Presence of four major haplotypes in human BCMA gene: lack of association with systemic lupus erythematosus and rheumatoid arthritis

Protein and functional isoform levels and genetic variants of the BAFF and APRIL pathway components in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is the prototype of an autoimmune disease. Belimumab, a monoclonal antibody targets BAFF, is the only biologic approved for SLE and active lupus nephritis. BAFF is a cytokine with a key-regulatory role in the B cell homeostasis, which acts by binding to three receptors: BAFF-R, TACI and BCMA. TACI and BCMA also bind APRIL. Many studies reported elevated soluble BAFF and APRIL levels in the sera of SLE patients, but other questions about the role of this system in the disease remain open. The study aimed to investigate the utility of the cytokine levels in serum and urine as biomarkers, the role of non-functional isoforms, and the association of gene variants with the disease. This case–control study includes a cohort (women, 18–60 years old) of 100 patients (48% with nephritis) and 100 healthy controls. We used ELISA assays to measure the cytokine concentrations in serum (sBAFF and sAPRIL) and urine (uBAFF and uAPRIL); TaqMan Gene Expression Assays to quantify the relative mRNA expression of ΔBAFF, βAPRIL, and εAPRIL, and next-generation sequencing to genotype the cytokine (TNFSF13 and TNFSF13B) and receptor (TNFRSF13B, TNFRSF17 and TNFRSF13C) genes. The statistical tests used were: Kruskal–Wallis (qualitative variables), the Spearman Rho coefficient (correlations), the Chi-square and SKAT (association of common and rare genetic variants, respectively). As expected, sBAFF and sAPRIL levels were higher in patients than in controls (p ≤ 0.001) but found differences between patient subgroups. sBAFF and sAPRIL significantly correlated only in patients with nephritis (r_s = 0.67, p ≤ 0.001) and βAPRIL levels were lower in patients with nephritis (p = 0.04), and ΔBAFF levels were lower in patients with dsDNA antibodies (p = 0.04). Rare variants of TNFSF13 and TNFRSF13B and TNFSF13 p.Gly67Arg and TNFRSF13B p.Val220Ala were associated with SLE. Our study supports differences among SLE patient subgroups with diverse clinical features in the BAFF/APRIL pathway. In addition, it suggests the involvement of genetic variants in the susceptibility to the disease.

Leptin promotes systemic lupus erythematosus by increasing autoantibody production and inhibiting immune regulation

Leptin is an adipocytokine that plays a key role in the modulation of immune responses and the development and maintenance of inflammation. Circulating levels of leptin are elevated in systemic lupus erythematosus (SLE) patients, but it is not clear whether this association can reflect a direct influence of leptin on the propathogenic events that lead to SLE. To investigate this possibility, we compared the extent of susceptibility to SLE and lupus manifestations between leptin-deficient (ob/ob) and H2-matched leptin-sufficient (wild-type, WT) mice that had been treated with the lupus-inducing agent pristane. Leptin deficiency protected ob/ob mice from the development of autoantibodies and renal disease and increased the frequency of immunoregulatory T cells (Tregs) compared with leptin-sufficient WT mice. The role of leptin in the development of SLE was confirmed in the New Zealand Black (NZB) × New Zealand White (NZW)F1 (NZB/W) mouse model of spontaneous SLE, where elevated leptin levels correlated with disease manifestations and the administration of leptin accelerated development of autoantibodies and renal disease. Conversely, leptin antagonism delayed disease progression and increased survival of severely nephritic NZB/W mice. At the cellular level, leptin promoted effector T-cell responses and facilitated the presentation of self-antigens to T cells, whereas it inhibited the activity of regulatory CD4 T cells. The understanding of the role of leptin in modulating autoimmune responses in SLE can open possibilities of leptin-targeted therapeutic intervention in the disease.

Genetic associations of leptin-related polymorphisms with systemic lupus erythematosus

Leptin is abnormally elevated in the plasma of patients with systemic lupus erythematosus (SLE), where it is thought to promote and/or sustain pro-inflammatory responses. Whether this association could reflect an increased genetic susceptibility to develop SLE is not known, and studies of genetic associations with leptin-related polymorphisms in SLE patients have been so far inconclusive. Here we genotyped DNA samples from 15,706 SLE patients and healthy matched controls from four different ancestral groups, to correlate polymorphisms of genes of the leptin pathway to risk for SLE. It was found that although several SNPs showed weak associations, those associations did not remain significant after correction for multiple testing. These data do not support associations between defined leptin-related polymorphisms and increased susceptibility to develop SLE.

Regulatory roles of the tumor necrosis factor receptor BCMA

B cell maturation antigen (BCMA) is a tumor necrosis family receptor (TNFR) member that is predominantly expressed on terminally differentiated B cells and, upon binding to its ligands B cell activator of the TNF family (BAFF) and a proliferation inducing ligand (APRIL), delivers pro-survival cell signals. Thus, BCMA is mostly known for its functional activity in mediating the survival of plasma cells that maintain long-term humoral immunity. The expression of BCMA has also been linked to a number of cancers, autoimmune disorders, and infectious diseases that suggest additional roles for BCMA activity. Despite recent advances in our understanding of the roles for the related TNFR members BAFF-R and transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), the signaling pathway used by BCMA for mediating plasma cell survival as well as its putative function in certain disease states are not well understood. By examining the expression, regulation, and signaling targets of BCMA, we may gain further insight into this receptor and how it operates within cells in both health and disease. This information is important for identifying new therapeutic targets that may be relevant in treating diseases that involve the BAFF/APRIL cytokine network.

Association between -871C>T promoter polymorphism in the B-cell activating factor gene and the response to rituximab in rheumatoid arthritis patients

OBJECTIVE

To determine whether a functional single-nucleotide polymorphism in the B-cell activating factor (BAFF) gene correlates with the response to treatment with rituximab (RTX) in RA.

METHODS

SMART is a randomized open trial (NCT01126541) assessing two strategies of re-treatment in patients responding to 1-g infusion of RTX with MTX on days 1 and 15 after failure, intolerance or contraindication to TNF blockers. Among the 224 patients included, 115 provided informed consent, could be genotyped and were included in an ancillary study of SMART assessing European League Against Rheumatism (EULAR) response rate after the first course of RTX according to BAFF-871C>T polymorphism. Baseline clinical factors (patients and disease characteristics) and biologic factors (ESR, CRP, RF, anti-citrullinated peptide antibodies, serum immunoglobulins) were collected. Univariate analyses were performed to assess whether BAFF-871C>T polymorphism was associated with EULAR response at week 24. Results with P < 0.15 obtained in univariate analyses were then included in multivariate analysis adjusted on DAS28 level.

RESULTS

Ninety-three patients (81%) were responders, of whom 31 (27%) were good responders. CC genotype was significantly associated with a higher response rate [92% of responders vs 64% for TT genotype, odds ratio (OR) = 6.9; 95% CI 1.6, 29.6; P = 0.03]. These results were also confirmed in RF-positive patients (96% vs 58%, P = 0.006). In multivariate analysis, C allele carriage was independently associated with response to RTX (OR = 4.1; 95% CI 1.3, 12.7; P = 0.017).

CONCLUSION

The BAFF-871C>T polymorphism seems to influence the response to RTX in RA patients after failure or intolerance to TNF blockers.

B lymphocyte stimulator expression in pediatric systemic lupus erythematosus and juvenile idiopathic arthritis patients

OBJECTIVE

To assess the expression of B lymphocyte stimulator (BLyS) in patients with pediatric systemic lupus erythematosus (SLE) or juvenile idiopathic arthritis (JIA).

METHODS

Blood samples collected from patients with pediatric SLE (n = 56) and patients with JIA (n = 54) at the beginning and end of a 6-month interval were analyzed for plasma BLyS protein levels by enzyme-linked immunosorbent assay and for blood leukocyte full-length BLyS and DeltaBLyS messenger RNA (mRNA) levels by quantitative real-time polymerase chain reaction (normalized to 18S expression). Healthy siblings (n = 34) of these patients served as controls.

RESULTS

In pediatric SLE, plasma BLyS protein and blood leukocyte BLyS mRNA levels were each significantly elevated, and plasma BLyS protein levels, but not blood leukocyte BLyS mRNA levels, were correlated with disease activity. In contrast, plasma BLyS protein levels were normal in JIA despite blood leukocyte BLyS mRNA levels being elevated to degrees similar to those in pediatric SLE. Among JIA patients, neither BLyS parameter was correlated with disease activity. In both pediatric SLE and JIA, the BLyS expression profiles remained stable at 6 months.

CONCLUSION

Our findings indicate that, as previously noted in adult SLE, plasma BLyS protein and blood leukocyte BLyS mRNA levels are elevated in pediatric SLE. The correlation of plasma BLyS protein levels with disease activity points to BLyS as a candidate therapeutic target in pediatric SLE. Contrary to previous observations in adults with rheumatoid arthritis, plasma BLyS protein levels are normal in JIA despite elevated blood leukocyte BLyS mRNA levels. The absence of correlation between either of the BLyS parameters and disease activity in JIA calls for circumspection prior to assigning BLyS as a candidate therapeutic target in this disorder.

Markers of B-lymphocyte activation are elevated in patients with early rheumatoid arthritis and correlated with disease activity in the ESPOIR cohort

Introduction

Little is known about systemic B-cell activation in early rheumatoid arthritis (RA). We therefore evaluated the serum levels of markers of B-cell activation in patients included in the ESPOIR early arthritis cohort.

Methods

In the ESPOIR early arthritis cohort (at least 2 swollen joints for more than 6 weeks but less than 6 months), 710 patients were assessed at 1 year and either met the 1987 American College of Rheumatology criteria for RA (n = 578) or had undifferentiated arthritis (n = 132). Baseline serum samples of patients naïve to corticosteroid and disease-modifying antirheumatic drug treatment were assessed for beta2-microglobulin, IgG, IgA, IgM, immunoglobulin free light chains of immunoglobulins, and B-cell activating factor of the tumor necrosis factor family (BAFF). The BAFF gene 871T>C polymorphism was genotyped in all patients.

Results

All markers of B-cell activation except BAFF and IgM were significantly higher in patients with early RA than those with undifferentiated arthritis. Anti-cyclic citrullinated peptide (anti-CCP) and beta2-microglobulin were associated with a diagnosis of early RA in the multivariate analysis. Markers of B-cell activation, except BAFF, were associated with disease activity, rheumatoid factor and anti-CCP secretion. The BAFF gene polymorphism was not associated with early RA.

Conclusions

Markers of B-cell activation are elevated in patients with early RA, compared with undifferentiated arthritis, independently of any systemic increase in BAFF secretion, and correlate with disease activity. This study sheds new light on the early pathogenic role of B-lymphocytes in RA and suggests that targeting them might be a useful therapeutic strategy in early RA.

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.

No evidence for an association between the -871 T/C promoter polymorphism in the B-cell-activating factor gene and primary Sjögren's syndrome

Polyclonal B cell activation might be related to pathogenic over-expression of B-cell-activating factor (BAFF) in primary Sjögren's syndrome (pSS) and other autoimmune diseases. We therefore investigated whether BAFF over-expression in pSS could be a primary, genetically determined event that leads to the disease. The complete BAFF gene was sequenced in Caucasian pSS patients and control individuals. The only single nucleotide polymorphism frequently observed, namely -871 T/C in the promoter region, was then genotyped in 162 French patients with pSS and 90 French control individuals. No significant differences in allele (T allele frequency: 49.7% in patients with pSS versus 50% in controls; P = 0.94) and genotype frequencies of BAFF polymorphism were detected between pSS patients and control individuals. BAFF gene polymorphism was not associated with a specific pattern of antibody secretion either. T allele carriers had significantly increased BAFF protein serum levels (mean values of 8.6 and 5.7 ng/ml in patients with TT and TC genotypes, respectively, versus 3.3 ng/ml in patients with CC genotype; P = 0.01), although no correlation was observed between BAFF polymorphism and mRNA level. In conclusion, BAFF gene polymorphism is neither involved in genetic predisposition to pSS nor associated with a specific pattern of antibody production.

CD72 polymorphisms associated with alternative splicing modify susceptibility to human systemic lupus erythematosus through epistatic interaction with FCGR2B

We previously reported association of FCGR2B-Ile232Thr with systemic lupus erythematosus (SLE) in three Asian populations. Because polymorphism of CD72, another inhibitory receptor of B cells, was associated with murine SLE, we identified human CD72 polymorphisms, tested their association with SLE and examined genetic interaction with FCGR2B in the Japanese (160 SLE, 277 controls), Thais (87 SLE, 187 controls) and Caucasians (94 families containing SLE members). Four polymorphisms and six rare variations were detected. The former constituted two major haplotypes that contained one or two repeats of 13 nucleotides in intron 8 (designated as *1 and *2, respectively). Although association with susceptibility to SLE was not detected, the *1 allele was significantly associated with nephritis among the Japanese patients (P=0.024). RT-PCR identified a novel alternatively spliced (AS) transcript that was expressed at the protein level in COS-7 transfectants. The ratio of AS/common isoforms was strikingly increased in individuals with *2/*2 genotype when compared with *1/*1 (P=0.000038) or *1/*2 (P=0.0085) genotypes. Using the two Asian cohorts, significant association of FCGR2B-232Thr/Thr with SLE was observed only in the presence of CD72-*1/*1 genotype (OR 4.63, 95% CI 1.47-14.6, P=0.009 versus FCGR2B-232Ile/Ile plus CD72-*2/*2). Minigene assays demonstrated that the 13-nucleotide repeat and 4 bp deletion within the same haplotype of intron 8 could regulate alternative splicing. The AS isoform lacks exon 8, and is deduced to contain 49 amino acid changes in the membrane-distal portion of the extracellular domain, where considerable amino acid changes are known in CD72(c) allele associated with murine SLE. These results indicated that the presence of CD72-*2 allele decreases risk for human SLE conferred by FCGR2B-232Thr, possibly by increasing the AS isoform of CD72.

BAFF AND APRIL: a tutorial on B cell survival

BAFF, a member of the TNF family, is a fundamental survival factor for transitional and mature B cells. BAFF overexpression leads to an expanded B cell compartment and autoimmunity in mice, and elevated amounts of BAFF can be found in the serum of autoimmune patients. APRIL is a related factor that shares receptors with BAFF yet appears to play a different biological role. The BAFF system provides not only potential insight into the development of autoreactive B cells but a relatively simple paradigm to begin considering the balancing act between survival, growth, and death that affects all cells.

The TNF family members BAFF and APRIL: the growing complexity

B cell activating factor belonging to the TNF family (BAFF) and apoptosis-inducing ligand (APRIL) are two related members of the TNF ligand superfamily. Although they share two receptors, TACI and BCMA, transgenic and knockout mice in this system reveal that their functions are not redundant. BAFF is a critical survival/maturation factor for peripheral B cells and this activity is mediated through a BAFF-specific receptor, BAFF-R. Overexpression of BAFF has been linked to autoimmune disease and aspects of B cell neoplasia. APRIL appears to play a role in T-independent type II antigen responses and T cell survival, but can also induce proliferation/survival of non-lymphoid cells. Elevated expression of APRIL has been found in some tumor cell lines and in tumor tissue libraries. Therapies designed to inhibit the BAFF and APRIL pathways holds great promise for the future.

Variations in immune response genes and their associations with multifactorial immune disorders

There are three genetic methods often used for detecting genes contributing to susceptibility or resistance to multifactorial diseases: nonparametric linkage analysis, case-control association analysis, and transmission disequilibrium test. In this review, we present the theoretical basis that the case-control association study has the highest power of detecting disease genes if there is no population stratification between patients and controls. Taking advantage of the high power, we have carried out extensive case-control association analyses of candidate genes for the search of susceptibility genes to rheumatic diseases in the Japanese as well as in some other populations. Several new associations have been disclosed, including those of TNFR2, FCGR2B, and CD19 gene polymorphisms with systemic lupus erythematosus, in addition to some unexpected findings such as the common occurrence of NKG2-C null allele in the healthy population. Genome-wide association studies using single nucleotide polymorphisms (SNPs) or microsatellite polymorphisms have become realistic, and development of new high-throughput and cost-effective SNP typing technologies is urgently needed. At the same time, our observations may indicate that the 'classical' candidate gene approach will remain a strong alternative, even in the age of 'post genome-sequence'.

Genetic approaches to the investigation of rheumatoid arthritis

The investigation of genetic factors affecting the development or severity of rheumatoid arthritis may give new insights into the pathways involved in disease pathogenesis and lead to the identification of novel therapeutic targets. Recently, several novel approaches have been used in the attempt to unravel the complex association of rheumatoid arthritis with the human leukocyte antigen ( HLA ) gene region. It is clear that non-HLA genes are also involved in disease pathogenesis, and identifying them remains a challenge. In the past year, considerable headway has been made in this field, and some interesting strategies have been used. This review summarizes the results of many of the HLA and non-HLA studies and tries to draw lessons from the investigation of genetic susceptibility factors in other complex diseases.