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

Effect of scopoletin on phagocytic activity of U937-derived human macrophages: Insights from transcriptomic analysis

Scopoletin is a botanical coumarin. Notably, scopoletin effect on phagocytic activity has not been addressed on transcriptomic level. Accordingly, this study investigated the effect of scopoletin on phagocytosis-linked gene transcription. Whole phagocytosis transcriptional profiling of stimulated U937-derived macrophages (SUDMs) in response to scopoletin as compared to non-treated SUDMs was studied. Regarding scopoletin effect on 92 phagocytosis-linked genes, 12 of them were significantly affected (p-value < .05). Seven genes were downregulated (CDC42, FCGR1A/FCGR1C, ITGA9, ITGB3, PLCE1, RHOD & RND3) and five were upregulated (DIRAS3, ITGA1, PIK3CA, PIK3R3 & PLCD1). Moreover, scopoletin enhanced phagocytic activity of SUDMs. The current results highlighted the potential use of scopoletin as immunity booster and as an adjuvant remedy in management of some autoimmune reactions. To the best of our knowledge, this is the first report that unravels the effect of scopoletin on phagocytosis via transcriptomic analysis.

Serum levels of B-cell activating factor of the TNF family (BAFF) correlate with anti-Jo-1 autoantibodies levels and disease activity in patients with anti-Jo-1positive polymyositis and dermatomyositis

BACKGROUND

B-cell activating factor of the tumour necrosis factor family (BAFF) plays a role in autoantibody production and is elevated in dermatomyositis (DM) and anti-Jo-1-positive polymyositis (PM). We investigated the inter-relationships between serum levels of BAFF, anti-Jo-1 autoantibodies, and disease activity.

METHODS

Serum levels of BAFF and anti-Jo-1 antibodies measured by enzyme-linked immunosorbent assay (ELISA) were compared to levels of myoglobin, creatine kinase (CK), aminotransferases (alanine (ALT) and aspartate (AST)), C-reactive protein (CRP), and disease activity assessed by the Myositis Disease Activity Assessment Tool in 63 anti-Jo-1 antibody-positive DM/PM patients. Serial serum samples collected at 2 (46 cases) and 3-5 time points (23 cases) were included. Relationships between BAFF, anti-Jo-1, disease activity, CRP, and their longitudinal changes were evaluated using correlation analysis, multiple regression (MR), path analysis (PA), and hierarchical linear models (HLM).

RESULTS

Cross-sectional assessment demonstrated significant correlations between the levels of BAFF and anti-Jo-1 antibodies which were associated with levels of CK, myoglobin, AST, and CRP, as well as multivariate associations between BAFF, anti-Jo-1 antibodies, and CK levels. PA revealed direct effects of anti-Jo-1 antibodies on CK (β = 0.41) and both direct (β = 0.42) and indirect (through anti-Jo-1 antibodies; β = 0.17) effects of BAFF on CK. Changes in levels of both BAFF and anti-Jo-1 between two time points (Δ) were associated with Δmyoglobin and Δaminotransferases and changes of BAFF correlated with ΔCK, Δcutaneous, Δmuscle, Δglobal, and Δskeletal disease activities. The longitudinal analysis showed a high intra-individual variability of serum levels of BAFF over time (97%) which could predict 79% of the variance in anti-Jo-1 levels. The anti-Jo-1 variability was explained by inter-individual differences (68%). The close longitudinal relationship between levels of BAFF, anti-Jo-1, and disease activity was supported by high proportions of their variance explained with serum levels of CK and CRP or pulmonary and muscle activities.

CONCLUSION

Our findings of associations between levels of BAFF and anti-Jo-1 antibodies in serum and myositis activity suggest a role of this cytokine in disease-specific autoantibody production as part of disease mechanisms, and support BAFF as a potential target for intervention in anti-Jo-1-positive myositis patients.

B Cell-Extrinsic Myd88 and Fcer1g Negatively Regulate Autoreactive and Normal B Cell Immune Responses

MyD88 and FcR common γ-chain (Fcer1g, FcRγ) elicit proinflammatory responses to exogenous Ags. Deletion of these receptors in autoimmune models has generally led to reduced overall disease. In B cells, Myd88 is required for anti-DNA and anti-RNA autoantibody responses, whereas Fcer1g is not expressed in these cells. The roles of these receptors in myeloid cells during B cell autoimmune activation remain less clear. To investigate the roles of Myd88 and Fcer1g in non-B cells, we transferred anti-self-IgG (rheumatoid factor) B cells and their physiologic target Ag, anti-chromatin Ab, into mice lacking Fcer1g, Myd88, or both and studied the extrafollicular plasmablast response. Surprisingly, we found a markedly higher and more prolonged response in the absence of either molecule; this effect was accentuated in doubly deficient recipients, with a 40-fold increase compared with wild-type recipients at day 10. This enhancement was dependent on CD40L, indicating that Myd88 and FcRγ, presumably on myeloid APCs, were required to downregulate T cell help for the extrafollicular response. To extend the generality, we then investigated a classic T cell-dependent response to (4-hydroxy-3-nitrophenyl)acetyl conjugated to chicken γ globulin and found a similar effect. Thus, these results reveal novel regulatory roles in the B cell response for receptors that are typically proinflammatory.

IgG-Immune Complexes Promote B Cell Memory by Inducing BAFF

Memory B cell responses are vital for protection against infections but must also be regulated to prevent autoimmunity. Cognate T cell help, somatic hypermutation, and affinity maturation within germinal centers (GCs) are required for high-affinity memory B cell formation; however, the signals that commit GC B cells to the memory pool remain unclear. In this study, we identify a role for IgG-immune complexes (ICs), FcγRs, and BAFF during the formation of memory B cells in mice. We found that early secretion of IgG in response to immunization with a T-dependent Ag leads to IC-FcγR interactions that induce dendritic cells to secrete BAFF, which acts at or upstream of Bcl-6 in activated B cells. Loss of CD16, hematopoietic cell-derived BAFF, or blocking IC:FcγR regions in vivo diminished the expression of Bcl-6, the frequency of GC and memory B cells, and secondary Ab responses. BAFF also contributed to the maintenance and/or expansion of the follicular helper T cell population, although it was dispensable for their formation. Thus, early Ab responses contribute to the optimal formation of B cell memory through IgG-ICs and BAFF. Our work defines a new role for FcγRs in GC and memory B cell responses.

Allelic-dependent expression of an activating Fc receptor on B cells enhances humoral immune responses

B cells are pivotal regulators of acquired immune responses, and recent work in both experimental murine models and humans has demonstrated that subtle changes in the regulation of B cell function can substantially alter immunological responses. The balance of negative and positive signals in maintaining an appropriate B cell activation threshold is critical in B lymphocyte immune tolerance and autoreactivity. FcγRIIb (CD32B), the only recognized Fcγ receptor on B cells, provides immunoglobulin G (IgG)-mediated negative modulation through a tyrosine-based inhibition motif, which down-regulates B cell receptor-initiated signaling. These properties make FcγRIIb a promising target for antibody-based therapy. We report the discovery of allele-dependent expression of the activating FcγRIIc on B cells. Identical to FcγRIIb in the extracellular domain, FcγRIIc has a tyrosine-based activation motif in its cytoplasmic domain. In both human B cells and B cells from mice transgenic for human FcγRIIc, FcγRIIc expression counterbalances the negative feedback of FcγRIIb and enhances humoral responses to immunization in mice and to BioThrax vaccination in a human anthrax vaccine trial. Moreover, the FCGR2C-ORF allele is associated with the risk of development of autoimmunity in humans. FcγRIIc expression on B cells challenges the prevailing paradigm of unidirectional negative feedback by IgG immune complexes via the inhibitory FcγRIIb, is a previously unrecognized determinant in human antibody/autoantibody responses, and opens the opportunity for more precise personalized use of B cell-targeted antibody-based therapy.

Differential expression of BAFF and its receptors in discoid lupus erythematosus patients

BACKGROUND

B-cell activating factor of the TNF family (BAFF) promotes the maturation and survival of B cells. Because BAFF levels are elevated in systemic lupus erythematosus (SLE) patients, BAFF has been the target of emerging therapies for SLE, such as belimumab. Levels of BAFF and its receptors in discoid lupus erythematosus (DLE) patients are unknown.

OBJECTIVE

To compare skin and blood mRNA and protein levels of BAFF and its receptors BAFF-R, TACI, and BCMA in DLE subjects with (DLE+/SLE+ (N=28)) and without SLE (DLE+/SLE- (N=35)), psoriasis subjects (N=11), and normal subjects (N=42).

METHODS

We used quantitative real-time PCR to measure blood and skin BAFF, BAFF-R, TACI, and BCMA mRNA, sandwich ELISAs to measure sera BAFF, and immunohistochemistry to evaluate BAFF and BAFF-R skin protein expression.

RESULTS

BAFF mRNA and protein levels were highest in DLE+/SLE+blood, followed by DLE+/SLE-, psoriasis, and normal blood. BAFF protein also correlated with anti-nuclear antibodies, and autoantibodies against double-stranded DNA, single-stranded DNA, and ribonucleoprotein, and Systemic Lupus Erythematosus Disease Activity Index scores in DLE patients. While showing no difference between DLE+/SLE+ and DLE+/SLE- skin, BAFF and its receptors mRNA were up-regulated in DLE skin vs. normal and psoriasis skin. DLE skin had higher percentages of BAFF-R⁺ inflammatory cells, likely T cells and macrophages, than psoriasis and normal skin.

CONCLUSIONS

BAFF may be a serologic marker of systemic disease in DLE patients. BAFF and its receptors are elevated in DLE skin, suggesting that targeted therapies against these proteins could treat refractory DLE patients.

BAFF-ling autoantibodies

There is emerging evidence that autoantibodies directed against cytokines modulate the severity of autoimmune disease. Identification of cytokine-targeted autoantibodies in patients can be informative for diagnosis and predicting clinical outcome. In this issue of the JCI, Price and colleagues used a multiplex protein microarray to identify autoantibodies in serum from SLE patients. They found autoantibodies directed against the B cell-activating factor (BAFF) were associated with greater disease severity. This study highlights the contribution of cytokine-directed autoantibodies in disease and describes a valuable tool for identifying autoantibodies against serum antigens.

Pentraxins: structure, function, and role in inflammation

The pentraxins are an ancient family of proteins with a unique architecture found as far back in evolution as the Horseshoe crab. In humans the two members of this family are C-reactive protein and serum amyloid P. Pentraxins are defined by their sequence homology, their pentameric structure and their calcium-dependent binding to their ligands. Pentraxins function as soluble pattern recognition molecules and one of the earliest and most important roles for these proteins is host defense primarily against pathogenic bacteria. They function as opsonins for pathogens through activation of the complement pathway and through binding to Fc gamma receptors. Pentraxins also recognize membrane phospholipids and nuclear components exposed on or released by damaged cells. CRP has a specific interaction with small nuclear ribonucleoproteins whereas SAP is a major recognition molecule for DNA, two nuclear autoantigens. Studies in autoimmune and inflammatory disease models suggest that pentraxins interact with macrophage Fc receptors to regulate the inflammatory response. Because CRP is a strong acute phase reactant it is widely used as a marker of inflammation and infection.

C-reactive protein inhibits plasmacytoid dendritic cell interferon responses to autoantibody immune complexes

OBJECTIVE

C-reactive protein (CRP) is a serum pattern recognition molecule that binds to apoptotic cells and nucleoprotein autoantigens and Fcγ receptors (FcγR). In systemic lupus erythematosus (SLE), immune complexes (ICs) containing nucleoprotein autoantigens activate plasmacytoid dendritic cells (PDCs) to produce type I interferon (IFN), which contributes to disease pathogenesis. Autoantibody ICs are taken up by PDCs through FcγR type IIa into endosomes, where the nucleic acid components activate Toll-like receptor 7 (TLR-7) or TLR-9. The objective of this study was to investigate the effect of CRP on PDC and monocyte responses to nucleoprotein autoantigens and ICs.

METHODS

Peripheral blood mononuclear cells (PBMCs), purified monocytes, and PDCs were isolated from healthy volunteers and stimulated with autoantibody ICs containing apoptotic cells, small nuclear RNPs (snRNPs), or DNA, or directly with TLR-7 and TLR-9 agonists. Supernatants were analyzed for IFNα and cytokine levels by enzyme-linked immunosorbent assay and multiplex assay. Small nuclear RNPs were fluorescence-labeled, and the effect of CRP on binding, uptake, and intracellular localization of autoantibody snRNP complexes was measured by flow cytometry and confocal microscopy.

RESULTS

CRP bound to autoantigen did not induce IFNα in PBMCs or PDCs, whereas complexes formed with autoantibody did. Significantly, CRP inhibited the IFNα response to both anti-U1 RNP-snRNP complexes and anti-DNA-DNA complexes, but not to other TLR-7 and TLR-9 agonists. CRP directly inhibited PDC IFNα release, promoted PDC differentiation, and increased late endosome localization of autoantigen in PDCs and monocytes.

CONCLUSION

CRP is a regulator of the type I IFN response to SLE ICs. CRP increased the intracellular processing of ICs in late endosomes, which is associated with decreased synthesis of type I IFN after intracellular TLR activation.

Intravenous immunoglobulin inhibits BAFF production in chronic inflammatory demyelinating polyneuropathy - a new mechanism of action?

Chronic-inflammatory demyelinating polyneuropathy (CIDP) is an immune-mediated disease treated with intravenous immunoglobulin (IVIg). The underlying mechanism of action remains incompletely understood. The B-cell activating factor BAFF contributes to B-cell homeostasis and (auto-)antibody production. BAFF was recently identified as one key molecule in the development of autoimmune diseases. Herein, we demonstrate that BAFF serum levels are elevated in CIDP patients. IVIg treatment resulted in a significant decrease of BAFF serum level. In vitro, IVIg inhibited BAFF in monocytes. Consequently, we identified BAFF as a new target for IVIg in CIDP treatment and provide a new, Fcγ-receptor independent, mechanism of action for IVIg.

Serine phosphorylation of FcγRI cytoplasmic domain directs lipid raft localization and interaction with protein 4.1G

The high-affinity IgG receptor (CD64, FcγRI) has several special capacities, including the receptor-stimulated cleavage of the cell surface B cell-activating factor of the TNF superfamily (TNFSF13B). With the use of the yeast two-hybrid system, we and others have shown that FcγRI interacts with protein 4.1G (EPB41L2). Our mutational analyses identified two required 4.1G-interacting regions in the FcγRI CY and one FcγRI-interacting site in the C-terminus of protein 4.1G. Herein, we explore mechanism(s) that may regulate the interaction between protein 4.1G and FcγRI CY and influence FcγRI membrane mobility and function. We show that FcγRI CY interacts with protein 4.1G in vitro and that FcγRI coimmunoprecipitates protein 4.1G in freshly isolated human PBMC. With the use of immunostaining, we show that FcγRI colocalizes with protein 4.1G in unstimulated U937 cells, in which the FcγRI CY is constitutively serine-phosphorylated, but significant uncoupling occurs following FcγRI cross-linking, suggesting phosphoserine-regulated interaction. In vitro, protein 4.1G interacted preferentially with CK2-phosphorylated FcγRI CY, and compared with WT FcγRI, a nonphosphorylatable FcγRI mutant receptor was excluded from lipid rafts, suggesting a key role for protein 4.1G in targeting phosphorylated FcγRI to rafts. These data are consistent with a phosphoserine-dependent tethering role for protein 4.1G in maintaining FcγRI in lipid rafts and provide insight into the unique phosphoserine-based regulation of receptor signaling by FcγRI CY.

Blood stage Plasmodium falciparum antigens induce T cell independent immunoglobulin production via B cell activation factor of the TNF family (BAFF) pathway

T independent (TI) antigens (Ags) activate monocytes to produce a cytokine, termed B cell activation factor (BAFF), involved in immunoglobulin (Ig) production. This study aimed to investigate whether the soluble schizont fraction of Plasmodium falciparum antigen (sPfAg) and hemozoin (HZ) could act as TI Ag to induce P. falciparum (Pf) specific Ig production via BAFF pathway. Co-cultures of monocytes and naïve B cells from 6 healthy donors were stimulated with sPfAg (10mg/ml) or HZ (10μM). At interval times, the expressions of BAFF on activated monocytes, BAFF receptor (BAFF-R) and proliferation nuclear Ag in activated B cells were determined by flow cytometry. The soluble BAFF (sBAFF), total and specific IgG levels in the supernatants were assessed by enzyme-linked immunosorbent assay (ELISA). The finding revealed both sPfAg and HZ could activate monocytes to express BAFF on surface and release sBAFF in the supernatant within 72h of stimulation. The B cells responded to specific activation, indicated by BAFF-R expression on the surface within 72h, marked proliferation on day 7, and final production of total and specific IgG during days 7-12. Comparing to sPfAg, HZ stimulated monocyte and B cell co-culture to express higher levels of BAFF and sBAFF during 24-48h, more BAFF-R on HZ activated B cells within 24h and induced marked proliferation of B cells with higher Pf specific IgG level. However, stimulation with sPfAg showed a more significant correlation between BAFF expression on the activated monocytes at 72h and the Pf specific IgG level on day 12 (r=0.961, p=0.039, Pearson Correlation). In conclusion, it is possible that both sPfAg and HZ stimulated B cells to produce specific IgG with BAFF involvement.

Targeting BAFF in autoimmunity

BAFF and APRIL are TNF-like cytokines that support survival and differentiation of B cells. The early appreciation that overexpression of BAFF leads to B cell expansion and a lupus-like syndrome in mice, and the demonstration that BAFF inhibition delays lupus onset in spontaneous mouse models of SLE and other autoimmune diseases has rapidly led to the development of strategies for inhibiting both BAFF and APRIL. The commercialization of this new class of drugs has proceeded in parallel with the continuing elucidation of the biology of the cytokines and their receptors. Recent studies have uncovered a role for BAFF in enhancing both innate and adaptive immune responses and in amplifying aberrant pathways that arise during inflammation. Two phase III studies of an anti-BAFF antibody have yielded positive, although modest, results in SLE and alternate inhibitors are being tested in a variety of autoimmune diseases in which BAFF may play a pathogenic role.

Requirement for BAFF and APRIL during B cell development in GALT

The effects of B cell-activating factor belonging to the TNF family (BAFF) on B cell maturation and survival in the mouse are relatively well understood. In contrast, little is known about the role of BAFF in B cell development in other mammals, such as rabbits, that use GALT to develop and maintain the B cell compartment. We examined the expression and requirement of BAFF and a proliferation-inducing ligand (APRIL) during peripheral B cell development in young rabbits. By neutralizing BAFF and APRIL in neonates with a soluble decoy receptor, transmembrane activator calcium modulator and cyclophilin ligand interactor-Fc, we found a marked reduction in the number of peripheral B cells, but found no change in the bone marrow (BM) compartment. In the appendix, the size and number of proliferating B cell follicles were greatly reduced, demonstrating that although BAFF/APRIL is dispensable for B cell development in BM, it is required for B cell development in GALT. We found that all rabbit B cells expressed BAFF receptor 3, but did not bind rBAFF, suggesting that the BAFF-binding receptors (BBRs) are bound by endogenous soluble BAFF. Further, we found that B cells themselves express BAFF, suggesting that the soluble BAFF bound to BBRs may be endogenously produced and stimulate B cells in an autocrine fashion. Additionally, we propose that this chronic occupancy of BBRs on B cells may provide a tonic and/or survival signal for the maintenance of peripheral B cells in adults after B lymphopoiesis is arrested in BM.

BAFF is increased in renal transplant patients following treatment with alemtuzumab

Alemtuzumab is a monoclonal antibody that depletes T and B cells and is used as induction therapy for renal transplant recipients. Without long-term calcineurin inhibitor (CNI) therapy, alemtuzumab-treated patients have a propensity to develop alloantibody and may undergo antibody-mediated rejection (AMR). In pursuit of a mechanistic explanation, we analyzed peripheral B cells and serum of these patients for BAFF (Blys) and BAFF-R, factors known to be integral for B-cell activation, survival, and homeostasis. Serum BAFF levels of 22/24 alemtuzumab-treated patients were above normal range, with average levels of 1967 pg/mL compared to 775 pg/mL in healthy controls (p = 0.006). BAFF remained elevated 2 years posttransplant in 78% of these patients. BAFF-R on CD19(+) B cells was significantly downregulated, suggesting ligand/receptor engagement. BAFF mRNA expression was increased 2-7-fold in CD14(+) cells of depleted patients, possibly linking monocytes to the BAFF dysregulation. Addition of recombinant BAFF to mixed lymphocyte cultures increased B-cell activation to alloantigen, as measured by CD25 and CD69 coexpression on CD19(+) cells. Of note, addition of sirolimus (SRL) augmented BAFF-enhanced B-cell activation whereas CNIs blocked it. These data suggest associations between BAFF/BAFF-R and AMR in alemtuzumab-treated patients.

Cracking the BAFF code

The tumour necrosis factor (TNF) family members B cell activating factor (BAFF) and APRIL (a proliferation-inducing ligand) are crucial survival factors for peripheral B cells. An excess of BAFF leads to the development of autoimmune disorders in animal models, and high levels of BAFF have been detected in the serum of patients with various autoimmune conditions. In this Review, we consider the possibility that in mice autoimmunity induced by BAFF is linked to T cell-independent B cell activation rather than to a severe breakdown of B cell tolerance. We also outline the mechanisms of BAFF signalling, the impact of ligand oligomerization on receptor activation and the progress of BAFF-depleting agents in the clinical setting.

Fcgamma receptors: structure, function and role as genetic risk factors in SLE

Over 30 years ago, receptors for the Fc region of IgG (FcgammaR) were implicated in the pathogenesis of systemic lupus erythematosus (SLE). Since those pioneering studies, our knowledge of the structure and function of these FcgammaRs has increased dramatically. We now know that FcgammaR contributes to the regulation of acquired immunity and to the regulation of innate immune responses where FcgammaRs act as specific receptors for innate opsonins (CRP and SAP). Our understanding of the genomic architecture of the genes encoding the FcgammaR has also witnessed remarkable advances. Numerous functionally relevant single-nucleotide polymorphism (SNP) variants and copy number (CN) variants have been characterized in the FcgammaR genes. Many of these variants have also been shown to associate with risk to development of SLE and some have been associated with disease progression. This review will provide an overview of the FcgammaR in relation to SLE, including consideration of the role of genetic variants in FcgammaR in SLE pathogenesis. The difficulties in assessing genetic variation in these genes will be discussed. To enhance our understanding of the functional roles of these receptors in SLE, future research will need to integrate our knowledge of SNP variants, CN variants and the functional diversity of these receptors.

Selective regulation of autoreactive B cells by FcgammaRIIB

FcgammaRIIB is an inhibitory receptor which plays a role in limiting B cell and DC activation. Since FcgammaRIIB is known to dampen the signaling strength of the BCR, we wished to determine the impact of FcgammaRIIB on the regulation of BCRs which differ in their affinity for DNA. For these studies, FcgammaRIIB deficient BALB/c mice were bred with mice expressing the transgene-encoded H chain of the R4A anti-DNA antibody which gives rise to BCRs which express high, low or no affinity for DNA. The deletion of FcgammaRIIB in R4A BALB/c mice led to an alteration in the B cell repertoire, allowing for the expansion and activation of high affinity DNA-reactive B cells. By 6-8 months of age, R4A x FcgammaRIIB-/- BALB/c mice spontaneously developed anti-DNA antibody titers. These mice also displayed an induction of IFN-inducible genes and an elevation in levels of the B cell survival factor, BAFF. These data demonstrate that FcgammaRIIB preferentially limits activation of high affinity autoreactive B cells and can influence the activation of DC through an immune complex-mediated mechanism.