Space, selection, and surveillance: setting boundaries with BLyS

The emerging role of BLyS/APRIL in autoimmune diseases: Biological characteristics, functions, and therapeutic potential

Autoimmune diseases (AIDs) are common diseases in the world. Some cases are difficult to cure and can only delay the progression of the diseases. The B lymphocyte stimulator (BLyS)/a proliferation-inducing ligand (APRIL) plays an important role in B cell homeostasis, regulation of both innate and adaptive immune responses. After binding to their receptors, BLyS/APRIL primarily affects the survival and development of marginal, transitional, and mature B cells. Of note, elevated BLyS/APRIL is seen in many AIDs, such as systemic lupus erythematosus, rheumatoid arthritis, immunoglobulin A nephropathy, etc. Moreover, there is evidence that blocking these two cytokines can control the number of serum autoantibodies, promote the depletion of B lymphocytes, inhibit the activation of T cells and dendritic lymphocytes, and reduce inflammatory stress. Currently, some clinical studies are underway targeting BLyS/APRIL inhibitors for the treatment of AIDs. However, due to the scattered knowledge on the relationship between BLyS/APRIL and AIDs, it is necessary to sort out the existing data. Therefore, in this review, we describe the basic biological characteristics and functions of BLyS/APRIL in AIDs, summarize the potential clinical applications of related inhibitors, especially monoclonal antibodies and recombinant fusion proteins targeting BLyS/APRIL in AIDs, and also outline promising research directions.

Past and present discovery of the BAFF/APRIL system - A bibliometric study from 1999 to 2023

Cytokines from the Tumour Necrosis Factor (TNF) family are important regulators of both physiological and pathological processes. The discovery of novel TNF ligands and receptors, BAFF and APRIL, have opened up new possibilities for scientists to explore the effect of these cytokines on the human immune system. The role of BAFF/APRIL system in B lymphocytes is particularly important for survival and maintenance of homeostasis. Aberrant expression of the system is associated with various immunological disorders. Hence, this study provides a comprehensive overview of the past and present BAFF/APRIL system research development in a bibliometric perspective. To our best knowledge, this is the first ever bibliometric analysis conducted focusing on the BAFF/APRIL system. A total of 1055 relevant documents were retrieved from WoSCC. Microsoft Excel, VOSviewer, and Biblioshiny of R studio were bibliometric tools used to analyse the scientific literature. From 1999, the annual publications showed an upward trend, with Journal of Immunology being the most productive journal. USA leads the race for BAFF/APRIL system research developments. Pascal Schneider, a senior researcher affiliated with University of Lausanne, Switzerland was recognised as the most productive author and institution in the BAFF/APRIL system research field. The research focus transitioned from focusing on the role of the system in B cell biology, to immunological disorders and finally to development of BAFF/APRIL targeting drugs. Despite several studies elucidating briefly the pathway mechanism of BAFF/APRIL system in B-cell selection, substantial research on the mechanism of action in disease models and T cell activation and development of immunomodulating drugs from natural origins remains largely unexplored. Therefore, future research focusing on these areas are crucial for the deeper understanding of the system in disease manifestations and progression allowing a better treatment management for various immunological disorders.

BLyS neutralization results in selective anti-HLA alloantibody depletion without successful desensitization

Pre-existing anti-HLA allo-antibodies (allo-Abs) are a major barrier to successful kidney transplantation, resulting in an elevated risk for antibody-mediated rejection (AMR) and eventual graft loss. The cytokine B lymphocyte stimulator (BLyS) promotes B cell maturation and plasma cell survival; consequently, anti-BLyS therapy represents a potential therapeutic opportunity in diminishing pre-existing allo-Abs. Here we report that in our 1-year pilot trial, BLyS neutralization failed to reduce total anti-HLA allo-Ab levels in highly sensitized candidates awaiting kidney transplant in a clinically meaningful way. Additionally, we performed a post hoc analysis using sera from trial candidates which revealed selective depletion of anti-HLA class I and class II Abs in response to belimumab treatment, restricted to certain allele specificities and IgG subclasses. Altogether, we observed that BLyS blockade only results in selective depletion of anti-HLA Abs recognizing a few discrete HLA allele specificities.

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.

Age-Associated B Cells

The age-associated B cell subset has been the focus of increasing interest over the last decade. These cells have a unique cell surface phenotype and transcriptional signature, and they rely on TLR7 or TLR9 signals in the context of Th1 cytokines for their formation and activation. Most are antigen-experienced memory B cells that arise during responses to microbial infections and are key to pathogen clearance and control. Their increasing prevalence with age contributes to several well-established features of immunosenescence, including reduced B cell genesis and damped immune responses. In addition, they are elevated in autoimmune and autoinflammatory diseases, and in these settings they are enriched for characteristic autoantibody specificities. Together, these features identify age-associated B cells as a subset with pivotal roles in immunological health, disease, and aging. Accordingly, a detailed understanding of their origins, functions, and physiology should make them tractable translational targets in each of these settings.

Ancient BCMA-like Genes Herald B Cell Regulation in Lampreys

The TNF superfamily ligands BAFF and APRIL interact with three receptors, BAFFR, BCMA, and TACI, to play discrete and crucial roles in regulating B cell selection and homeostasis in mammals. The interactions between these ligands and receptors are both specific and redundant: BAFFR binds BAFF, whereas BCMA and TACI bind to either BAFF or APRIL. In a previous phylogenetic inquiry, we identified and characterized a BAFF-like gene in lampreys, which, with hagfish, are the only extant jawless vertebrates, both of which have B-like and T-like lymphocytes. To gain insight into lymphocyte regulation in jawless vertebrates, in this study we identified two BCMA-like genes in lampreys, BCMAL1 and BCMAL2, which were found to be preferentially expressed by B-like lymphocytes. In vitro analyses indicated that the lamprey BAFF-like protein can bind to a BCMA-like receptor Ig fusion protein and to both BCMAL1- and BCMAL2-transfected cells. Discriminating regulatory roles for the two BCMA-like molecules are suggested by their differential expression before and after activation of the B-like lymphocytes in lampreys. Our composite results imply that BAFF-based mechanisms for B cell regulation evolved before the divergence of jawed and jawless vertebrates.

Here, There, and Anywhere? Arguments for and against the Physical Plasma Cell Survival Niche

To maintain Ab titers, individual plasma cells must survive for extended periods, perhaps even for the life of the host. Although it is clear that plasma cell survival requires cell extrinsic signals, the nature and source of these signals remains open for debate. It is commonly postulated that plasma cells only gain access to these signals within specialized regulatory microenvironments, or niches, in the bone marrow or in the gut. In this review we discuss current concepts and information surrounding plasma cell survival niches, and consider two opposing models to explain long-term serologic immunity.

Klhl6 Deficiency Impairs Transitional B Cell Survival and Differentiation

Klhl6 belongs to the KLHL gene family, which is composed of an N-terminal BTB-POZ domain and four to six Kelch motifs in tandem. Several of these proteins function as adaptors of the Cullin3 E3 ubiquitin ligase complex. In this article, we report that Klhl6 deficiency induces, as previously described, a 2-fold reduction in mature B cells. However, we find that this deficit is centered on the inability of transitional type 1 B cells to survive and to progress toward the transitional type 2 B cell stage, whereas cells that have passed this step generate normal germinal centers (GCs) upon a T-dependent immune challenge. Klhl6-deficient type 1 B cells showed a 2-fold overexpression of genes linked with cell proliferation, including most targets of the anaphase-promoting complex/cyclosome complex, a set of genes whose expression is precisely downmodulated upon culture of splenic transitional B cells in the presence of BAFF. These results thus suggest a delay in the differentiation process of Klhl6-deficient B cells between the immature and transitional stage. We further show, in the BL2 Burkitt's lymphoma cell line, that KLHL6 interacts with Cullin3, but also that it binds to HBXIP/Lamtor5, a protein involved in cell-cycle regulation and cytokinesis. Finally, we report that KLHL6, which is recurrently mutated in B cell lymphomas, is an off-target of the normal somatic hypermutation process taking place in GC B cells in both mice and humans, thus leaving open whether, despite the lack of impact of Klhl6 deficiency on GC B cell expansion, mutants could contribute to the oncogenic process.

Current and Emerging Therapies for Lupus Nephritis

The introduction of corticosteroids and later, cyclophosphamide dramatically improved survival in patients with proliferative lupus nephritis, and combined administration of these agents became the standard-of-care treatment for this disease. However, treatment failures were still common and the rate of progression to ESRD remained unacceptably high. Additionally, treatment was associated with significant morbidity. Therefore, as patient survival improved, the goals for advancing lupus nephritis treatment shifted to identifying therapies that could improve long-term renal outcomes and minimize treatment-related toxicity. Unfortunately, progress has been slow and the current approaches to the management of lupus nephritis continue to rely on high-dose corticosteroids plus a broad-spectrum immunosuppressive agent. Over the past decade, an improved understanding of lupus nephritis pathogenesis fueled several clinical trials of novel drugs, but none have been found to be superior to the combination of a cytotoxic agent and corticosteroids. Despite these trial failures, efforts to translate mechanistic advances into new treatment approaches continue. In this review, we discuss current therapeutic strategies for lupus nephritis, briefly review recent advances in understanding the pathogenesis of this disease, and describe emerging approaches developed on the basis of these advances that promise to improve upon the standard-of-care lupus nephritis treatments.

An Overview of B-1 Cells as Antigen-Presenting Cells

The role of B cells as antigen-presenting cells (APCs) has been extensively studied, mainly in relation to the activation of memory T cells. Considering the B cell subtypes, the role of B-1 cells as APCs is beginning to be explored. Initially, it was described that B-1 cells are activated preferentially by T-independent antigens. However, some reports demonstrated that these cells are also involved in a T-dependent response. The aim of this review is to summarize information about the ability of B-1 cells to play a role as APCs and to briefly discuss the role of the BCR and toll-like receptor signals in this process. Furthermore, some characteristics of B-1 cells, such as natural IgM production and phagocytic ability, could interfere in the participation of these cells in the onset of an adaptive response.

Pharmacokinetics, pharmacodynamics, short term efficacy and safety of RCT-18, a novel BLyS/APRIL fusion protein, in patients with rheumatoid arthritis

AIM

RCT-18 is a recombinant fusion protein that interferes with the selection and survival of mature B-lymphocytes by inhibiting B-lymphocyte stimulator and a proliferation-inducing ligand.

METHODS

This single blind, randomized, placebo controlled, clinical pharmacological study explored the short term efficacy and safety of RCT-18 in 21 rheumatoid arthritis (RA) patients with three different dosing regimens. The pharmacological behaviour of RCT-18 was also characterized through a six level biomarker cascade approach to identify potential predictors for clinical responses.

RESULTS

Nine out of 10 patients (>80%) experienced moderate to good EULAR response at the end of 3 months with once or twice weekly doses of 180 mg RCT-18, whereas weekly administration of 360 mg RCT-18 or placebo, however, only resulted in moderate improvement in one patient in each group. Absence of IgM-type rheumatoid factor reduction, recovery of IgM 2 weeks after drug cessation, lack of decrease in the count of CD27(+) B-lymphocytes and a DAS28 change from baseline <6 in 4-6 weeks after the treatment initiation may indicate poor clinical response. No anti-drug antibody of RCT-18 was detected. The active treatments were well tolerated, although more mild to moderate infections were reported in patients receiving RCT-18.

CONCLUSION

The study results support further development of RCT-18 in RA patients and provide important information for future dose selection.

BAFF receptors and ligands create independent homeostatic niches for B cell subsets

The BAFF family of receptors and ligands controls B cell homeostasis and selection. Recent studies reveal distinct sources and roles for systemic versus locally produced BAFF. Moreover, the notion that differential BAFF receptor expression patterns establish independent homeostatic and selective niches has been strengthened. Finally, unique roles for BAFF family members in the regulation of antigen experienced and innate B cell subsets have been revealed. Herein, we overview current knowledge in these areas, emphasizing recent findings that inform these ideas.

Differences in mouse and human nonmemory B cell pools

Identifying cross-species similarities and differences in immune development and function is critical for maximizing the translational potential of animal models. Coexpression of CD21 and CD24 distinguishes transitional and mature B cell subsets in mice. In this study, we validate these markers for identifying analogous subsets in humans and use them to compare the nonmemory B cell pools in mice and humans, across tissues, and during fetal/neonatal and adult life. Among human CD19(+)IgM(+) B cells, the CD21/CD24 schema identifies distinct populations that correspond to transitional 1 (T1), transitional 2 (T2), follicular mature, and marginal zone subsets identified in mice. Markers specific to human B cell development validate the identity of marginal zone cells and the maturation status of human CD21/CD24 nonmemory B cell subsets. A comparison of the nonmemory B cell pools in bone marrow, blood, and spleen in mice and humans shows that transitional B cells comprise a much smaller fraction in adult humans than mice. T1 cells are a major contributor to the nonmemory B cell pool in mouse bone marrow, in which their frequency is more than twice that in humans. Conversely, in spleen, the T1:T2 ratio shows that T2 cells are proportionally ∼ 8-fold higher in humans than in mice. Despite the relatively small contribution of transitional B cells to the human nonmemory pool, the number of naive follicular mature cells produced per transitional B cell is 3- to 6-fold higher across tissues than in mice. These data suggest differing dynamics or mechanisms produce the nonmemory B cell compartments in mice and humans.

Persistence and selection of an expanded B-cell clone in the setting of rituximab therapy for Sjögren's syndrome

Introduction

Subjects with primary Sjögren’s syndrome (SjS) have an increased risk of developing B-cell lymphoma and may harbor monoclonal B-cell expansions in the peripheral blood. Expanded B-cell clones could be pathogenic, and their persistence could exacerbate disease or predispose toward the development of lymphoma. Therapy with anti-CD20 (rituximab) has the potential to eliminate expanded B-cell clones and thereby potentially ameliorate disease. This study was undertaken to identify and track expanded B-cell clones in the blood of subjects with primary SjS who were treated with rituximab.

Methods

To determine whether circulating B-cell clones in subjects with primary SjS emerge or remain after B cell-depleting therapy with rituximab, we studied the antibody heavy-chain repertoire. We performed single-memory B-cell and plasmablast sorting and antibody heavy-chain sequencing in six rituximab-treated SjS subjects over the course of a 1-year follow-up period.

Results

Expanded B-cell clones were identified in four out of the six rituximab-treated SjS subjects, based upon the independent amplification of sequences with identical or highly similar VH, DH, and JH gene segments. We identified one SjS subject with a large expanded B-cell clone that was present prior to therapy and persisted after therapy. Somatic mutations in the clone were numerous but did not increase in frequency over the course of the 1-year follow-up, suggesting that the clone had been present for a long period of time. Intriguingly, a majority of the somatic mutations in the clone were silent, suggesting that the clone was under chronic negative selection.

Conclusions

For some subjects with primary SjS, these data show that (a) expanded B-cell clones are readily identified in the peripheral blood, (b) some clones are not eliminated by rituximab, and (c) persistent clones may be under chronic negative selection or may not be antigen-driven. The analysis of sequence variation among members of an expanded clone may provide a novel means of measuring the chronicity and selection of expanded B-cell populations in humans.

Neutrophil-mediated IFN activation in the bone marrow alters B cell development in human and murine systemic lupus erythematosus

Inappropriate activation of type I IFN plays a key role in the pathogenesis of autoimmune disease, including systemic lupus erythematosus (SLE). In this study, we report the presence of IFN activation in SLE bone marrow (BM), as measured by an IFN gene signature, increased IFN regulated chemokines, and direct production of IFN by BM-resident cells, associated with profound changes in B cell development. The majority of SLE patients had an IFN signature in the BM that was more pronounced than the paired peripheral blood and correlated with both higher autoantibodies and disease activity. Pronounced alterations in B cell development were noted in SLE in the presence of an IFN signature with a reduction in the fraction of pro/pre-B cells, suggesting an inhibition in early B cell development and an expansion of B cells at the transitional stage. These B cell changes strongly correlated with an increase in BAFF and APRIL expression in the IFN-high BM. Furthermore, we found that BM neutrophils in SLE were prime producers of IFN-α and B cell factors. In NZM lupus-prone mice, similar changes in B cell development were observed and mediated by IFN, given abrogation in NZM mice lacking type-I IFNR. BM neutrophils were abundant, responsive to, and producers of IFN, in close proximity to B cells. These results indicate that the BM is an important but previously unrecognized target organ in SLE with neutrophil-mediated IFN activation and alterations in B cell ontogeny and selection.

New roles for the BLyS/BAFF family in antigen-experienced B cell niches

BLyS family members govern selection and survival of cells in the pre-immune B cell compartment, and emerging evidence suggests similar roles in antigen-experienced B cell pools. We review the features of this family, with particular emphasis on recent findings of how BLyS influences affinity maturation in germinal centers, which lie at the intersection of the pre-immune and antigen-experienced B cell compartments. We propose a model whereby tolerogenic selection at the transitional stage and affinity maturation in the germinal center employ the same BLyS driven mechanism.

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.

The role of B lymphocyte stimulator in B cell biology: implications for the treatment of lupus

B lymphocyte stimulator (BLyS; also known as B cell activating factor (BAFF)) plays a key role in peripheral B cell tolerance. Mounting evidence indicates that B cell tolerance can be either broken or modulated by deliberately manipulating BLyS levels, and belimumab, a BLyS-neutralizing antibody, was recently approved for the treatment of systemic lupus erythematosus (SLE). Thus, intense investigation has focused on understanding how therapeutics targeting BLyS may work, and accumulating evidence suggests multiple points of action. BLyS signaling, in conjunction with B cell receptor (BCR) signaling, determines the size and quality of the mature primary B cell compartment. Moreover, BLyS family members play roles in antigen-experienced B cell selection and differentiation. Together, these findings have implications for the continued development of novel therapeutics that target BLyS.

Homeostatic T cell proliferation after islet transplantation

Pancreatic islet transplantation in patients with type 1 diabetes mellitus is performed under immunosuppression to avoid alloreactive T cell responses and to control the reactivation of autoreactive memory T cells. However, lymphopenia associated with immunosuppression and T cell depletion can induce a paradoxical expansion of lymphocyte subsets under the influence of homeostatic proliferation. Homeostatic T cell proliferation is mainly driven by the IL-7/IL-7 receptor axis, a molecular pathway which is not affected by standard immune-suppressive drugs and, consequently, represents a novel potential target for immuno-modulatory strategies. In this review, we will discuss how homeostatic T cell proliferation can support autoimmunity recurrence after islet transplantation and how it can be targeted by new therapeutic approaches.

Roles for BLyS family members in meeting the distinct homeostatic demands of innate and adaptive B cells

B-1 and B-2 B cell populations have different progenitors, receptor diversity, anatomic location, and functions – suggesting vastly differing requisites for homeostatic regulation. There is evidence that the B lymphocyte stimulator (BLyS) family of cytokines and receptors, key factors in the homeostatic regulation of B-2 B cell subsets, is also a major player in the B-1 compartment. Here we review the development and differentiation of these two primary B cell lineages and their immune functions. We discuss evidence that BLyS or a proliferation-inducing ligand (APRIL) availability in different anatomic sites, coupled with signature BLyS receptor expression patterns on different B cell subsets, may be important for homeostatic regulation of B-1 as well as B-2 populations. Finally, we extend our working model of B cell homeostasis to integrate B-1s.

Multiple regulatory mechanisms control B-1 B cell activation

B-1 cells constitute a unique subset of B cells identified in several species including mice and humans. B-1 cells are further subdivided into B-1a and B-1b subsets as the former but not the later express CD5. The B-1a subset contributes to innate type of immune responses while the B-1b B cell subset contributes to adaptive responses. B-1 cell responses to B cell receptor (BCR) as well as Toll-like receptor (TLR) ligation are tightly regulated due to the cross-reactivity of antigen specific receptors on B-1 cells to self-antigens. B-1 cells are elevated in several autoimmune diseases. CD5 plays a major role in down regulation of BCR responses in the B-1a cell subset. Reduced amplification of BCR induced signals via CD19 and autoregulation of BCR and TLR responses by B-1 cell produced IL-10 appear to have a role in regulation of both B-1a and B-1b B cell responses. Siglec G receptors and Lyn kinase also regulate B-1 cell responses but their differential role in the two B-1 cell subsets is unknown.

Antibodies against human BLyS and APRIL attenuate EAE development in marmoset monkeys

B lymphocyte stimulator (BLyS, also indicated as BAFF (B-cell activating factor) and CD257), and A Proliferation Inducing Ligand (APRIL, CD256) are two members of the TNF superfamily with a central role in B cell survival. Antibodies against these factors have potential therapeutic relevance in autoimmune inflammatory disorders with a proven pathogenic contribution of B cells, such as multiple sclerosis (MS). In the current study we performed a multi-parameter efficacy comparison of monoclonal antibodies against human anti-BLyS and anti-APRIL in a common marmoset (Callithrix jacchus) model of experimental autoimmune encephalomyelitis (EAE). A MS-like disease was induced by immunization with recombinant human myelin/oligodendrocyte glycoprotein (rhMOG) in complete Freund’s adjuvant. The results show that the anti-BLyS and anti-APRIL antibody cause significant depletion of circulating CD20+ B cells, but a small subset of CD20 + CD40^high B cells was not depleted. Induction of CD20+ B cell depletion from lymph nodes was only observed in the anti-BLyS treated monkeys. Both antibodies had a significant inhibitory effect on disease development, but all monkeys developed clinically evident EAE. Anti-BLyS treated monkeys were sacrificed with the same clinical signs as saline-treated monkeys, but nevertheless displayed significantly reduced spinal cord demyelination. This effect was not observed in the anti-APRIL treated monkeys. The two antibodies had a different effect on T cell subset activation and the profiles of ex vivo released cytokines. In conclusion, treatment with anti-BLyS and anti-APRIL delays the development of neurological disease in a relevant preclinical model of MS. The two mAbs achieve this effect via different mechanisms.

Murine islet allograft tolerance upon blockade of the B-lymphocyte stimulator, BLyS/BAFF

BACKGROUND

Immunologic rejection is a major barrier to successful long-term outcomes in clinical transplantation. The importance of B lymphocytes-and their secretory products, alloantibodies-in the pathogenesis of allograft rejection is accepted. Furthermore, it is now clear that the dominant regulator of peripheral B-cell homeostasis and tolerance is the B-lymphocyte stimulator (BLyS), also referred to as the B-cell activating factor (BAFF). Recently, a novel class of clinical immunotherapeutic agents specific for BLyS, and its family of cytokines, has emerged for the treatment of B-cell-mediated diseases. In this study, we demonstrate the potential utility of BLyS-directed immunotherapy in preventing allograft rejection using a murine islet transplantation model.

METHODS

A transient period of mature peripheral B-cell depletion was induced by means of in vivo BLyS neutralization using a murine analog of the monoclonal antibody, Benlysta. Subsequently, fully major histocompatibility complex-mismatched islets were transplanted into naïve diabetic mice followed by a short course of rapamycin.

RESULTS

After BLyS neutralization, indefinite islet allograft survival was achieved. Induction therapy with rapamycin was necessary, but not sufficient, for the achievement of this long-term graft survival. The tolerant state was associated with (1) abrogation of the donor-specific antibody response, (2) transient preponderance of immature/transitional B cells in all lymphoid organs, (3) impaired CD4 T-cell activation during the period of B-cell depletion, and (4) presence of a "regulatory" cytokine milieu.

CONCLUSIONS

In vivo BLyS neutralization effectively induces humoral tolerance and promotes long-term islet allograft survival in mice. Therefore, B-lymphocyte-directed immunotherapy targeting the homeostatic regulator, BLyS, may be effective in promoting transplantation tolerance.

Clinical applications for biomarkers of acute and chronic graft-versus-host disease

Acute and chronic graft-versus-host disease (aGVHD, cGVHD) are serious complications of allogeneic hematopoietic cell transplantation. The complex pathophysiology of these disease processes is associated with immune system activation, the release of cytokines and chemokines, and alterations in cell populations. The blood levels of specific protein and cellular levels in patients with GVHD have correlated with the development, diagnosis, and prognosis of GVHD. Here, we review the most promising biomarkers for aGVHD and cGVHD with clinical relevance. The utility of GVHD biomarkers in clinical care of allogeneic hematopoietic cell transplantation recipients needs to be proven through clinical trials, and potential approaches to trial design are discussed.

Lymphodepletion and homeostatic proliferation: implications for transplantation

Control of the alloimmune response requires elimination and/or suppression of alloreactive immune cells. Lymphodepleting induction therapies are increasingly used to accomplish this goal, both as part of tolerance induction protocols or to reduce the requirements for maintenance immunosuppression in the peritransplant setting. However, it is well recognized that lymphopenia induces compensatory proliferation of immune cells, generally termed ``homeostatic proliferation,'' which favors the emergence of memory T cells. Paradoxically therefore, the result may be a situation that favors graft rejection and/or makes tolerance difficult to achieve or sustain. Yet all depletion is not alike, particularly with respect to the timing of reconstitution and the types of cells that repopulate the host. Thus, to design more effective induction strategies it is important to understand the homeostatic mechanisms, which exist to maintain a balanced repertoire of naïve and memory T and B cells in the periphery and how they respond to lymphodepletion. Here we will review the biology of homeostatic proliferation stimulated by lymphopenia, the effects of specific depleting agents on reconstitution of the T- and B-cell immune repertoire, drawing from both from animal models and human experience, and potential strategies to enhance allodepletion while minimizing the adverse effects of homeostatic proliferation.

Bone marrow B cell precursor number after allogeneic stem cell transplantation and GVHD development

Patients without chronic graft-versus-host disease (cGVHD) have robust B cell reconstitution and are able to maintain B cell homeostasis after allogeneic hematopoietic stem cell transplantation (HSCT). To determine whether B lymphopoiesis differs before cGVHD develops, we examined bone marrow (BM) biopsies for terminal deoxynucleotidyl transferase (TdT) and PAX5 immunostaining early post-HSCT at day 30 when all patients have been shown to have high B cell activating factor (BAFF) levels. We found significantly greater numbers of BM B cell precursors in patients who did not develop cGVHD compared with those who developed cGVHD (median = 44 vs 2 cells/high powered field [hpf]; respectively; P < .001). Importantly, a significant increase in precursor B cells was maintained when patients receiving high-dose steroid therapy were excluded (median = 49 vs 20 cells/hpf; P = .017). Thus, we demonstrate the association of BM B cell production capacity in human GVHD development. Increased BM precursor B cell number may serve to predict good clinical outcome after HSCT.

Resolve, revise, and relax: the 3 Rs of B cell repertoire adjustment

Competition for limited, cell extrinsic survival factors is a general feature of peripheral selection checkpoints involved in B lymphocyte maturation and activation. Perhaps the best-characterized example involves BLyS (B lymphocyte stimulator), which modulates the size and composition of mature naïve B cell pools, but evidence for analogous competitive checkpoints is emerging for both germinal center B cells and plasma cells. Here we discuss how deliberate alteration of BLyS levels might be used to manipulate B cell repertoire selection in order to restore self-tolerance in autoimmunity, remodel the repertoire to accommodate neo-self antigens introduced through transplantation and gene therapy, or expand repertoire diversity to reveal novel, therapeutically useful specificities.

Essential role for B cells in transplantation tolerance

T lymphocytes are the primary targets of immunotherapy in clinical transplantation. However, B lymphocytes are detrimental to graft survival by virtue of their capacity to present antigen to T cells via the indirect pathway of allorecognition and the generation of donor specific alloantibody. Furthermore, the long-term survival of organ allografts remains challenged by chronic rejection, a process in which activated B cells have been found to play a significant role. Therefore, the achievement of transplantation tolerance will likely require induction of both T and B cell tolerance to alloantigens. Moreover, human and animal investigations have shown that subsets of B cells, Transitional and Regulatory, are inherently tolerogenic. Developing therapeutic strategies that exploit these populations may be key to achieving transplantation tolerance. In this review we describe the current evidence for the essential role of B cells in transplant tolerance and discuss emerging B cell directed strategies to achieve allograft tolerance.

Syk inhibition with fostamatinib leads to transitional B lymphocyte depletion

Cell signaling initiated by the B cell receptor is critical to normal development of B lymphocytes, most notably at the transitional B cell stage. Inhibition of this signaling pathway with the syk inhibitor, fostamatinib, has produced significant efficacy in lymphoid malignancies and autoimmune conditions. Here, we demonstrate that short-term use of fostamatinib impairs B lymphocyte development at the transitional stage without affecting mature B cell populations. Additionally, IL-10 producing B cells remained relatively constant throughout the treatment period. These findings provide insight into the mechanism of action of B cell receptor inhibition in autoimmune disease. As the development of agents targeting B cell receptor signaling proceeds, monitoring for long-term consequences as well as functional evaluation of B cell subsets may further improve our understanding of this rapidly growing class of novel agents.

The coin toss of B cells in rejection and tolerance: danger versus defense

Transplantation is the preferred therapy for the end stage organ disease. Since the introduction of organ transplantation into medical practice in 1953 [1], significant progress has been achieved in patient and graft survival rates due to improvements in surgical techniques and more targeted immunosuppressive medications [2]. Nevertheless, current gaps in the management of the transplant patient stem from an incomplete understanding about the heterogeneity of the injury response in organ transplantation, at different rates and different time points after transplantation, as well as our inability to monitor the immunologic threshold of risk versus safety in each individual patient. Recent advances in immunology/transplantation biology with the advent of high throughput "omic" assays such as gene microarrays, proteomics, metabolomics, antibiomics, chemical genomics and functional imaging with nanoparticles, offers us unique methods to interrogate and decipher the variability and unpredictability of the immune response in organ transplantation (Fig. 1) [3]. Recent studies using these applications [3-8] have uncovered a critical and pivotal role for specific B cell lineages in organ injury [9] and organ acceptance [10,11] (Fig. 2). The availability of specific therapies against some of these defined B cell populations provides for an exciting new field of B cell targeted manipulation that can both abrogate the allospecific injury response, as well as promote allospecific graft accommodation and health.

Primary B cell repertoire remodeling to achieve humoral transplantation tolerance

The current mainstay of immunotherapy in clinical transplantation is T lymphocyte directed. However, it has long been appreciated that the emergence of an alloimmune response mounted by the B lymphocyte compartment and detectable as donor-specific antibodies is a critical challenge to long-term graft survival. Thus, achieving robust transplantation tolerance will require induction of tolerance in both the T- and B-cell compartments. Here we propose that the natural developmental propensity of the B-lymphocyte compartment acquisition of tolerance to self-antigens can be recapitulated to achieve humoral transplantation tolerance. It is our contention B-lymphocyte directed induction immunotherapy would be an important component of emerging strategies for induction of Transplantation tolerance.

A B-cell subset uniquely responsive to innate stimuli accumulates in aged mice

We have discovered a distinct mature B-cell subset that accumulates with age, which we have termed age-associated B cells. These cells comprise up to 30% of mature B cells by 22 months. Despite sharing some features with other mature B-cell subsets, they are refractory to BCR and CD40 stimulation. Instead, they respond to TLR9 or TLR7 stimulation and divide maximally on combined BCR and TLR ligation, leading to Ig production and preferential secretion of IL-10 and IL-4. Although similar to follicular B cells in both B-lymphocyte stimulator (BLyS) receptor expression and BLyS binding capacity, these cells do not rely on BLyS for survival. They are neither cycling nor the result of intrinsically altered B lymphopoiesis in aged BM, but instead appear to be generated from mature B cells that exhaustively expand during the individual's lifetime. Finally, they present Ag effectively and favor polarization to a TH17 profile. Together, these findings reveal that while the magnitude of the mature primary B-cell niche is maintained with age, it is increasingly occupied by cells refractory to BCR-driven activation yet responsive to innate receptor stimulation.

Targeting BLyS in rheumatic disease: the sometimes-bumpy road from bench to bedside

PURPOSE OF REVIEW

BLyS family ligands and receptors are key players in the selection and survival of most mature B lymphocytes. The fundamental role of BLyS in transitional B cell selection, coupled with the relative BLyS-independence of memory B cells and plasma cells, suggests that BLyS may be a useful therapeutic target in strategies directed against preimmune B cell pools. Several agents that target BLyS are in clinical trials now, and we summarize recent results here, with a focus on systemic lupus erythematosus (SLE).

RECENT FINDINGS

Belimumab, a human neutralizing anti-BLyS monoclonal antibody, has delivered moderate but positive results in two separate phase III clinical trials for SLE, and was recently recommended for approval by an FDA advisory panel. Additional agents targeting BLyS or other members of this cytokine receptor family are also being tested in clinical trials.

SUMMARY

Together, these trials should yield novel therapies for a debilitating and often intractable illness and offer insights that in turn should foster subsequent generations of personalized, targeted therapies for rheumatic diseases.

Recovery of B-cell homeostasis after rituximab in chronic graft-versus-host disease

Investigation of the effects of rituximab (anti-CD20) on B-cell-activating factor of the tumor necrosis factor family (BAFF) and B cells would better define the significance of B-cell homeostasis in chronic graft-versus-host disease (cGVHD) pathophysiology. We studied 20 cGVHD patients at a median of 25 months after rituximab treatment when most patients had recovered total B-cell numbers. A total of 55% of patients had stable/improved cGVHD, and total B-cell numbers in these patients were significantly higher compared with rituximab-unresponsive patients. Although total B-cell number did not differ significantly between cGVHD groups before rituximab, there was a proportional increase in B-cell precursors in patients who later had stable/improved cGVHD. After rituximab, BAFF levels increased in all patients. Coincident with B-cell recovery in the stable/improved group, BAFF/B-cell ratios and CD27(+) B-cell frequencies decreased significantly. The peripheral B-cell pool in stable/improved cGVHD patients was largely composed of naive IgD(+) B cells. By contrast, rituximab-unresponsive cGVHD patients had persistent elevation of BAFF and a predominance of circulating B cells possessing an activated BAFF-R(Lo)CD20(Lo) cell surface phenotype. Thus, naive B-cell reconstitution and decreased BAFF/B-cell ratios were associated with clinical response after rituximab in cGVHD. Our findings begin to delineate B-cell homeostatic mechanisms important for human immune tolerance.

Acquisition of humoral transplantation tolerance upon de novo emergence of B lymphocytes

A major obstacle to transplantation tolerance is humoral immunity. In this paper, we demonstrate that the intrinsic developmental propensity of the B lymphocyte compartment for acquisition of self-tolerance can be harnessed to induce humoral unresponsiveness to transplanted alloantigens. In the current study, when transitional B cells developed in the presence of donor lymphoid cells, the mature B lymphocyte compartment failed to mount a donor-specific alloantibody response to an organ transplant--despite unrestrained acute T cell-mediated allograft rejection. Specifically, we generated an experimental system wherein a B6 strain B cell compartment developed de novo in the presence of F1 (B6xBALB/c) lymphoid cells and in a T cell-deficient setting. Following establishment of a steady-state B cell compartment, these B6 mice were transplanted with heterotopic cardiac allografts from allogeneic BALB/c donors. The mice were then inoculated with purified syngeneic B6 T cells. As expected, all cardiac allografts were acutely rejected. However, the B lymphocyte compartment of these mice was completely inert in its capacity to form a BALB/c-specific alloantibody response. Using an alloantigen-specific Ig transgenic system, we demonstrated that this profound degree of humoral tolerance was caused by clonal deletion of alloreactive specificities from the primary B cell repertoire. Thus, de novo B cell compartment development at the time of transplantation is of critical importance in recipient repertoire "remodeling" to a humoral tolerant state.

B-lymphocyte homeostasis and BLyS-directed immunotherapy in transplantation

Current strategies for immunotherapy after transplantation are primarily T-lymphocyte directed and effectively abrogate acute rejection. However, the reality of chronic allograft rejection attests to the fact that transplantation tolerance remains an elusive goal. Donor-specific antibodies are considered the primary cause of chronic rejection. When naive, alloreactive B-cells encounter alloantigen and are activated, a resilient "sensitized" state, characterized by the presence of high-affinity antibody, is established. Here, we will delineate findings that support transient B-lymphocyte depletion therapy at the time of transplantation to preempt sensitization by eliminating alloreactive specificities from the recipient B-cell pool (ie, "repertoire remodeling"). Recent advances in our understanding of B-lymphocyte homeostasis provide novel targets for immunomodulation in transplantation. Specifically, the tumor necrosis factor-related cytokine BLyS is the dominant survival factor for "tolerance-susceptible" transitional and "preimmune" mature follicular B-cells. The transitional phenotype is the intermediate through which all newly formed B-cells pass before maturing into the follicular subset, which is responsible for mounting an alloantigen-specific antibody response. Systemic BLyS levels dictate the stringency of negative selection during peripheral B-cell repertoire development. Thus, targeting BLyS will likely provide an opportunity for repertoire-directed therapy to eliminate alloreactive B-cell specificities in transplant recipients, a requirement for the achievement of humoral tolerance and prevention of chronic rejection. In this review, the fundamentals of preimmune B-cell selection, homeostasis, and activation will be described. Furthermore, new and current B-lymphocyte-directed therapy for antibody-mediated rejection and the highly sensitized state will be discussed. Overall, our objective is to propose a rational approach for induction of humoral transplantation tolerance by remodeling the primary B-cell repertoire of the allograft recipient.

Expansion of immunoglobulin-secreting cells and defects in B cell tolerance in Rag-dependent immunodeficiency

The contribution of B cells to the pathology of Omenn syndrome and leaky severe combined immunodeficiency (SCID) has not been previously investigated. We have studied a mut/mut mouse model of leaky SCID with a homozygous Rag1 S723C mutation that impairs, but does not abrogate, V(D)J recombination activity. In spite of a severe block at the pro–B cell stage and profound B cell lymphopenia, significant serum levels of immunoglobulin (Ig) G, IgM, IgA, and IgE and a high proportion of Ig-secreting cells were detected in mut/mut mice. Antibody responses to trinitrophenyl (TNP)-Ficoll and production of high-affinity antibodies to TNP–keyhole limpet hemocyanin were severely impaired, even after adoptive transfer of wild-type CD4⁺ T cells. Mut/mut mice produced high amounts of low-affinity self-reactive antibodies and showed significant lymphocytic infiltrates in peripheral tissues. Autoantibody production was associated with impaired receptor editing and increased serum B cell–activating factor (BAFF) concentrations. Autoantibodies and elevated BAFF levels were also identified in patients with Omenn syndrome and leaky SCID as a result of hypomorphic RAG mutations. These data indicate that the stochastic generation of an autoreactive B cell repertoire, which is associated with defects in central and peripheral checkpoints of B cell tolerance, is an important, previously unrecognized, aspect of immunodeficiencies associated with hypomorphic RAG mutations.

B-lymphocyte stimulator/a proliferation-inducing ligand heterotrimers are elevated in the sera of patients with autoimmune disease and are neutralized by atacicept and B-cell maturation antigen-immunoglobulin

Introduction

B-lymphocyte stimulator (BLyS) and a proliferation-inducing ligand (APRIL) are members of the tumor necrosis factor (TNF) family that regulate B-cell maturation, survival, and function. They are overexpressed in a variety of autoimmune diseases and reportedly exist in vivo not only as homotrimers, but also as BLyS/APRIL heterotrimers.

Methods

A proprietary N-terminal trimerization domain was used to produce recombinant BLyS/APRIL heterotrimers. Heterotrimer biologic activity was compared with that of BLyS and APRIL in a 4-hour signaling assay by using transmembrane activator and CAML interactor (TACI)-transfected Jurkat cells and in a 4-day primary human B-cell proliferation assay. A bead-based immunoassay was developed to quantify native heterotrimers in human sera from healthy donors (n = 89) and patients with systemic lupus erythematosus (SLE; n = 89) or rheumatoid arthritis (RA; n = 30). Heterotrimer levels were compared with BLyS and APRIL homotrimer levels in a subset of these samples.

Results

The recombinant heterotrimers consisted mostly of one BLyS and two APRIL molecules. Heterotrimer signaling did not show any significant difference compared with APRIL in the TACI-Jurkat assay. Heterotrimers were less-potent inducers of B-cell proliferation than were homotrimeric BLyS or APRIL (EC₅₀, nMol/L: BLyS, 0.02; APRIL, 0.17; heterotrimers, 4.06). The soluble receptor fusion proteins atacicept and B-cell maturation antigen (BCMA)-immunoglobulin (Ig) neutralized the activity of BLyS, APRIL, and heterotrimers in both cellular assays, whereas B-cell activating factor belonging to the TNF family receptor (BAFF-R)-Ig neutralized only the activity of BLyS. In human sera, significantly more patients with SLE had detectable BLyS (67% versus 18%; P < 0.0001), APRIL (38% versus 3%; P < 0.0002), and heterotrimer (27% versus 8%; P = 0.0013) levels compared with healthy donors. Significantly more patients with RA had detectable APRIL, but not BLyS or heterotrimer, levels compared with healthy donors (83% versus 3%; P < 0.0001). Heterotrimer levels weakly correlated with BLyS, but not APRIL, levels.

Conclusions

Recombinant BLyS/APRIL heterotrimers have biologic activity and are inhibited by atacicept and BCMA-Ig, but not by BAFF-R-Ig. A novel immunoassay demonstrated that native BLyS/APRIL heterotrimers, as well as BLyS and APRIL homotrimers, are elevated in patients with autoimmune diseases.

B-cell biology and related therapies in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a complex disease characterized by numerous autoantibodies and clinical involvement in multiple organ systems. The immunologic events triggering the onset and progression of clinical manifestations have not yet been fully defined, but a central role for B cells in the pathogenesis has been brought to the fore in the last several years. The breakdown of B-cell tolerance is likely a defining and early event in the disease process and may occur by multiple pathways, including alterations in factors that affect B-cell activation thresholds, B-cell longevity, and apoptotic cell processing. Antibody-dependent and -independent mechanisms of B cells are important in SLE. Thus, autoantibodies contribute to autoimmunity by multiple mechanisms including immune complex mediated type III hypersensitivity reactions, type II antibody-dependent cytotoxicity, and by instructing innate immune cells to produce pathogenic cytokines including interferon alpha, tumor necrosis factor, and interleukin 1. Recent data have highlighted the critical role of toll-like receptors as a link between the innate and adaptive immune system in SLE immunopathogenesis. Given the large body of evidence implicating abnormalities in the B-cell compartment in SLE, there has been a therapeutic focus on developing interventions that target the B-cell compartment. Several different approaches to targeting B cells have been used, including B-cell depletion with monoclonal antibodies against B-cell-specific molecules, induction of negative signaling in B cells, and blocking B-cell survival and activation factors. Overall, therapies targeting B cells are beginning to show promise in the treatment of SLE and continue to elucidate the diverse roles of B cells in this complex disease.

B-cell tolerance in transplantation: is repertoire remodeling the answer?

T lymphocytes are the primary targets of immunotherapy in clinical transplantation; however, B lymphocytes and their secreted alloantibodies are also highly detrimental to the allograft. Therefore, the achievement of sustained organ transplant survival will likely require the induction of B-lymphocyte tolerance. During development, acquisition of B-cell tolerance to self-antigens relies on clonal deletion in the early stages of B-cell compartment ontogeny. We contend that this mechanism should be recapitulated in the setting of alloantigens and organ transplantation to eliminate the alloreactive B-cell subset from the recipient. Clinically feasible targets of B-cell-directed immunotherapy, such as CD20 and B-lymphocyte stimulator (BLyS), should drive upcoming clinical trials aimed at remodeling the recipient B-cell repertoire.

Anticytokine therapy impacting on B cells in autoimmune diseases

PURPOSE OF REVIEW

To evaluate the impact of particular anticytokine therapies able to indirectly target B cells with emphasis on the tumor necrosis factor (TNF) family members, B cell activating factor/B lymphocyte stimulator (BAFF/BLyS) and a proliferation-inducing ligand (APRIL).

RECENT FINDINGS

Although blockade of TNF/lymphotoxin by etanercept has been shown to have an impact on memory B cells, recent studies of inhibiting the TNF family members BAFF/BlyS or simultaneously blocking BAFF/BlyS and APRIL in rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) clearly demonstrated biologic activity, including reductions of immunoglobulin levels. However, clear evidence of clinical activity by any of the compounds interfering with BAFF/BLys or APRIL has not yet been shown.

SUMMARY

Although anticytokine therapies mainly blocking signaling pathways of innate immunity, that is TNF-alpha, have shown efficacy in the treatment of arthritides and have partial effects on memory B cells, current studies evaluate effects on adaptive immunity by blocking BAFF/BlyS and/or APRIL which indirectly act on B and plasma cells.

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

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

Altered B-cell homeostasis and excess BAFF in human chronic graft-versus-host disease

Chronic graft-versus-host disease (cGVHD) causes significant morbidity and mortality in patients otherwise cured of malignancy after hematopoietic stem cell transplantation (HSCT). The presence of alloantibodies and high plasma B cell-activating factor (BAFF) levels in patients with cGVHD suggest that B cells play a role in disease pathogenesis. We performed detailed phenotypic and functional analyses of peripheral B cells in 82 patients after HSCT. Patients with cGVHD had significantly higher BAFF/B-cell ratios compared with patients without cGVHD or healthy donors. In cGVHD, increasing BAFF concentrations correlated with increased numbers of circulating pre-germinal center (GC) B cells and post-GC "plasmablast-like" cells, suggesting in vivo BAFF dependence of these 2 CD27(+) B-cell subsets. Circulating CD27(+) B cells in cGVHD comprised in vivo activated B cells capable of IgG production without requiring additional antigen stimulation. Serial studies revealed that patients who subsequently developed cGVHD had delayed reconstitution of naive B cells despite persistent BAFF elevation as well as proportional increase in CD27(+) B cells in the first year after HSCT. These studies delineate specific abnormalities of B-cell homeostasis in patients with cGVHD and suggest that BAFF targeting agents may be useful in this disease.