Normal B cell homeostasis requires B cell activation factor production by radiation-resistant cells

Transcriptome Analysis of BAFF/BAFF-R System in Murine Nephrotoxic Serum Nephritis

Chronic kidney disease (CKD) is an emerging cause for morbidity and mortality worldwide. Acute kidney injury (AKI) can transition to CKD and finally to end-stage renal disease (ESRD). Targeted treatment is still unavailable. NF-κB signaling is associated with CKD and activated by B cell activating factor (BAFF) via BAFF-R binding. In turn, renal tubular epithelial cells (TECs) are critical for the progression of fibrosis and producing BAFF. Therefore, the direct involvement of the BAFF/BAFF-R system to the pathogenesis of CKD is conceivable. We performed non-accelerated nephrotoxic serum nephritis (NTN) as the CKD model in BAFF KO (B6.129S2-Tnfsf13btm1Msc/J), BAFF-R KO (B6(Cg)-Tnfrsf13ctm1Mass/J) and wildtype (C57BL/6J) mice to analyze the BAFF/BAFF-R system in anti-glomerular basement membrane (GBM) disease using high throughput RNA sequencing. We found that BAFF signaling is directly involved in the upregulation of collagen III as BAFF ko mice showed a reduced expression. However, these effects were not mediated via BAFF-R. We identified several upregulated genes that could explain the effects of BAFF in chronic kidney injury such as Txnip, Gpx3, Igfbp7, Ccn2, Kap, Umod and Ren1. Thus, we conclude that targeted treatment with anti-BAFF drugs such as belimumab may reduce chronic kidney damage. Furthermore, upregulated genes may be useful prognostic CKD biomarkers.

Progression of Notch signaling regulation of B cells under radiation exposure

With the continuous development of nuclear technology, the radiation exposure caused by radiation therapy is a serious health hazard. It is of great significance to further develop effective radiation countermeasures. B cells easily succumb to irradiation exposure along with immunosuppressive response. The approach to ameliorate radiation-induced B cell damage is rarely studied, implying that the underlying mechanisms of B cell damage after exposure are eager to be revealed. Recent studies suggest that Notch signaling plays an important role in B cell-mediated immune response. Notch signaling is a critical regulator for B cells to maintain immune function. Although accumulating studies reported that Notch signaling contributes to the functionality of hematopoietic stem cells and T cells, its role in B cells is scarcely appreciated. Presently, we discussed the regulation of Notch signaling on B cells under radiation exposure to provide a scientific basis to prevent radiation-induced B cell damage.

Distinct fibroblast functions associated with fibrotic and immune-mediated inflammatory diseases and their implications for therapeutic development

Fibroblasts are ubiquitous cells that can adopt many functional states. As tissue-resident sentinels, they respond to acute damage signals and shape the earliest events in fibrotic and immune-mediated inflammatory diseases. Upon sensing an insult, fibroblasts produce chemokines and growth factors to organize and support the response. Depending on the size and composition of the resulting infiltrate, these activated fibroblasts may also begin to contract or relax thus changing local stiffness within the tissue. These early events likely contribute to the divergent clinical manifestations of fibrotic and immune-mediated inflammatory diseases. Further, distinct changes to the cellular composition and signaling dialogue in these diseases drive progressive fibroblasts specialization. In fibrotic diseases, fibroblasts support the survival, activation and differentiation of myeloid cells, granulocytes and innate lymphocytes, and produce most of the pathogenic extracellular matrix proteins. Whereas, in immune-mediated inflammatory diseases, sequential accumulation of dendritic cells, T cells and B cells programs fibroblasts to support local, destructive adaptive immune responses. Fibroblast specialization has clear implications for the development of effective induction and maintenance therapies for patients with these clinically distinct diseases.

Clinical advances in immunotherapy for immune-mediated glomerular diseases

BACKGROUND AND OBJECTIVE

Due to the suboptimal therapeutic efficacy and potential adverse effects associated with traditional immunosuppressive medications, there has been an increasing emphasis on the development and utilization of immunotherapies. This paper aims to provide clinicians with valuable insights for selecting appropriate therapeutic approaches and contribute to the development of novel immunotherapeutic drugs.

MAIN BODY

This paper categorizes the immunotherapeutic drugs that are used for the treatment of immune-mediated glomerular diseases into three groups: immunotherapies targeting antigen-presenting cells (anti-CD80), immunotherapies targeting T/B cells (anti-CD20, anti-CD22, BAFF and APRIL inhibitors, CD40-CD40L inhibitors, proteasome inhibitors, Syk inhibitors, and Btk inhibitors), and immunotherapies targeting the complement system (C5 inhibitors, C5a/C5aR inhibitors, C3 inhibitors, MASP2 inhibitors, factor B inhibitors, and factor D inhibitors). The article then provides a comprehensive overview of advances related to these immunotherapeutic drugs in clinical research.

CONCLUSION

Certain immunotherapeutic drugs, such as rituximab, belimumab, and eculizumab, have exhibited notable efficacy in treating specific immune-mediated glomerular diseases, thereby providing novel therapeutic approaches for patients. Nonetheless, the efficacy of numerous immunotherapeutic drugs remains to be substantiated.

Aging of lymphoid stromal architecture impacts immune responses

The secondary lymphoid organs (SLOs) undergo structural changes with age, which correlates with diminishing immune responses against infectious disease. A growing body of research suggests that the aged tissue microenvironment can contribute to decreased immune function, independent of intrinsic changes to hematopoietic cells with age. Stromal cells impart structural integrity, facilitate fluid transport, and provide chemokine and cytokine signals that are essential for immune homeostasis. Mechanisms that drive SLO development have been described, but their roles in SLO maintenance with advanced age are unknown. Disorganization of the fibroblasts of the T cell and B cell zones may reduce the maintenance of naïve lymphocytes and delay immune activation. Reduced lymphatic transport efficiency with age can also delay the onset of the adaptive immune response. This review focuses on recent studies that describe age-associated changes to the stroma of the lymph nodes and spleen. We also review recent investigations into stromal cell biology, which include high-dimensional analysis of the stromal cell transcriptome and viscoelastic testing of lymph node mechanical properties, as they constitute an important framework for understanding aging of the lymphoid tissues.

Transcriptome analysis of renal ischemia/reperfusion (I/R) injury in BAFF and BAFF-R deficient mice

Acute kidney injury (AKI) accompanies with high morbidity and mortality. Incomplete renal recovery can lead to chronic and finally end-stage kidney disease, which results in the requirement of lifelong dialysis or kidney transplantation. Consequently, finding predictive biomarker and therefore developing preventive therapeutic approaches is an urgent need. For this purpose, a better understanding of the mechanism underlying AKI is necessary. The cytokine BAFF (B cell activating factor) is related to AKI by supporting B cells, which in turn play an important role in inflammatory processes and the production of antibodies. In our study, we investigated the role of BAFF and its receptor BAFF-R in the early phase of AKI. Therefore, we performed the well-established ischemia/reperfusion (I/R) model in BAFF (B6.129S2-Tnfsf13btm1Msc/J) and BAFF-R (B6(Cg)-Tnfrsf13ctm1Mass/J) deficient mice. Transcriptome of ischemic and contralateral control kidneys was analyzed and compared to wildtype littermates. We detected the upregulation of Lcn2, Lyz2, Cd44, Fn1 and Il1rn in ischemic kidneys as well as the downregulation of Kl. Furthermore, we revealed different expression patterns in BAFF and BAFF-R knockout mice. Compared to wildtype littermates, up- and downregulation of each investigated gene were higher in BAFF-R knockout and lower in BAFF knockout. Our findings indicate a positive impact of BAFF knockout in early phase of AKI, while BAFF-R knockout seems to worsen I/R injury. In addition, our study shows for the first time a remarkable renal upregulation of Lyz2 in a murine I/R model. Therefore, we consider Lyz2 as conceivable predictive or early biomarker in case of I/R and AKI.

Post-transcriptional checkpoints in autoimmunity

Post-transcriptional regulation is a fundamental process in gene expression that has a role in diverse cellular processes, including immune responses. A core concept underlying post-transcriptional regulation is that protein abundance is not solely determined by transcript abundance. Indeed, transcription and translation are not directly coupled, and intervening steps occur between these processes, including the regulation of mRNA stability, localization and alternative splicing, which can impact protein abundance. These steps are controlled by various post-transcription factors such as RNA-binding proteins and non-coding RNAs, including microRNAs, and aberrant post-transcriptional regulation has been implicated in various pathological conditions. Indeed, studies on the pathogenesis of autoimmune and inflammatory diseases have identified various post-transcription factors as important regulators of immune cell-mediated and target effector cell-mediated pathological conditions. This Review summarizes current knowledge regarding the roles of post-transcriptional checkpoints in autoimmunity, as evidenced by studies in both haematopoietic and non-haematopoietic cells, and discusses the relevance of these findings for developing new anti-inflammatory therapies.

B cell-activating factor (BAFF) from dendritic cells, monocytes and neutrophils is required for B cell maturation and autoantibody production in SLE-like autoimmune disease

Purpose and methods

B cell-activating factor (BAFF) contributes to the pathogenesis of autoimmune diseases including systemic lupus erythematosus (SLE). Although several anti-BAFF Abs and derivatives have been developed for the treatment of SLE, the specific sources of BAFF that sustain autoantibody (auto-Ab) producing cells have not been definitively identified. Using BAFF-RFP reporter mice, we identified major changes in BAFF-producing cells in two mouse spontaneous lupus models (Tlr7 Tg mice and Sle1), and in a pristane-induced lupus (PIL) model.

Results

First, we confirmed that similar to their wildtype Tlr7 Tg and Sle1 mice counterparts, BAFF-RFP Tlr7 Tg mice and BAFF-RFP Sle1 mice had increased BAFF serum levels, which correlated with increases in plasma cells and auto-Ab production. Next, using the RFP reporter, we defined which cells had dysregulated BAFF production. BAFF-producing neutrophils (Nphs), monocytes (MOs), cDCs, T cells and B cells were all expanded in the spleens of BAFF-RFP Tlr7 Tg mice and BAFF-RFP Sle1 mice compared to controls. Furthermore, Ly6C^hi inflammatory MOs and T cells had significantly increased BAFF expression per cell in both spontaneous lupus models, while CD8^- DCs up-regulated BAFF expression only in the Tlr7 Tg mice. Similarly, pristane injection of BAFF-RFP mice induced increases in serum BAFF levels, auto-Abs, and the expansion of BAFF-producing Nphs, MOs, and DCs in both the spleen and peritoneal cavity. BAFF expression in MOs and DCs, in contrast to BAFF from Nphs, was required to maintain homeostatic and pristane-induced systemic BAFF levels and to sustain mature B cell pools in spleens and BMs. Although acting through different mechanisms, Nph, MO and DC sources of BAFF were each required for the development of auto-Abs in PIL mice.

Conclusions

Our findings underscore the importance of considering the relative roles of specific myeloid BAFF sources and B cell niches when developing treatments for SLE and other BAFF-associated autoimmune diseases.

SOCS1 Haploinsufficiency Presenting as Severe Enthesitis, Bone Marrow Hypocellularity, and Refractory Thrombocytopenia in a Pediatric Patient with Subsequent Response to JAK Inhibition

Haploinsufficiency of suppressor of cytokine signaling 1 (SOCS1) is a recently discovered autoinflammatory disorder with significant rheumatologic, immunologic, and hematologic manifestations. Here we report a case of SOCS1 haploinsufficiency in a 5-year-old child with profound arthralgias and immune-mediated thrombocytopenia unmasked by SARS-CoV-2 infection. Her clinical manifestations were accompanied by excessive B cell activity, eosinophilia, and elevated IgE levels. Uniquely, this is the first report of SOCS1 haploinsufficiency in the setting of a chromosomal deletion resulting in complete loss of a single SOCS1 gene with additional clinical findings of bone marrow hypocellularity and radiologic evidence of severe enthesitis. Immunologic profiling showed a prominent interferon signature in the patient's peripheral blood mononuclear cells, which were also hypersensitive to stimulation by type I and type II interferons. The patient showed excellent clinical and functional laboratory response to tofacitinib, a Janus kinase inhibitor that disrupts interferon signaling. Our case highlights the need to utilize a multidisciplinary diagnostic approach and consider a comprehensive genetic evaluation for inborn errors of immunity in patients with an atypical immune-mediated thrombocytopenia phenotype.

Polygenic autoimmune disease risk alleles impacting B cell tolerance act in concert across shared molecular networks in mouse and in humans

Most B cells produced in the bone marrow have some level of autoreactivity. Despite efforts of central tolerance to eliminate these cells, many escape to periphery, where in healthy individuals, they are rendered functionally non-responsive to restimulation through their antigen receptor via a process termed anergy. Broad repertoire autoreactivity may reflect the chances of generating autoreactivity by stochastic use of germline immunoglobulin gene segments or active mechanisms may select autoreactive cells during egress to the naïve peripheral B cell pool. Likewise, it is unclear why in some individuals autoreactive B cell clones become activated and drive pathophysiologic changes in autoimmune diseases. Both of these remain central questions in the study of the immune system(s). In most individuals, autoimmune diseases arise from complex interplay of genetic risk factors and environmental influences. Advances in genome sequencing and increased statistical power from large autoimmune disease cohorts has led to identification of more than 200 autoimmune disease risk loci. It has been observed that autoantibodies are detectable in the serum years to decades prior to the diagnosis of autoimmune disease. Thus, current models hold that genetic defects in the pathways that control autoreactive B cell tolerance set genetic liability thresholds across multiple autoimmune diseases. Despite the fact these seminal concepts were developed in animal (especially murine) models of autoimmune disease, some perceive a disconnect between human risk alleles and those identified in murine models of autoimmune disease. Here, we synthesize the current state of the art in our understanding of human risk alleles in two prototypical autoimmune diseases – systemic lupus erythematosus (SLE) and type 1 diabetes (T1D) along with spontaneous murine disease models. We compare these risk networks to those reported in murine models of these diseases, focusing on pathways relevant to anergy and central tolerance. We highlight some differences between murine and human environmental and genetic factors that may impact autoimmune disease development and expression and may, in turn, explain some of this discrepancy. Finally, we show that there is substantial overlap between the molecular networks that define these disease states across species. Our synthesis and analysis of the current state of the field are consistent with the idea that the same molecular networks are perturbed in murine and human autoimmune disease. Based on these analyses, we anticipate that murine autoimmune disease models will continue to yield novel insights into how best to diagnose, prognose, prevent and treat human autoimmune diseases.

Serum immune modulators associated with immune-related toxicities and efficacy of atezolizumab in patients with non-small cell lung cancer

PURPOSE

Identifying patients at high risk of immune-related adverse events (irAEs) that impede the achievement of durable efficacy of programmed cell death 1 (PD-1)/programmed death ligand 1 (PD-L1) blockade therapy is important in improving their management. Identification of a novel predictive factor of therapeutic benefit is also important in improving patient selection for treatment with PD-1/PD-L1 inhibitors. Further determinants driving response and linking with irAEs are urgently required.

METHODS

To address these unmet needs in the field, we explored whether 27 soluble checkpoint proteins and immunomodulatory proteins in serum at the therapy baseline and after week 3 were associated with irAE onset and therapeutic efficacy using MILLIPLEX Human Immuno-Oncology Checkpoint Protein Panel assays in a prospective, multicenter cohort of 81 patients with non-small cell lung cancer (NSCLC) receiving atezolizumab monotherapy.

RESULTS

By competing-risks regression analysis, we identified that high levels of B cell-activating factor (BAFF) at baseline were a significant and strong risk factor of irAEs (hazard ratio, 5.61; 95% confidence interval, 2.43-12.96; P < 0.0001). We also identified that increased inducible T cell co-stimulator (ICOS) during the first therapeutic cycle was an independent factor associated with prolonged progression-free survival and overall survival.

CONCLUSION

These findings are in keeping with the reported mechanistic basis of these molecules and may provide potential guidance for clinical decision-making to improve patient care. Further validation studies are warranted. Trial registration UMIN000035616 (January 28, 2019).

Multitier mechanics control stromal adaptations in the swelling lymph node

Lymph nodes (LNs) comprise two main structural elements: fibroblastic reticular cells that form dedicated niches for immune cell interaction and capsular fibroblasts that build a shell around the organ. Immunological challenge causes LNs to increase more than tenfold in size within a few days. Here, we characterized the biomechanics of LN swelling on the cellular and organ scale. We identified lymphocyte trapping by influx and proliferation as drivers of an outward pressure force, causing fibroblastic reticular cells of the T-zone (TRCs) and their associated conduits to stretch. After an initial phase of relaxation, TRCs sensed the resulting strain through cell matrix adhesions, which coordinated local growth and remodeling of the stromal network. While the expanded TRC network readopted its typical configuration, a massive fibrotic reaction of the organ capsule set in and countered further organ expansion. Thus, different fibroblast populations mechanically control LN swelling in a multitier fashion.

Sixt and colleagues show that different fibroblast populations in the lymph node mechanically control its swelling in a multitier fashion.

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.

The relationship between serum A proliferation-inducing ligand and B-cell activating factor levels with disease activity and organ involvement in systemic lupus erythematosus

Objectives

We aim to investigate the association between serum B-cell activating factor (BAFF) and A proliferation-inducing ligand (APRIL) levels with disease activity and clinical findings in SLE patients.

Methods

Seventy-nine patients with SLE and 27 healthy controls were included into the study. Serum BAFF and APRIL levels were measured by using ELISA. In 19 patients with active disease at the time of the assessment, BAFF/APRIL levels were reassessed after 6 months of follow-up and disease activity was evaluated by using SLEDAI-2K. The relationship between renal histopathology index scores and lupus nephritis (LN) classes with serum BAFF/APRIL levels was examined in 16 patients who had recent renal involvement and underwent biopsy during the study.

Results

Although both BAFF/APRIL levels were higher in patients with SLE compared to the control group ( p < 0.001), no correlation was found between BAFF/APRIL levels and SLEDAI scores. Serum BAFF levels were higher in patients with renal disease activity ( p = 0.01), and there was a significant correlation between APRIL levels and proteinuria (r = 0.42, p = 0.02). A weak inverse correlation was observed between BAFF and C3 levels (r = 0.25, p = 0.02). No correlation was found between BAFF/APRIL levels and renal SLEDAI scores, renal histopathology, activity, and chronicity index scores. In the active disease group after treatment, there was no significant change in serum BAFF levels, but a significant increase in serum APRIL levels was observed.

Conclusion

These results suggest that both cytokines are involved in the pathogenesis of SLE and that serum BAFF can be valuable as a biomarker in SLE especially in patients with renal activity.

BAFF Attenuates Immunosuppressive Monocytes in the Melanoma Tumor Microenvironment

Emerging evidence indicates B-cell activating factor (BAFF, Tnfsf13b) to be an important cytokine for antitumor immunity. In this study, we generated a BAFF-overexpressing B16.F10 melanoma cell model and found that BAFF-expressing tumors grow more slowly in vivo than control tumors. The tumor microenvironment (TME) of BAFF-overexpressing tumors had decreased myeloid infiltrates with lower PD-L1 expression. Monocyte depletion and anti-PD-L1 antibody treatment confirmed the functional importance of monocytes for the phenotype of BAFF-mediated tumor growth delay. RNA sequencing analysis confirmed that monocytes isolated from BAFF-overexpressing tumors were characterized by a less exhaustive phenotype and were enriched for in genes involved in activating adaptive immune responses and NF-κB signaling. Evaluation of patients with late-stage metastatic melanoma treated with inhibitors of the PD-1/PD-L1 axis demonstrated a stratification of patients with high and low BAFF plasma levels. Patients with high BAFF levels experienced lower responses to anti-PD-1 immunotherapies. In summary, these results show that BAFF, through its effect on tumor-infiltrating monocytes, not only impacts primary tumor growth but can serve as a biomarker to predict response to anti-PD-1 immunotherapy in advanced disease. SIGNIFICANCE: The BAFF cytokine regulates monocytes in the melanoma microenvironment to suppress tumor growth, highlighting the importance of BAFF in antitumor immunity.

Role of B-Cell Activating Factor (BAFF) in Inflammatory Bowel Disease

As early commencement of inflammatory bowel disease (IBD) treatment has been shown to substantially improve outcomes, it is of utmost importance to make a timely diagnosis of this disease. Despite undisputed sensitivity of fecal calprotectin, the most widely accepted IBD biomarker, in discriminating between irritable bowel syndrome (IBS) and IBD, as well as recognized role in monitoring disease activity and response to therapy, perhaps the biggest setback of calprotectin use in IBD is lack of specificity. Therefore, an additional biomarker in IBD is warranted. B-cell activating factor (BAFF), a member of the tumor necrosis factor (TNF) superfamily, recently emerged as a viable candidate for this role. So far, overproduction of BAFF has been observed in various autoimmune diseases, most notably in systemic lupus erythematosus, where BAFF-inhibitor belimumab was approved for treatment. As BAFF levels were also shown to correlate with indices of IBD, in this review we aimed to summarize the current evidence with respect to the role of BAFF in diagnosis and assessing the activity of IBD, as well as putative therapeutic implications that may arise from exploring of this relation.

Review article: targeting the B cell activation system in autoimmune hepatitis

BACKGROUND

The B cell activation system, consisting of B cell activating factor and a proliferation-inducing ligand, may have pathogenic effects in autoimmune hepatitis.

AIMS

To describe the biological actions of the B cell activation system, indicate its possible role in autoimmune diseases, and evaluate its prospects as a therapeutic target in autoimmune hepatitis METHODS: English abstracts were identified in PubMed by multiple search terms. Full length articles were selected for review, and secondary and tertiary bibliographies were developed.

RESULTS

The B cell activating factor is crucial for the maturation and survival of B cells, and it can co-stimulate T cell activation, proliferation, and survival. It can also modulate the immune response by inducing interleukin 10 production by regulatory B cells. A proliferation-inducing ligand modulates and diversifies the antibody response by inducing class-switch recombination in B cells. It can also increase the proliferation, survival, and antigen activation of T cells. These immune stimulatory actions can be modulated by inducing proliferation of regulatory T cells. The B cell activation system has been implicated in diverse autoimmune diseases, and therapeutic blockade is a management strategy now being evaluated in autoimmune hepatitis.

CONCLUSIONS

The B cell activation system has profound effects on B and T cell function in autoimmune diseases. Blockade therapy is being actively evaluated in autoimmune hepatitis. Clarification of the critical pathogenic components of the B cell activation system will improve the targeting, efficacy, and safety of blockade therapy in this disease.

Homeostatic apoptosis prevents competition-induced atrophy in follicular B cells

While the intrinsic apoptosis pathway is thought to play a central role in shaping the B cell lineage, its precise role in mature B cell homeostasis remains elusive. Using mice in which mature B cells are unable to undergo apoptotic cell death, we show that apoptosis constrains follicular B (FoB) cell lifespan but plays no role in marginal zone B (MZB) cell homeostasis. In these mice, FoB cells accumulate abnormally. This intensifies intercellular competition for BAFF, resulting in a contraction of the MZB cell compartment, and reducing the growth, trafficking, and fitness of FoB cells. Diminished BAFF signaling dampens the non-canonical NF-κB pathway, undermining FoB cell growth despite the concurrent triggering of a protective p53 response. Thus, MZB and FoB cells exhibit a differential requirement for the intrinsic apoptosis pathway. Homeostatic apoptosis constrains the size of the FoB cell compartment, thereby preventing competition-induced FoB cell atrophy.

Function, occurrence and inhibition of different forms of BAFF

B cell activating factor (BAFF or BLyS), an important cytokine for B cell survival and humoral immune responses, is targeted in the clinic for the treatment of systemic lupus erythematosus. This review focuses on the structure, function and inhibition profiles of membrane-bound BAFF, soluble BAFF 3-mer and soluble BAFF 60-mer, all of which have distinct properties. BAFF contains a loop region not required for receptor binding but essential for receptor activation via promotion of BAFF-to-BAFF contacts. This loop region additionally allows formation of BAFF 60-mer, in which epitopes of the BAFF inhibitor belimumab are inaccessible. If 60-mer forms in humans, it is predicted to be short-lived and to act locally because adult serum contains a BAFF 60-mer dissociating activity. Cord blood contains elevated levels of BAFF, part of which displays attributes of 60-mer, suggesting a role for this form of BAFF in the development of foetal or neonate B cells.

B cell/stromal cell crosstalk in health, disease, and treatment: Follicular lymphoma as a paradigm

Stromal cells organize specific anatomic compartments within bone marrow (BM) and secondary lymphoid organs where they finely regulate the behavior of mature normal B cells. In particular, lymphoid stromal cells (LSCs) form a phenotypically heterogeneous compartment including various cell subsets variably supporting B-cell survival, activation, proliferation, and differentiation. In turn, activated B cells trigger in-depth remodeling of LSC networks within lymph nodes (LN) and BM. Follicular lymphoma (FL) is one of the best paradigms of a B-cell neoplasia depending on a specific tumor microenvironment (TME), including cancer-associated fibroblasts (CAFs) emerging from the reprogramming of LN LSCs or poorly characterized local BM precursors. FL-CAFs support directly malignant B-cell growth and orchestrate FL permissive cell niche by contributing, through a bidirectional crosstalk, to the recruitment and polarization of immune TME subsets. Recent studies have highlighted a previously unexpected level of heterogeneity of both FL B cells and FL TME, underlined by FL-CAF plasticity. A better understanding of the signaling pathways, molecular mechanisms, and kinetic of stromal cell remodeling in FL would be useful to delineate new predictive markers and new therapeutic approaches in this still fatal malignancy.

BAFF promotes heightened BCR responsiveness and manifestations of chronic GVHD after allogeneic stem cell transplantation

Patients with chronic graft-versus-host disease (cGVHD) have increased B cell-activating factor (BAFF) levels, but whether BAFF promotes disease after allogeneic bone marrow transplantation (allo-BMT) remains unknown. In a major histocompatibility complex-mismatched model with cGVHD-like manifestations, we first examined B-lymphopenic μMT allo-BMT recipients and found that increased BAFF levels in cGVHD mice were not merely a reflection of B-cell number. Mice that later developed cGVHD had significantly increased numbers of recipient fibroblastic reticular cells with higher BAFF transcript levels. Increased BAFF production by donor cells also likely contributed to cGVHD, because BAFF transcript in CD4+ T cells from diseased mice and patients was increased. cGVHD manifestations in mice were associated with high BAFF/B-cell ratios and persistence of B-cell receptor (BCR)-activated B cells in peripheral blood and lesional tissue. By employing BAFF transgenic (Tg) mice donor cells, we addressed whether high BAFF contributed to BCR activation in cGVHD. BAFF increased NOTCH2 expression on B cells, augmenting BCR responsiveness to surrogate antigen and NOTCH ligand. BAFF Tg B cells had significantly increased protein levels of the proximal BCR signaling molecule SYK, and high SYK protein was maintained by BAFF after in vitro BCR activation or when alloantigen was present in vivo. Using T cell-depleted (BM only) BAFF Tg donors, we found that BAFF promoted cGVHD manifestations, circulating GL7+ B cells, and alloantibody production. We demonstrate that pathologic production of BAFF promotes an altered B-cell compartment and augments BCR responsiveness. Our findings compel studies of therapeutic targeting of BAFF and BCR pathways in patients with cGVHD.

B-cell activating factor (BAFF) expression is associated with Crohn's disease and can serve as a potential prognostic indicator of disease response to Infliximab treatment

BACKGROUND

Several studies correlated elevated B-cell activating factor (BAFF) levels and its polymorphisms (SNPs) in patients with autoimmunity. Limited data existed regarding the role of BAFF in Crohn's Disease (CD) susceptibility and/or treatment response to infliximab.

AIM

This study aims to evaluate BAFF expression in CD patients, investigate if its expression can predict response to infliximab treatment, and examine the association of BAFF SNPs with CD susceptibility.

METHODS

One hundred twelve CD patients and 164 healthy controls were recruited. Serum BAFF levels were determined using an enzyme-linked immunosorbent assay. Participants were genotyped for rs9514828, rs1041569 and rs2893321 SNPs.

RESULTS

Serum BAFF concentration was elevated in CD patients (472.86 ± 223.60 pg/ml) compared with controls (128.16 ± 70.10 pg/ml) before treatment. Responders to IFX treatment had increased serum BAFF levels at baseline (610.03 ± 167.55 pg/ml) compared to non-responders (267.09 ± 107 pg/ml). In responders, BAFF concentration reduced after IFX administration, while increased in non-responders. The rs1041569, TA and AA genotypes frequencies, and the minor allele A were increased significantly in CD patients, indicating an association of the SNP with CD susceptibility.

CONCLUSIONS

Our study suggests that BAFF could be a potential biomarker of CD, while SNP rs1041569 was associated with CD susceptibility.

Rate of Immune Complex Cycling in Follicular Dendritic Cells Determines the Extent of Protecting Antigen Integrity and Availability to Germinal Center B Cells

Follicular dendritic cells (FDCs) retain immune complexes (ICs) for prolonged time periods and are important for germinal center (GC) reactions. ICs undergo periodic cycling in FDCs, a mechanism supporting an extended half-life of Ag. Based on experimental data, we estimated that the average residence time of PE-ICs on FDC surface and interior were 21 and 36 min, respectively. GC simulations show that Ag cycling might impact GC dynamics because of redistribution of Ag on the FDC surface and by protecting Ag from degradation. Ag protection and influence on GC dynamics varied with Ag cycling time and total Ag concentration. Simulations predict that blocking Ag cycling terminates the GC reaction and decreases plasma cell production. Considering that cycling of Ag could be a target for the modulation of GC reactions, our findings highlight the importance of understanding the mechanism and regulation of IC cycling in FDCs.

The BAFF / APRIL system as therapeutic target in multiple sclerosis

INTRODUCTION

The complex system of BAFF (B-cell-activating factor of the TNF family) and APRIL (A proliferation-inducing ligand) has been studied in animal models of autoimmune diseases such as those resembling human systemic lupus erythematosus and Sjogren's syndrome and multiple sclerosis (MS). Accumulating evidence suggests that BAFF and APRIL have a physiological role in B cell immunity regulation, however inappropriate production of these factors may represent a key event which disrupts immune tolerance which is associated with systemic autoimmune diseases.

AREAS COVERED

We provide an update on the latest studies of the BAFF/APRIL system in multiple sclerosis, as well as on related clinical trials.

EXPERT OPINION

Experimental and clinical evidence suggests that increased BAFF levels may interfere directly and indirectly with B cell immunity; this can lead to breakdown of immune tolerance, the production of autoantibodies and continuous local intracerebral inflammation and brain tissue destruction. A more comprehensive understanding of the cell/molecular mechanism immune reactions specifically regulated by BAFF/APRIL in MS would better elucidate the specific cell phenotype targeted by actual anti-BAFF/APRIL therapies; this may enable the identification of either specific biomarkers of MS subgroups that would benefit of anti-BAFF/APRIL treatments or new targets of MS-specific anti-BAFF/APRIL therapies.

Phage Display Derived Monoclonal Antibodies: From Bench to Bedside

Monoclonal antibodies (mAbs) have become one of the most important classes of biopharmaceutical products, and they continue to dominate the universe of biopharmaceutical markets in terms of approval and sales. They are the most profitable single product class, where they represent six of the top ten selling drugs. At the beginning of the 1990s, an in vitro antibody selection technology known as antibody phage display was developed by John McCafferty and Sir. Gregory Winter that enabled the discovery of human antibodies for diverse applications, particularly antibody-based drugs. They created combinatorial antibody libraries on filamentous phage to be utilized for generating antigen specific antibodies in a matter of weeks. Since then, more than 70 phage–derived antibodies entered clinical studies and 14 of them have been approved. These antibodies are indicated for cancer, and non-cancer medical conditions, such as inflammatory, optical, infectious, or immunological diseases. This review will illustrate the utility of phage display as a powerful platform for therapeutic antibodies discovery and describe in detail all the approved mAbs derived from phage display.

The predictive value of serum free light chain level early after allogeneic hematopoietic stem cell transplantation for chronic graft-versus-host disease, a preliminary study

OBJECTIVE

Serum free light chain (FLC) level is closely associated with the functional state of B lymphocytes, and many studies have shown that delayed reconstitution of B lymphocytes contributed to chronic graft-versus-host disease (cGVHD). This study assessed the predictive value of FLC levels in serum collected early after allogeneic hematopoietic stem cell transplantation (allo-HSCT) for cGVHD.

METHODS

Sixty-two patients who had undergone allo-HSCT were retrospectively reviewed. The correlations between the FLC levels and the development of cGVHD were explored.

RESULTS

Of the 62 patients, 33 cases developed cGVHD, with the prevalence of 53.2%. With Seattle classification, 19 cases had limited cGVHD while 14 cases contracted extensive cGVHD. While with NIH classification, 17 cases had mild cGVHD, 6 cases moderate cGVHD, and 10 cases severe cGVHD. Multivariant statistical analysis showed that the FLC levels were not associated with all severities of cGVHD but were correlated with the development of extensive or moderate to severe cGVHD (P = .01 and .038, respectively).

CONCLUSIONS

Serum FLC levels early after HSCT may reflect the functional state of B-cell reconstitution. Patients with low serum FLC Level early post-allo-HSCT tend to develop extensive cGVHD or moderate to severe cGVHD.

BAFF Produced by Neutrophils and Dendritic Cells Is Regulated Differently and Has Distinct Roles in Antibody Responses and Protective Immunity against West Nile Virus

B cell activating factor (BAFF) is essential for B cells to develop and respond to Ags. Dysregulation of BAFF contributes to the development of some autoimmune diseases and malignancies. Little is known about when, where, and how BAFF is produced in vivo and about which BAFF-producing cells contribute to B cell responses. To better understand BAFF functions, we created BAFF reporter (BAFF-RFP) mice and Baff floxed (Bafffl/fl ) mice. Splenic and bone marrow neutrophils (Nphs) from BAFF-RFP mice expressed the highest constitutive levels of BAFF; other myeloid subsets, including conventional dendritic cells (cDCs) and monocyte (MO) subsets, expressed lower levels. Treatment of BAFF-RFP mice with polyinosinic:polycytidylic acid increased BAFF expression in splenic Ly6Chi inflammatory MOs, CD11bhi activated NK subset, and in bone marrow myeloid precursors. Postinfection with West Nile virus (WNV), BAFF increased in CD8- cDCs and Nphs, and BAFF+ CD11bhi NK cells expanded in draining lymph nodes. The cell- and tissue-specific increases in BAFF expression were dependent on type I IFN signaling. MAVS also was required or contributed to BAFF expression in dendritic cell and MO subsets, respectively. Mice with deletion of Baff in either cDCs or Nphs had reduced Ab responses after NP-Ficoll immunization; thus, BAFF produced by both cDCs and Nphs contributes to T cell-independent Ab responses. Conversely, mice with a cDC Baff deficiency had increased mortality after WNV infection and decreased WNV-specific IgG and neutralizing Ab responses. BAFF produced by Nphs and cDCs is regulated differently and has key roles in Ab responses and protective immunity.

Effect of rituximab treatment on T and B cell subsets in lymph node biopsies of patients with rheumatoid arthritis

Objectives

The exact underlying mechanism of rituximab treatment in patients with RA is poorly defined and knowledge about the effect of B cell depletion on immune cells in secondary lymphoid organs is lacking. We analysed lymphoid tissue responses to rituximab in RA patients.

Methods

Fourteen RA patients received 2 × 1000 mg rituximab intravenously, and lymph node (LN) biopsies were obtained before and 4 weeks after the first infusion. Tissues were examined by flow cytometry, immunohistochemistry and quantitative PCR. LN biopsies from five healthy individuals (HC) served as controls.

Results

LN biopsies of RA patients showed increased frequencies of CD21⁺CD23⁺IgD^highIgM^variable follicular B cells and CD3⁺CD25⁺CD69⁺ early activated, tissue resident T cells when compared with HCs. After treatment, there was incomplete depletion of LN B cells. There was a significant decrease in CD27⁻IgD⁺ naïve B cells, and CD27⁺IgD⁺ unswitched memory B cells including the CD27⁺IgD⁺IgM⁺ subset and follicular B cells. Strikingly, CD27⁺IgD⁻ switched memory B cells persisted in LN biopsies after rituximab treatment. In the T cell compartment, a significant decrease was observed in the frequency of early activated, tissue resident T cells after rituximab treatment, but late activated T cells persisted. B cell proliferation inducing cytokine IL-21 was higher expressed in LN biopsies of RA patients compared with HC and expression was not affected by rituximab treatment.

Conclusion

Rituximab does not cure RA, possibly due to persistence of switched memory B cells in lymphoid tissues suggesting that factors promoting B cell survival and differentiation need to be additionally targeted.

Essential cell-extrinsic requirement for PDIA6 in lymphoid and myeloid development

In a forward genetic screen of N-ethyl-N-nitrosourea (ENU)–induced mutant mice for aberrant immune function, we identified mice with a syndromic disorder marked by growth retardation, diabetes, premature death, and severe lymphoid and myeloid hypoplasia together with diminished T cell–independent (TI) antibody responses. The causative mutation was in Pdia6, an essential gene encoding protein disulfide isomerase A6 (PDIA6), an oxidoreductase that functions in nascent protein folding in the endoplasmic reticulum. The immune deficiency caused by the Pdia6 mutation was, with the exception of a residual T cell developmental defect, completely rescued in irradiated wild-type recipients of PDIA6-deficient bone marrow cells, both in the absence or presence of competition. The viable hypomorphic allele uncovered in these studies reveals an essential role for PDIA6 in hematopoiesis, but one extrinsic to cells of the hematopoietic lineage. We show evidence that this role is in the proper folding of Wnt3a, BAFF, IL-7, and perhaps other factors produced by the extra-hematopoietic compartment that contribute to the development and lineage commitment of hematopoietic cells.

Phage Display Derived Monoclonal Antibodies: From Bench to Bedside

Monoclonal antibodies (mAbs) have become one of the most important classes of biopharmaceutical products, and they continue to dominate the universe of biopharmaceutical markets in terms of approval and sales. They are the most profitable single product class, where they represent six of the top ten selling drugs. At the beginning of the 1990s, an in vitro antibody selection technology known as antibody phage display was developed by John McCafferty and Sir. Gregory Winter that enabled the discovery of human antibodies for diverse applications, particularly antibody-based drugs. They created combinatorial antibody libraries on filamentous phage to be utilized for generating antigen specific antibodies in a matter of weeks. Since then, more than 70 phage-derived antibodies entered clinical studies and 14 of them have been approved. These antibodies are indicated for cancer, and non-cancer medical conditions, such as inflammatory, optical, infectious, or immunological diseases. This review will illustrate the utility of phage display as a powerful platform for therapeutic antibodies discovery and describe in detail all the approved mAbs derived from phage display.

The role of BAFF and APRIL in rheumatoid arthritis

Development and activation of B cells quickly became clear after identifying new ligands and receptors in the tumor necrosis factor superfamily. B cell-activating factor (BAFF) and a proliferation-inducing ligand (APRIL) are the members of membrane proteins Type 2 family released by proteolytic cleavage of furin to form active, soluble homotrimers. Except for B cells, ligands are expressed by all such immune cells like T cells, dendritic cells, monocytes, and macrophages. BAFF and APRIL have two common receptors, namely TNFR homolog transmembrane activator and Ca2+ modulator and CAML interactor (TACI) and B cell-maturation antigen. BAFF alone can also be coupled with a third receptor called BAFFR (also called BR3 or BLyS Receptor). These receptors are often expressed by immune cells in the B-cell lineage. The binding of BAFF or APRIL to their receptors supports B cells differentiation and proliferation, immunoglobulin production and the upregulation of B cell-effector molecules expression. It is possible that the overexpression of BAFF and APRIL contributes to the pathogenesis of autoimmune diseases. In BAFF transgenic mice, there is a pseudo-autoimmune manifestation, which is associated with an increase in B-lymphocytes, hyperglobulinemia, anti-single stranded DNA, and anti-double-stranded DNA antibodies, and immune complexes in their peripheral blood. Furthermore, overexpressing BAFF augments the number of peripheral B220+ B cells with a normal proliferation rate, high levels of Bcl2, and prolonged survival and hyperactivity. Therefore, in this review article, we studied BAFF and APRIL as important mediators in B-cell and discussed their role in rheumatoid arthritis.

Reverse Signaling of Tumor Necrosis Factor Superfamily Proteins in Macrophages and Microglia: Superfamily Portrait in the Neuroimmune Interface

The tumor necrosis factor (TNF) superfamily (TNFSF) is a protein superfamily of type II transmembrane proteins commonly containing the TNF homology domain. The superfamily contains more than 20 protein members, which can be released from the cell membrane by proteolytic cleavage. Members of the TNFSF function as cytokines and regulate diverse biological processes, including immune responses, proliferation, differentiation, apoptosis, and embryogenesis, by binding to TNFSF receptors. Many TNFSF proteins are also known to be responsible for the regulation of innate immunity and inflammation. Both receptor-mediated forward signaling and ligand-mediated reverse signaling play important roles in these processes. In this review, we discuss the functional expression and roles of various reverse signaling molecules and pathways of TNFSF members in macrophages and microglia in the central nervous system (CNS). A thorough understanding of the roles of TNFSF ligands and receptors in the activation of macrophages and microglia may improve the treatment of inflammatory diseases in the brain and periphery. In particular, TNFSF reverse signaling in microglia can be exploited to gain further insights into the functions of the neuroimmune interface in physiological and pathological processes in the CNS.

Stromal cell networks coordinate immune response generation and maintenance

Secondary lymphoid organs (SLO), including the spleen and lymph nodes (LN) are a meeting place for immune cells to initiate adaptive immune responses. Lymphocytes constantly circulate between SLO through the blood and lymph in search of their cognate antigen and are activated within the organized microarchitecture of SLO. Lymphoid stromal cells (LSC) of mesenchymal and endothelial origin construct and support the microarchitecture of SLO by defining distinct compartments and providing signals that can either promote or inhibit immune responses. Here, we discuss recent studies indicating that LSC, including fibroblastic reticular cells (FRC), contribute substantially to immune responses and may tune responses to secondary challenge.

Developing role of B cells in the pathogenesis and treatment of chronic GVHD

Chronic graft-versus-host disease (cGVHD) is a major complication affecting the long-term survival of patients after allogeneic haematopoietic stem cell transplantation. The mechanism of cGVHD is unclear, and while previous studies have primarily focused on T cells, the role of B cells in the pathogenesis of cGVHD has been less reported. However, current studies on cGVHD are increasingly focused on the important role of B cells. In this review, we will introduce the newest studies and examine the role of B cells in cGVHD in detail with respect to the following aspects: altered B cell subpopulations, aberrant B cell signalling pathways, autoantibodies and T-B cell interactions. Treatment strategies for the targeting of B cells during cGVHD will also be discussed.

Interleukin-33 Contributes Toward Loss of Tolerance by Promoting B-Cell-Activating Factor of the Tumor-Necrosis-Factor Family (BAFF)-Dependent Autoantibody Production

Breaking tolerance is a key event leading to autoimmunity, but the exact mechanisms responsible for this remain uncertain. Here we show that the alarmin IL-33 is able to drive the generation of autoantibodies through induction of the B cell survival factor BAFF. A temporary, short-term increase in IL-33 results in a primary (IgM) response to self-antigens. This transient DNA-specific autoantibody response was dependent on the induction of BAFF. Notably, radiation resistant cells and not myeloid cells, such as neutrophils or dendritic cells were the major source of BAFF and were critical in driving the autoantibody response. Chronic exposure to IL-33 elicited dramatic increases in BAFF levels and resulted in elevated numbers of B and T follicular helper cells as well as germinal center formation. We also observed class-switching from an IgM to an IgG DNA-specific autoantibody response. Collectively, the results provide novel insights into a potential mechanism for breaking immune-tolerance via IL-33-mediated induction of BAFF.

Systemic ST6Gal-1 Is a Pro-survival Factor for Murine Transitional B Cells

Humoral immunity depends on intrinsic B cell developmental programs guided by systemic signals that convey physiologic needs. Aberrant cues or their improper interpretation can lead to immune insufficiency or a failure of tolerance and autoimmunity. The means by which such systemic signals are conveyed remain poorly understood. Hence, further insight is essential to understanding and treating autoimmune diseases and to the development of improved vaccines. ST6Gal-1 is a sialyltransferase that constructs the α2,6-sialyl linkage on cell surface and extracellular glycans. The requirement for functional ST6Gal-1 in the development of humoral immunity is well documented. Canonically, ST6Gal-1 resides within the intracellular ER-Golgi secretory apparatus and participates in cell-autonomous glycosylation. However, a significant pool of extracellular ST6Gal-1 exists in circulation. Here, we segregate the contributions of B cell intrinsic and extrinsic ST6Gal-1 to B cell development. We observed that B cell-intrinsic ST6Gal-1 is required for marginal zone B cell development, while B cell non-autonomous ST6Gal-1 modulates B cell development and survival at the early transitional stages of the marrow and spleen. Exposure to extracellular ST6Gal-1 ex vivo enhanced the formation of IgM-high B cells from immature precursors, and increased CD23 and IgM expression. Extrinsic sialylation by extracellular ST6Gal-1 augmented BAFF-mediated activation of the non-canonical NF-kB, p38 MAPK, and PI3K/AKT pathways, and accelerated tyrosine phosphorylation after B cell receptor stimulation. in vivo, systemic ST6Gal-1 did not influence homing of B cells to the spleen but was critical for their long-term survival and systemic IgG levels. Circulatory ST6Gal-1 levels respond to inflammation, infection, and malignancy in mammals, including humans. In turn, we have shown previously that systemic ST6Gal-1 regulates inflammatory cell production by modifying bone marrow myeloid progenitors. Our data here point to an additional role of systemic ST6Gal-1 in guiding B cell development, which supports the concept that circulating ST6Gal-1 is a conveyor of systemic cues to guide the development of multiple branches of immune cells.

Identification of a new subset of lymph node stromal cells involved in regulating plasma cell homeostasis

Antibody-secreting plasma cells (PCs) arise rapidly during adaptive immunity to control infections. The early PCs are retained within the reactive lymphoid organ where their localization and homeostasis rely on extrinsic factors, presumably produced by local niche cells. While myeloid cells have been proposed to form those niches, the contribution by colocalizing stromal cells has remained unclear. Here, we characterized a subset of fibroblastic reticular cells (FRCs) that forms a dense meshwork throughout medullary cords of lymph nodes (LNs) where PCs reside. This medullary FRC type is shown to be anatomically, phenotypically, and functionally distinct from T zone FRCs, both in mice and humans. By using static and dynamic imaging approaches, we provide evidence that medullary FRCs are the main cell type in contact with PCs guiding them in their migration. Medullary FRCs also represent a major local source of the PC survival factors IL-6, BAFF, and CXCL12, besides also producing APRIL. In vitro, medullary FRCs alone or in combination with macrophages promote PC survival while other LN cell types do not have this property. Thus, we propose that this FRC subset, together with medullary macrophages, forms PC survival niches within the LN medulla, and thereby helps in promoting the rapid development of humoral immunity, which is critical in limiting early pathogen spread.

A loop region of BAFF controls B cell survival and regulates recognition by different inhibitors

The B cell survival factor (TNFSF13B/BAFF) is often elevated in autoimmune diseases and is targeted in the clinic for the treatment of systemic lupus erythematosus. BAFF contains a loop region designated the flap, which is dispensable for receptor binding. Here we show that the flap of BAFF has two functions. In addition to facilitating the formation of a highly active BAFF 60-mer as shown previously, it also converts binding of BAFF to TNFRSF13C (BAFFR) into a signaling event via oligomerization of individual BAFF-BAFFR complexes. Binding and activation of BAFFR can therefore be targeted independently to inhibit or activate the function of BAFF. Moreover, structural analyses suggest that the flap of BAFF 60-mer temporarily prevents binding of an anti-BAFF antibody (belimumab) but not of a decoy receptor (atacicept). The observed differences in profiles of BAFF inhibition may confer distinct biological and clinical efficacies to these therapeutically relevant inhibitors.

BAFF is an important cytokine for B cell survival, and is a therapeutic target for autoimmune disorders. Here the authors show that a 'flap' region of BAFF converts BAFFR binding events into survival signals and, with structural data, that this ‘flap’ differentially modulates binding of drugs such as belimumab or atacicept.

BAFF and APRIL expression as an autoimmune signature of membranous nephropathy

Background

Based on the fact that B-cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL) have a regulatory role in B cell biology, excessive levels of these cytokines can promote autoimmune pathogenesis. However, the expression and implication remain unresolved in cases of membranous nephropathy (MN).

Results

The plasma BAFF levels of the primary MN patients were higher than those of healthy controls but lower than those of secondary MN patients, whereas the APRIL levels were similar between the MN patients and healthy controls. The BAFF levels were higher in relapse cases, whereas the APRIL levels were higher in the patients who did not experience remission compared with the counterpart patients. The ectopic expression of BAFF and APRIL was observed in the glomeruli or circulating B cells of MN patients, and this high expression trend was similar to that of lupus patients.

Conclusions

Expression profile of BAFF and APRIL in MN is similar to that of other autoimmune disease, which affects the kidney outcomes.

Methods

Plasma BAFF and APRIL levels were measured upon kidney biopsy in patients with primary (n = 89) and secondary MN (n = 13), and the results were compared with the levels in healthy controls (n = 111). The kidney outcomes (e.g., remission and relapse) were traced for the median of 3 years. Aberrant expression of the cytokines was evaluated in the kidney and circulating B cells using immunohistochemistry and flow cytometry analyses, respectively.

Targeting BAFF and APRIL in systemic lupus erythematosus and other antibody-associated diseases

The B cell-stimulating molecules, BAFF (B cell activating factor) and APRIL (a proliferation-inducing ligand), are critical factors in the maintenance of the B cell pool and humoral immunity. In addition, BAFF and APRIL are involved in the pathogenesis of a number of human autoimmune diseases, with elevated levels of these cytokines detected in the sera of patients with systemic lupus erythematosus (SLE), IgA nephropathy, Sjögren's syndrome, and rheumatoid arthritis. As such, both molecules are rational targets for new therapies in B cell-driven autoimmune diseases, and several inhibitors of BAFF or BAFF and APRIL together have been investigated in clinical trials. These include the BAFF/APRIL dual inhibitor, atacicept, and the BAFF inhibitor, belimumab, which is approved as an add-on therapy for patients with active SLE. Post hoc analyses of these trials indicate that baseline serum levels of BAFF and BAFF/APRIL correlate with treatment response to belimumab and atacicept, respectively, suggesting a role for the two molecules as predictive biomarkers. It will, however, be important to refine future testing to identify active forms of BAFF and APRIL in the circulation, as well as to distinguish between homotrimer and heteromer configurations. In this review, we discuss the rationale for dual BAFF/APRIL inhibition versus single BAFF inhibition in autoimmune disease, by focusing on the similarities and differences between the physiological and pathogenic roles of the two molecules. A summary of the preclinical and clinical data currently available is also presented.

An aberrant NOTCH2-BCR signaling axis in B cells from patients with chronic GVHD

B-cell receptor (BCR)-activated B cells contribute to pathogenesis in chronic graft-versus-host disease (cGVHD), a condition manifested by both B-cell autoreactivity and immune deficiency. We hypothesized that constitutive BCR activation precluded functional B-cell maturation in cGVHD. To address this, we examined BCR-NOTCH2 synergy because NOTCH has been shown to increase BCR responsiveness in normal mouse B cells. We conducted ex vivo activation and signaling assays of 30 primary samples from hematopoietic stem cell transplantation patients with and without cGVHD. Consistent with a molecular link between pathways, we found that BCR-NOTCH activation significantly increased the proximal BCR adapter protein BLNK. BCR-NOTCH activation also enabled persistent NOTCH2 surface expression, suggesting a positive feedback loop. Specific NOTCH2 blockade eliminated NOTCH-BCR activation and significantly altered NOTCH downstream targets and B-cell maturation/effector molecules. Examination of the molecular underpinnings of this "NOTCH2-BCR axis" in cGVHD revealed imbalanced expression of the transcription factors IRF4 and IRF8, each critical to B-cell differentiation and fate. All-trans retinoic acid (ATRA) increased IRF4 expression, restored the IRF4-to-IRF8 ratio, abrogated BCR-NOTCH hyperactivation, and reduced NOTCH2 expression in cGVHD B cells without compromising viability. ATRA-treated cGVHD B cells had elevated TLR9 and PAX5, but not BLIMP1 (a gene-expression pattern associated with mature follicular B cells) and also attained increased cytosine guanine dinucleotide responsiveness. Together, we reveal a mechanistic link between NOTCH2 activation and robust BCR responses to otherwise suboptimal amounts of surrogate antigen. Our findings suggest that peripheral B cells in cGVHD patients can be pharmacologically directed from hyperactivation toward maturity.

Crosstalk between signals initiated from TLR4 and cell surface BAFF results in synergistic induction of proinflammatory mediators in THP-1 cells

Cellular response to stimulation is mediated by meshwork of signaling pathways that may share common signaling adaptors. Here, we present data demonstrating that signaling pathways initiated from the membrane-bound form of B-cell activating factor (BAFF) can crosstalk with lipopolysaccharide (LPS)-induced signaling for synergistic expression of proinflammatory mediators in the human macrophage-like cell line THP-1. Co-treatment of the cells with BAFF-specific monoclonal antibody and LPS resulted in enhanced mitogen-activated protein kinase (MAPK)/mitogen- and stress-activated protein kinase (MSK)-mediated phosphorylation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) p65 subunit (Ser276), which then interacts with CREB binding protein (CBP) for subsequent acetylation. Simultaneously, the phosphorylation of cyclic AMP-response element binding protein (CREB) was enhanced through the combined action of phosphatidylinositol-3-kinase (PI3K)/AKT and MAPK/MSK pathways, and the resulting phospho-CREB interacted with the NF-κB/CBP complex. Transfection of CREB-specific siRNA inhibited the BAFF-mediated enhancing effect indicating that the formation of the CREB/NF-κB/CBP complex is required for the synergistic induction of the proinflammatory genes. These findings indicate that BAFF-mediated reverse signaling can modulate LPS-induced inflammatory activation through regulation of NF-κB and CREB activity and point out the necessity to re-evaluate the role of BAFF in diseases where its expression is high in macrophages.

A Chemoattractant-Guided Walk Through Lymphopoiesis: From Hematopoietic Stem Cells to Mature B Lymphocytes

B lymphocytes develop from hematopoietic stem cells (HSCs) in specialized bone marrow niches composed of rare mesenchymal lineage stem/progenitor cells (MSPCs) and sinusoidal endothelial cells. These niches are defined by function and location: MSPCs are mostly perisinusoidal cells that together with a small subset of sinusoidal endothelial cells express stem cell factor, interleukin-7 (IL-7), IL-15, and the highest amounts of CXCL12 in bone marrow. Though rare, MSPCs are morphologically heterogeneous, highly reticular, and form a vast cellular network in the bone marrow parenchyma capable of interacting with large numbers of hematopoietic cells. HSCs, downstream multipotent progenitor cells, and common lymphoid progenitor cells utilize CXCR4 to fine-tune access to critical short-range growth factors provided by MSPCs for their long-term maintenance and/or multilineage differentiation. In later stages, developing B lymphocytes use CXCR4 to navigate the bone marrow parenchyma, and predominantly cannabinoid receptor-2 for positioning within bone marrow sinusoids, prior to being released into peripheral blood circulation. In the final stages of differentiation, transitional B cells migrate to the spleen where they preferentially undergo further rounds of differentiation until selection into the mature B cell pool occurs. This bottleneck purges up to 97% of all developing B cells in a peripheral selection process that is heavily controlled not only by the intensity of BCR signaling and access to BAFF but also by the proper functioning of the B cell motility machinery.

Altered B cell homeostasis and Toll-like receptor 9-driven response in patients affected by autoimmune polyglandular syndrome Type 1: Altered B cell phenotype and dysregulation of the B cell function in APECED patients

APECED is a T-cell mediated disease with increased frequencies of CD8+ effector and reduction of FoxP3+ T regulatory cells. Antibodies against affected organs and neutralizing to cytokines are found in the peripheral blood. The contribution of B cells to multiorgan autoimmunity in Aire-/- mice was reported opening perspectives on the utility of anti-B cell therapy. We aimed to analyse the B cell phenotype of APECED patients compared to age-matched controls. FACS analysis was conducted on PBMC in basal conditions and following CpG stimulation. Total B and switched memory (SM) B cells were reduced while IgM memory were increased in patients. In those having more than 15 years from the first clinical manifestation the defect included also mature and transitional B cells; total memory B cells were increased, while SM were unaffected. In patients with shorter disease duration, total B cells were unaltered while SM and IgM memory behaved as in the total group. A defective B cell proliferation was detected after 4day-stimulation. In conclusion APECED patients show, in addition to a significant alteration of the B cell phenotype, a dysregulation of the B cell function involving peripheral innate immune mechanisms particularly those with longer disease duration.

IFN type I and II induce BAFF secretion from human decidual stromal cells

B cell activating factor (BAFF) is a critical cytokine for maturation of immature B cells. In murine lymph nodes, BAFF is mainly produced by podoplanin-expressing stromal cells. We have previously shown that circulating BAFF levels are maximal at birth, and that farmers’ children exhibit higher BAFF levels in cord blood than non-farmers’ children. Here, we sought to investigate whether maternal-derived decidual stromal cells from placenta secrete BAFF and examine what factors could stimulate this production. We found that podoplanin is expressed in decidua basalis and in the underlying villous tissue as well as on isolated maternal-derived decidual stromal cells. Decidual stromal cells produced BAFF when stimulated with IFN-γ and IFN-α, and NK cells and NK-T-like cells competent of IFN-γ production were isolated from the decidua. Finally, B cells at different maturational stages are present in decidua and all expressed BAFF-R, while stromal cells did not. These findings suggest that decidual stromal cells are a cellular source of BAFF for B cells present in decidua during pregnancy.

Urinary B cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL): potential biomarkers of active lupus nephritis

B cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL) help in B cell activation, maintenance and plasma cell survival. B cell infiltration has been demonstrated in kidneys of patients with lupus nephritis (LN). Serum levels of BAFF and APRIL have shown inconsistent relationships with lupus disease activity. We evaluated urinary levels of BAFF and APRIL as biomarker for LN. Thirty-six patients with proliferative lupus nephritis (AN), 10 with active lupus without nephritis (AL) and 15 healthy controls (HC) were studied. APRIL and BAFF levels were measured in both serum and urine using enzyme-linked immunosorbent assay (ELISA). Urine levels were normalized for urinary creatinine excretion. Urine levels were correlated with conventional disease activity markers and histology. Levels were reassessed in 20 AN patients at 6 months after treatment with cyclophosphamide. Urinary APRIL (uAPRIL) and BAFF (uBAFF) levels were raised significantly in AN. uAPRIL, but not uBAFF, correlated moderately with renal Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) in AN (r = 0·36, P < 0·05). On receiver operator curve (ROC) analysis, uBAFF and uAPRIL showed an area under the curve (AUC) of 0·825 and 0·781, respectively, in differentiating between nephritis and non-nephritis, which performed better than low C3, C4 and raised anti-dsDNA antibodies. There was no correlation of serum levels with uBAFF (r = 0·187, P = 0·261) and uAPRIL (r = 0·114, P = 0·494). uAPRIL levels reduced after treatment (mean 125 pg/mg to 36 pg/mg, P < 0·05). uBAFF levels reduced in 16 responders while two of four non-responders had increase in levels. Thus, uBAFF and uAPRIL are potential biomarkers of proliferative lupus nephritis.

B Cell-Activating Factor (BAFF)-Targeted B Cell Therapies in Inflammatory Bowel Diseases

Inflammatory bowel diseases (IBD) involve dysregulated immune responses to gut antigens in genetically predisposed individuals. While a better elucidation of IBD pathophysiology has considerably increased the number of treatment options, the need for more effective therapeutic strategies remains a pressing priority. Defects of both non-hematopoietic (epithelial and stromal) and hematopoietic (lymphoid and myeloid) cells have been described in patients with IBD. Within the lymphoid system, alterations of the T cell compartment are viewed as essential in the pathogenesis of IBD. However, growing evidence points to the additional perturbations of the B cell compartment. Indeed, the intestinal lamina propria from IBD patients shows an increased presence of antibody-secreting plasma cells, which correlates with enhanced pro-inflammatory immunoglobulin G production and changes in the quality of non-inflammatory IgA responses. These B cell abnormalities are compounded by the emergence of systemic antibody responses to various autologous and microbial antigens, which predates the clinical diagnosis of IBD and identifies patients with complicated disease. It is presently unclear whether such antibody responses play a pathogenetic role, as B cell depletion with the CD20-targeting monoclonal antibody rituximab did not ameliorate ulcerative colitis in a clinical trial. However, it must be noted that unresponsiveness to rituximab is also observed also in some patients with autoimmune disorders usually responsive to B cell-depleting therapies. In this review, we discussed mechanistic aspects of B cell-based therapies and their potential role in IBD with a special interest on BAFF and BAFF-targeting therapies buoyed by the success of anti-BAFF treatments in rheumatologic disorders.