similar to T-cell-sufficient BAFF Tg mice, and the disease was linked to innate activation of B cells and production of proinflammatory autoantibody isotypes. In conclusion, dysregulated innate activation of B cells alone can drive disease independently of T cells, and as such this aspect represents a new pathogenic mechanism in autoimmunity.

FcgammaRIIB signals inhibit BLyS signaling and BCR-mediated BLyS receptor up-regulation

These studies investigate how interactions between the BCR and FcgammaRIIB affect B lymphocyte stimulator (BLyS) recep-tor expression and signaling. Previous studies showed that BCR ligation up-regulates BLyS binding capacity in mature B cells, reflecting increased BLyS receptor levels. Here we show that FcgammaRIIB coaggregation dampens BCR-induced BLyS receptor up-regulation. This cross-regulation requires BCR and FcgammaRIIB coligation, and optimal action relies on the Src-homology-2 (SH2)-containing inositol 5 phosphase-1 (SHIP1). Subsequent to FcgammaRIIB/BCR coaggregation, the survival promoting actions of BLyS are attenuated, reflecting reduced BLyS receptor signaling capacity in terms of Pim 2 maintenance, noncanonical NF-kappaB activation, and Bcl-xL levels. These findings link the negative regulatory functions of FcgammaRIIB with BLyS-mediated B-cell survival.

Biologic activity and safety of belimumab, a neutralizing anti-B-lymphocyte stimulator (BLyS) monoclonal antibody: a phase I trial in patients with systemic lupus erythematosus

Introduction

This trial evaluated the safety, biologic activity, and pharmacokinetics of belimumab, a fully human monoclonal antibody that inhibits the biologic activity of the soluble form of the essential B-cell survival factor B-lymphocyte stimulator (BLyS) in patients with systemic lupus erythematosus (SLE).

Methods

Seventy patients with mild-to-moderate SLE were enrolled in a phase I, double-blind, randomized study and treated with placebo (n = 13) or belimumab (n = 57) at four different doses (1.0, 4.0, 10, and 20 mg/kg) as a single infusion or two infusions 21 days apart. Patients were followed for 84 to 105 days to assess adverse events, pharmacokinetics, peripheral blood B-cell counts, serology, and SLE disease activity. Data from the study were summarized using descriptive statistics. χ² type tests were used to analyze discrete variables. The Kruskal-Wallis test, the Wilcoxon test, and the analysis of covariance were used to analyze the continuous variables, as appropriate. The analysis was performed on all randomized patients who received study agent.

Results

The incidences of adverse events and laboratory abnormalities were similar among the belimumab and placebo groups. Belimumab pharmacokinetics were linear across the 1.0 to 20 mg/kg dose range. Long terminal elimination half-life (8.5 to 14.1 days), slow clearance (7 ml/day per kg), and small volume of distribution (69 to 112 ml/kg) were consistent with a fully human antibody. Significant reductions in median percentages of CD20⁺ B cells were observed in patients treated with a single dose of belimumab versus placebo (day 42: P = 0.0042; and day 84: P = 0.0036) and in patients treated with two doses of belimumab versus placebo (day 105: P = 0.0305). SLE disease activity did not change after one or two doses of belimumab.

Conclusions

Belimumab was well tolerated and reduced peripheral B-cell levels in SLE patients. These data support further studies of belimumab in autoimmune disorders.

Trial Registration

NCT00657007 [clinicaltrials.gov].

Prevention of murine antiphospholipid syndrome by BAFF blockade

OBJECTIVE

This study was undertaken to determine whether BAFF blockade can be used to prevent or treat antiphospholipid syndrome in a mouse model.

METHODS

Eight- and 12-week-old (NZW x BXSB)F(1) mice were treated with BAFF-R-Ig or TACI-Ig alone or in addition to a short course of CTLA-4Ig. Mice were monitored for thrombocytopenia and proteinuria. Sera were tested for anticardiolipin antibodies (aCL), BAFF levels, and levels of soluble vascular cell adhesion molecule and E-selectin. Mice were killed at 17, 22, or 32 weeks of age, and kidneys and hearts were subjected to histologic examination. Spleen cells were phenotyped and enzyme-linked immunospot assays for autoantibody-producing B cells were performed.

RESULTS

Both BAFF-R-Ig and TACI-Ig prevented disease onset and significantly prolonged survival. Treated mice had significantly smaller spleens than controls, with fewer B cells and fewer activated and memory T cells. BAFF blockade did not prevent the development of aCL, and there was only a modest delay in the development of thrombocytopenia. However, treated mice had significantly less nephritis and myocardial infarcts than did controls.

CONCLUSION

Our findings suggest that aCL are generated in the germinal center, which is relatively independent of BAFF. Effector function of antiplatelet antibodies was only modestly affected by BAFF blockade. In contrast, myocardial infarctions were prevented, suggesting that triggering of thromboses requires both autoantibodies and mediators of inflammation. Similarly, renal damage requires both immune complexes and effector cells. The dissociation between autoantibody production and inflammation that may occur with B cell-depleting therapies underscores the role of B cells as effector cells in the autoimmune response.

Raised intrathecal levels of APRIL and BAFF in patients with systemic lupus erythematosus: relationship to neuropsychiatric symptoms

Introduction

The tumour necrosis factor (TNF) family ligands BAFF (B-cell activating factor of TNF family) and APRIL (a proliferation-inducing ligand) are essential for B-cell survival and function. Elevated serum levels of BAFF and APRIL have been reported earlier in patients with systemic lupus erythematosus (SLE). Since autoantibody formation in the central nervous system (CNS) is a distinct feature of neuropsychiatric SLE (NPSLE), we have investigated whether NPSLE is associated with an enhanced intrathecal production of APRIL and BAFF.

Methods

Levels of BAFF and APRIL in cerebrospinal fluid (CSF) and serum from healthy controls, SLE patients without CNS involvement, and patients with NPSLE were determined by enzyme-linked immunosorbent assay. Interleukin-6 (IL-6) levels were determined by an IL-6-specific bioassay.

Results

SLE patients had levels of APRIL in CSF that were more than 20-fold higher and levels of BAFF in CSF that were more than 200-fold higher than those of healthy controls. Separate analyses of SLE patients with and without CNS involvement revealed that NPSLE patients had enhanced levels of APRIL in CSF. BAFF and APRIL were likely produced locally in the CNS as CSF and serum levels did not correlate. Moreover, CSF levels of APRIL correlated with BAFF but not with IL-6, suggesting that APRIL and BAFF in the CNS are regulated together but that they are produced independently of IL-6.

Conclusion

To our knowledge this is the first study to show elevated levels of BAFF and APRIL in CSF of SLE patients. APRIL was augmented in NPSLE patients compared with SLE patients without CNS involvement. APRIL and BAFF antagonists breeching the blood-brain barrier therefore could have beneficial effects on SLE patients, in particular patients with NPSLE.

The roles of the classical and alternative nuclear factor-kappaB pathways: potential implications for autoimmunity and rheumatoid arthritis

Nuclear factor-κB (NF-κB) is an inducible transcription factor controlled by two principal signaling cascades, each activated by a set of signal ligands: the classical/canonical NF-κB activation pathway and the alternative/noncanonical pathway. The former pathway proceeds via phosphorylation and degradation of inhibitor of NF-κB (IκB) and leads most commonly to activation of the heterodimer RelA/NF-κB1(p50). The latter pathway proceeds via phosphorylation and proteolytic processing of NF-κB2 (p100) and leads to activation, most commonly, of the heterodimer RelB/NF-κB2 (p52). Both pathways play critical roles at multiple levels of the immune system in both health and disease, including the autoimmune inflammatory response. These roles include cell cycle progression, cell survival, adhesion, and inhibition of apoptosis. NF-κB is constitutively activated in many autoimmune diseases, including diabetes type 1, systemic lupus erythematosus, and rheumatoid arthritis (RA). In this review we survey recent developments in the involvement of the classical and alternative pathways of NF-κB activation in autoimmunity, focusing particularly on RA. We discuss the involvement of NF-κB in self-reactive T and B lymphocyte development, survival and proliferation, and the maintenance of chronic inflammation due to cytokines such as tumor necrosis factor-α, IL-1, IL-6, and IL-8. We discuss the roles played by IL-17 and T-helper-17 cells in the inflammatory process; in the activation, maturation, and proliferation of RA fibroblast-like synovial cells; and differentiation and activation of osteoclast bone-resorbing activity. The prospects of therapeutic intervention to block activation of the NF-κB signaling pathways in RA are also discussed.

Effect of interleukin-2 on synthesis of B cell activating factor belonging to the tumor necrosis factor family (BAFF) in human peripheral blood mononuclear cells

B cell activating factor belonging to the tumor necrosis factor family (BAFF) is a cytokine, indispensable for B cell survival, maturation, and activation. Over-expression of BAFF leads to lupus like disease in mice and the serum level of BAFF is elevated in human lupus. However, little is known about BAFF synthesis and its regulation. In this study, we examined the effects of a series of inflammatory cytokines on BAFF production in human peripheral blood mononuclear cells (PBMCs) in vitro. We found interleukin-2 (IL-2) strongly and dose-dependently stimulated BAFF synthesis in PBMCs, and an anti-IL-2 antibody neutralized the effect. Furthermore, T and NK cells produced BAFF with IL-2 stimulation. From these observations, IL-2 is one of the regulatory cytokines having a positive effect on BAFF synthesis in human peripheral T and NK cells. Persistent over-production of IL-2 might lead to up-regulation of BAFF synthesis in PBMCs in pathological conditions such as lupus.

Immune opsonins modulate BLyS/BAFF release in a receptor-specific fashion

TNF ligand superfamily member 13B (B lymphocyte stimulator (BLyS), B cell activating factor (BAFF)) promotes primary B cell proliferation and Ig production. While the soluble form of BLyS/BAFF is thought to be the primary biologically active form, little is known about the regulation of its cleavage and processing. We provide evidence that Fcgamma receptor cross-linking triggers a rapid release of soluble, biologically active BLyS/BAFF from myeloid cells. Surprisingly, this function is primarily mediated by FcgammaRI, but not FcgammaRIIa as defined by specific mAb, and can be initiated by both IgG and C reactive protein as ligands. The generation of a B cell proliferation and survival factor by both innate and adaptive immune opsonins through engagement of an Fcgamma receptor, which can also enhance Ag uptake and presentation, provides a unique opportunity to facilitate Ab production. These results provide a mechanism by which Fcgamma receptors can elevate circulating BLyS levels and promote autoantibody production in immune complex-mediated autoimmune diseases.

B lymphocyte stimulator expression in patients with rheumatoid arthritis treated with tumour necrosis factor alpha antagonists: differential effects between good and poor clinical responders

OBJECTIVE

To assess the effects of tumour necrosis factor (TNF) antagonist therapy on B lymphocyte stimulator (BLyS) expression in patients with rheumatoid arthritis (RA).

METHODS

Blood from 38 patients with RA from a single centre was collected prior to and following initiation of TNF antagonist therapy. Plasma BLyS protein levels, blood leukocyte BLyS mRNA levels and disease activity were longitudinally monitored. Twelve patients with RA who either refused or were felt not to be candidates for TNF antagonist therapy and five normal healthy volunteers served as TNF antagonist-naïve controls.

RESULTS

Baseline plasma BLyS protein levels, but not blood leukocyte BLyS mRNA levels, were elevated in patients with RA. Plasma BLyS protein levels declined following initiation of TNF antagonist therapy in good responders (GR) to TNF antagonist therapy but not in poor responders (PR). By contrast, the erythrocyte sedimentation rate (ESR) declined in response to TNF antagonist therapy in GR and PR. TNF antagonist therapy did not promote change in blood leukocyte BLyS mRNA levels in either GR or PR, suggesting that the TNF antagonist-associated changes in circulating BLyS protein levels reflected changes in local BLyS production in the affected joints rather than changes in systemic BLyS production. BLyS expression did not change over time in either the normal or RA control groups.

CONCLUSIONS

A good clinical response to TNF antagonist therapy in patients with RA is associated with a decline in plasma BLyS protein levels. Increased BLyS expression in affected joints may contribute to ongoing disease activity, and reduction of such expression may help promote a favourable clinical response to TNF antagonist therapy.

Global T cell dysregulation in non-autoimmune-prone mice promotes rapid development of BAFF-independent, systemic lupus erythematosus-like autoimmunity

In otherwise non-autoimmune-prone C57BL/6 (B6) mice rendered genetically deficient in CD152 (CTLA-4), polyclonal hypergammaglobulinemia with increased levels of systemic lupus erythematosus (SLE)-associated IgG autoantibodies, glomerular IgG and C3 deposition, and interstitial nephritis all developed by 3-5 wk of age. Remarkably, superimposing genetic deficiency of BAFF (B cell-activating factor belonging to the TNF family) onto CD152 deficiency did not substantially attenuate humoral autoimmunity and immunopathology in these mice, despite the resulting marked reduction in B-lineage cells. Although superimposing a BAFF transgene (resulting in constitutive BAFF overexpression) onto CD152-deficient mice did lead to increases in B-lineage cells and serum levels of certain SLE-associated IgG autoantibodies, renal immunopathology remained largely unaffected. Taken together, these results demonstrate that global T cell dysregulation, even in an otherwise non-autoimmune-prone host, can promote systemic humoral autoimmunity and immunopathology in a BAFF-independent manner. Moreover, supraphysiologic expression of BAFF in the setting of ongoing autoimmunity does not necessarily lead to greater immunopathology. These findings may help explain the limited clinical efficacy appreciated to date of BAFF antagonists in human SLE.