The BLyS family of ligands and receptors: an archetype for niche-specific homeostatic regulation

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.

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.

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.

BLyS antagonists and peptide tolerance induction

The major impediment to drug development for systemic lupus erythematosus (SLE) is its heterogeneity. The unpredictable manner in which lupus targets different organs at varying intensity makes the study of new drugs and the optimization of their administration extremely difficult. With the advent of novel, targeted biologic agents for SLE, it can be hoped that more strategic, lupus-relevant immune modulation will lead to safer and more effective treatments. Two alternative new approaches to lupus treatment are reviewed. The first involves selective inhibition of a single protein (BLyS), which may play a central role in host defense and in the pathogenesis of SLE. Although this approach is finely targeted to the inhibition of a single protein which is known to be upregulated in SLE patients, the positioning of BLyS at a critical hub in the immune response suggests that more global adverse repercussions on immunity might still occur. The second strategy is the use of peptides designed to specifically induce tolerance in limited autoreactive immune responses. Immune repercussions might, at least in theory, be almost nonexistent with this kind of approach. Concerns that limited induction of tolerance might have equally limited impact on the complex immune disorder of SLE are not borne out in preliminary murine data. Specific development programs are ongoing using both of these strategies and have recently entered human trials.

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.

Phase II study of personalized peptide vaccination for previously treated advanced colorectal cancer

The prognosis of advanced colorectal cancer (aCRC) remains poor, and development of new therapeutic approaches, including immunotherapy, is needed urgently. Herein we report on our phase II study of personalized peptide vaccination (PPV) in 60 previously treated patients with aCRC, who had failed at least one regimen of standard chemotherapy and/or targeted therapy. For PPV, a maximum of four HLA-matched peptides were individually selected from a pool of 31 different peptide candidates based on preexisting host immunity, and administered subcutaneously without severe adverse events. Boosting of IgG and cytotoxic T lymphocyte (CTL) responses specific to the administered peptides was observed in 49% and 63%, respectively, of the patients, who completed the first cycles of six vaccinations. Median overall survival (OS) time was 498 days, with 1- and 2-year survival rates of 53% and 22%, respectively. Multivariate Cox regression analysis of prevaccination factors showed that plasma IL6, IP-10, and BAFF levels were significantly prognostic for OS [hazard ratio (HR), 1.508, P = 0.043; HR, 1.579, P = 0.024; HR, 0.509, P = 0.002, respectively]. In addition, increased peptide-specific CTL responses after vaccination were significantly predictive of favorable OS (HR, 0.231; P = 0.021), suggesting a causal relationship between biologic and clinical efficacy of PPV. On the basis of the safety profile and potential clinical efficacy, we believe that clinical trials of PPV would be warranted for previously treated patients with aCRC.

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.

Roles for TNF-receptor associated factor 3 (TRAF3) in lymphocyte functions

TRAF3 is an adapter protein that serves and regulates the functions of several types of receptors, located both inside the cell and at the plasma membrane. These include members of the TNF receptor superfamily (TNFR-SF), toll-like receptors (TLR), and cytokine receptors. It has become increasingly evident that the roles and functions of TRAF3 are highly context-dependent. TRAF3 can serve distinct roles for different receptors in the same cell, and also has highly cell-type-dependent functions. This review focuses upon the current state of knowledge regarding how TRAF3 regulates the biology and effector functions of B and T lymphocytes, two major cell types of the adaptive immune response in which TRAF3 has markedly distinct roles.

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.

BLyS levels correlate with vaccine-induced antibody titers in patients with glioblastoma lymphodepleted by therapeutic temozolomide

B lymphocyte stimulator (BLyS) is a cytokine involved in differentiation and survival of follicular B cells along with humoral response potentiation. Lymphopenia is known to precipitate dramatic elevation in serum BLyS; however, the use of this effect to enhance humoral responses following vaccination has not been evaluated. We evaluated BLyS serum levels and antigen-specific antibody titers in 8 patients undergoing therapeutic temozolomide (TMZ)-induced lymphopenia, with concomitant vaccine against a tumor-specific mutation in the epidermal growth factor receptor (EGFRvIII). Our studies demonstrate that TMZ-induced lymphopenia corresponded with spikes in serum BLyS that directly preceded the induction of anti-EGFRvIII antigen-specific antibody titers, in some cases as high as 1:2,000,000. Our data are the first clinical observation of BLyS serum elevation and greatly enhanced humoral immune responses as a consequence of chemotherapy-induced lymphopenia. These observations should be considered for the development of future vaccination strategies in the setting of malignancy.

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.

Belimumab--an anti-BLyS human monoclonal antibody for rheumatoid arthritis

INTRODUCTION

B lymphocyte stimulator (BLyS) is a major regulatory factor that controls the development and survival of B cells. Elevated serum levels of BLyS have been associated with rheumatoid arthritis (RA). Belimumab is a fully human monoclonal antibody that inhibits BLyS and it is being developed for the treatment of RA. This review aims to summarize up-to-date pharmacological and clinical data of belimumab in the treatment of RA.

AREAS COVERED

A literature search was performed on PubMed using keywords, including belimumab, LymphoStat-B, benlysta, BLyS inhibitor, rheumatoid arthritis and autoimmune disease. References of relevant studies were searched by hand. Abstracts of international conferences up to October 2012 were also included. Belimumab was well tolerated in the treatment of RA over 24 weeks. It significantly increased American College of Rheumatology (ACR)20 responses at week 24, especially in patients with high disease activity, positive rheumatoid factor, no anti-TNF treatment experience and those who had failed methotrexate therapy. However, belimumab failed to demonstrate significantly improved ACR50 and ACR70 responses in the single Phase II clinical trial of RA.

EXPERT OPINION

These results suggest that the clinical efficacy of belimumab for RA needs to be further investigated in future clinical trials. Careful patient selection may be necessary for belimumab to achieve optimal clinical outcomes in RA.

cDNA cloning, recombinant expression and bioactivity of Père David's deer BAFF

B cell activating factor (BAFF), a member of the TNF family, is a critical cytokine for the survival, proliferation, maturation, and differentiation of B cells. In the present study, Père David's deer BAFF (miBAFF) was amplified from Elaphurus davidianus using RT-PCR. This is the first BAFF cloned from a member of Cervidae family. The open reading frame (ORF) of the miBAFF cDNA consists of 843 bases that encode a 280-amino acid protein bearing typical TNF homology domain. Sequence alignment shows that miBAFF shares 39.3%-97% sequence homology with the BAFF sequences of other mammals. Comparative protein modeling predicted that the 3D structure of the soluble mature portion of miBAFF (misBAFF) is very similar to that of human BAFF (hsBAFF). Recombinant misBAFF fused to a SUMO-tag was efficiently expressed in Escherichia coli BL21 (DE3) cells. The protein molecular weight of ~36 KDa was determined using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting. In vitro, purified misBAFF was shown to promote the survival and proliferation of Père David's deer peripheral blood lymphocytes and mouse B cells. These results indicate that miBAFF plays an important role in the survival/proliferation of mouse B cells and, shows highly conserved evolutionarily, leading to functional cross-reactivity that exists between mouse and Père David's deer BAFF.

B cell maintenance and function in aging

In this review we discuss the changes that occur in the B lymphocyte compartment of mice and humans as they progress to old age, focusing on recent advances in this important area of research. Primary areas considered include increased morbidity and mortality in the elderly following infection, and decreased responsiveness to vaccines that evoke primary humoral immune responses, as well as those that evoke responses by memory B cells generated following vaccination and natural infection earlier in life. We then consider some of the mechanisms that may underlie these observed declines in humoral immune function. This includes a discussion of alterations in B cell repertoire and subcompartment distribution, as well as defects in B lymphopoiesis, cell development and homeostasis that may contribute to these alterations, and ultimately to declining protective quality of antibodies produced in the elderly.

The Nogo-66 receptor family in the intact and diseased CNS

The Nogo-66 receptor family (NgR) consists in three glycophosphatidylinositol (GPI)-anchored receptors (NgR1, NgR2 and NgR3), which are primarily expressed by neurons in the central and peripheral mammalian nervous system. NgR1 was identified as serving as a high affinity binding protein for the three classical myelin-associated inhibitors (MAIs) Nogo-A, myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (OMgp), which limit axon regeneration and sprouting in the injured brain. Recent studies suggest that NgR signaling may also play an essential role in the intact adult CNS in restricting axonal and synaptic plasticity and are involved in neurodegenerative diseases, particularly in Alzheimer's disease pathology through modulation of β-secretase cleavage. Here, we outline the biochemical properties of NgRs and their functional roles in the intact and diseased CNS.

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.

What's New, What's Hot in Solid Organ Transplantation? Summary of the American Transplant Congress 2011

Breakthroughs in basic and clinical science in solid organ transplantation were presented at the American Transplant Congress 2011. Key areas of presentation included the pathogenesis of late allograft failure, immune regulation and tolerance, pathways in allograft injury, electing appropriate patients for transplantation, determining the best allocation schemes to maximize effective utilization, organ preservation, monitoring the alloimmune response and immunosuppressive management. In this review, we present highlights of the meeting. These presentations demonstrate the exciting promise in translating from the bench to affect patient care.

Protective antiviral antibody responses in a mouse model of influenza virus infection require TACI

Antiviral Abs, for example those produced in response to influenza virus infection, are critical for virus neutralization and defense against secondary infection. While the half-life of Abs is short, Ab titers can last a lifetime due to a subset of the Ab-secreting cells (ASCs) that is long lived. However, the mechanisms governing ASC longevity are poorly understood. Here, we have identified a critical role for extrinsic cytokine signals in the survival of respiratory tract ASCs in a mouse model of influenza infection. Irradiation of mice at various time points after influenza virus infection markedly diminished numbers of lung ASCs, suggesting that they are short-lived and require extrinsic factors in order to persist. Neutralization of the TNF superfamily cytokines B lymphocyte stimulator (BLyS; also known as BAFF) and a proliferation-inducing ligand (APRIL) reduced numbers of antiviral ASCs in the lungs and bone marrow, whereas ASCs in the spleen and lung-draining lymph node were surprisingly unaffected. Mice deficient in transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), a receptor for BLyS and APRIL, mounted an initial antiviral B cell response similar to that generated in WT mice but failed to sustain protective Ab titers in the airways and serum, leading to increased susceptibility to secondary viral challenge. These studies highlight the importance of TACI signaling for the maintenance of ASCs and protection against influenza virus infection.

Emerging biotherapies for Sjögren's syndrome

IMPORTANCE OF THE FIELD

Sjögren's syndrome (SS) is an autoimmune epithelitis. This exocrinopathy is frequently associated with extraglandular complications, and the patients are at risk of developing B cell lymphoma. Given the lack of disease-modifying drugs, and the fact that SS is a quintessential B-cell mediated disease, attention has recently been focused on biotherapies.

AREAS COVERED IN THIS REVIEW

Despite negative grounds, TNF-alpha antagonists have been tested in the disease, and proven not be efficient. However, B-cell depleting therapy using anti-CD20 antibodies such as rituximab, which is a chimeric mAb, has shown promise in the field, while anti-CD22 mAb seems to be less active.

WHAT THE READER WILL GAIN

New treatments against the B-cell activating factor of the TNF family are about to be tested, or replaced by receptor immunoglobulin decay protein.

TAKE HOME MESSAGE

B-cell depleting therapies seem promising in SS, but no data are, thus far, available on treatments targeting B-cell activating factor of the TNF family.

Selective blockade of BAFF for the prevention and treatment of systemic lupus erythematosus nephritis in NZM2410 mice

OBJECTIVE

To determine whether BAFF or combined BAFF/APRIL blockade is effective in a mouse model of systemic lupus erythematosus (SLE) nephritis characterized by rapidly progressive glomerulosclerosis.

METHODS

NZM2410 mice at early and late stages of SLE nephritis were treated with a short course of BAFF-R-Ig or TACI-Ig fusion protein. Proteinuria and serologic profile were evaluated every 2 weeks. Immunohistochemical, flow cytometric, and enzyme-linked immunospot analyses of the spleen, kidney, and bone marrow were performed after 8 weeks and after 33 weeks.

RESULTS

A short course of selective blockade of BAFF alone was sufficient to prevent and treat SLE nephritis in NZM2410 mice, despite the formation of pathogenic autoantibodies. Decreases in spleen size and B cell depletion persisted for more than 33 weeks after treatment and resulted in secondary decreases in CD4 memory T cell formation and activation of splenic and peripheral monocytes. Immune complex deposition in the kidneys was dissociated from renal damage and from activation of renal endothelial and resident dendritic cells.

CONCLUSION

Selective blockade of BAFF alone, which resulted in B cell depletion and splenic collapse, was sufficient to prevent and treat the disease in this model of noninflammatory SLE nephritis. This shows that the inflammatory microenvironment may be a determinant of the outcome of B cell modulation strategies.

The in vitro derivation of phenotypically mature and diverse B cells from immature spleen and bone marrow precursors

The capacity of immature B cells of the spleen and bone marrow to differentiate in vitro into cells representing mature end stage cells was investigated using B-cell activating factor belonging to the TNF family (BAFF) and Notch pathway activators. Immature splenic and bone marrow B cells were found, in the presence of both of these activators, to mature into cells with follicular mature (FM) and marginal zone (MZ) cell phenotypes. Such cells were functionally responsive to B-cell-specific activation. The derivation in vitro of cells with an MZ phenotype was more robust from CD23(-) populations than CD23(+) immature/transitional B cells, suggesting a direct immature/T1 B cell to MZ cell differentiation pathway. Transcript analysis of the in vitro-derived B-cell populations demonstrated expression profiles similar to maturing B cells in vivo. FACS-purified populations of B220(+)CD19(+)CD21(-)CD23(-) cells from bone marrow of 2-wk-old mice gave rise to populations of CD21(+)CD23(-) cells with MZ cell phenotypes as well as CD21(+)CD23(+) cells with FM cell phenotypes in percentages similar to those found in vivo. These data suggest that the commitment to an MZ and FM B cell phenotype is set prior to immature B-cell release from the marrow.

BLyS and APRIL expression in peripheral blood mononuclear cells of cryptococcal meningitis patients and their clinical significance

OBJECTIVE

To investigate the levels of APRIL, BlyS and receptors as TACI, BCMA and BAFF-R in peripheral blood mononuclear cells (PBMC) of cryptococcal meningitis (CM) patients and its clinical significance.

METHODS

PBMC from 30 CM patients and 32 healthy controls were isolated. The mRNA levels of APRIL, BLyS and BLyS receptors were detected by fluorescent quantitation PCR. The effect of PBMC from CM patients on in vitro growth of Cryptococcus neoformans was compared in presence and absence of BLyS.

RESULTS

PBMC of CM patients exhibited significantly lower BLyS, TACI and BCMA mRNA levels but significantly higher BAFF-R mRNA levels than controls. Growth of C. neoformans was significantly slower in presence of BLyS than its absence.

CONCLUSION

Levels of BlyS and its receptors correlated with cryptococcal meningitis progression, and provide new clues for monitoring CM conditions and its effective therapy.

Transmembrane BAFF from rheumatoid synoviocytes requires interleukin-6 to induce the expression of recombination-activating gene in B lymphocytes

OBJECTIVE

B cells that accumulate in the synovial tissue of rheumatoid arthritis (RA) patients revise their receptors due to coordinate expression of recombination-activating gene 1 (RAG-1) and RAG-2 genes. The aim of this study was to determine the mechanisms that control this re-expression.

METHODS

B cells from healthy control subjects were cocultured with fibroblast-like synoviocytes (FLS) from patients with RA and osteoarthritis (OA). Re-expression of RAG messenger RNA (mRNA) and proteins was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and indirect immunofluorescence. Activity of RAG enzymes was evaluated by flow cytometry to measure variations in immunoglobulin kappa and lambda light chain expression and by ligation-mediated-PCR to assess specific DNA breaks. Blocking antibodies, short hairpin RNA, and recombinant cytokine were used to identify the molecules involved in RAG re-expression.

RESULTS

RA FLS, but not OA FLS, induced B cells to re-express RAG mRNA and proteins. Enzymes were functional, since the kappa-to-lambda ratios decreased and specific DNA breaks were detectable after coculture with RA FLS. Transmembrane BAFF provided the first signal of RAG re-expression, since its down-regulation in RA FLS prevented RAG gene transcription in B cells. The failure of transmembrane BAFF from OA FLS to induce RAG suggests that a second signal was provided by RA FLS. Interleukin-6 (IL-6) is a candidate, since blockade of its receptors precluded transcription of RAG genes by RA FLS. Unless supplemented with IL-6, OA FLS were unable to induce RAG gene expression in normal B cells.

CONCLUSION

Two independent signals are required for the induction of RAG gene expression in B cells that infiltrate the synovium of patients with RA.

Identification of BLyS (B lymphocyte stimulator), a non-myelin-associated protein, as a functional ligand for Nogo-66 receptor

B lymphocyte stimulator (BLyS), a tumor necrosis factor family protein essential for B cell development, was previously shown to be expressed at an elevated level in the CNS of multiple sclerosis patients. Although it may be involved in CNS diseases, its exact functions in CNS remain unknown. We hypothesize that BLyS may be a negative regulator for neuronal functions. Here Nogo-66 receptor (NgR) is identified as a high affinity receptor for BLyS, which inhibits dorsal root ganglion outgrowth in culture. The inhibition by BLyS can be reversed by a truncated NgR or by removal of glycosylphosphatidylinositol-linked proteins from neurons. More importantly, the inhibitory effect by BLyS is significantly diminished for neurons isolated from NgR(-/-) mice. Furthermore, expressions of BLyS and NgR are also found to be associated with astrocytes and macrophages/microglial cells at spinal cord injury sites. Thus, BLyS can function independently of myelin-associated inhibitors and likely serves as a redundant NgR ligand that negatively influences axonal outgrowth in CNS.

Reduced c-myc expression levels limit follicular mature B cell cycling in response to TLR signals

The splenic B cell compartment is comprised of two major, functionally distinct, mature B cell subsets, i.e., follicular mature (FM) and marginal zone (MZ) B cells. Whereas MZ B cells exhibit a robust proliferative response following stimulation with the TLR4 ligand LPS, FM B cells display markedly delayed and reduced levels of proliferation to the identical stimulus. The current study was designed to identify a potential mechanism(s) accounting for this differential responsiveness. In contrast to the delay in cell cycle entry, FM and MZ B cells exhibited nearly identical LPS-driven alterations in the expression level of cell surface activation markers. Furthermore, both the NF-kappaB and mTOR signaling cascades were similarly activated by LPS stimulation in FM vs MZ B cells, while inducible activation of ERK and AKT were nearly absent in both subsets. MZ B cells, however, exhibited higher basal levels of phospho-AKT and pS6, consistent with a preactivated status. Importantly, both basal and LPS activation-induced c-myc expression was markedly reduced in FM vs MZ B cells and enforced c-myc expression fully restored the defective proliferative response in FM B cells. These data support a model wherein TLR responses in FM B cells are tightly regulated by limiting c-myc levels, thereby providing an important checkpoint to control nonspecific FM B cell activation in the absence of cognate Ag.

Lunatic and manic fringe cooperatively enhance marginal zone B cell precursor competition for delta-like 1 in splenic endothelial niches

Notch2 activation induced by Delta-like-1 (DL1) drives development of splenic marginal zone (MZ) B cells, an innate-like lineage that protects against sepsis. DL1 interacts with Notch2 weakly, but it is not known whether enhancement of DL1-induced Notch2 activation by Fringe glycosyltransferases is important for MZ B cell development. Furthermore, DL1-expressing cells that promote MZ B cell development have not been identified. We show that Lunatic Fringe (Lfng) and Manic Fringe (Mfng) cooperatively enhanced the DL1-Notch2 interaction to promote MZ B cell development. We also identified radio-resistant red pulp endothelial cells in the splenic MZ that express high amounts of DL1 and promoted MZ B generation. Finally, MZ B cell precursor competition for DL1 homeostatically regulated entry into the MZ B cell pool. Our study has revealed that the Fringe-Notch2 interaction has important functions in vivo and provides insights into mechanisms regulating MZ B cell development.

In vivo BLyS/BAFF neutralization ameliorates islet-directed autoimmunity in nonobese diabetic mice

B lymphocytes are required for the pathogenesis of autoimmune diabetes in NOD mice. Previous studies established that a lymphopenic transitional (TR) B cell compartment reduces the competitive constraint on the entry of newly emerging TR B cells into the splenic follicle (FO), thereby disrupting a peripheral negative selection checkpoint in NOD mice. Thus, development of clinically feasible immunotherapeutic approaches for restoration of appropriate negative selection is essential for the prevention of anti-islet autoimmunity. In this study we hypothesized that in vivo neutralization of the B lymphocyte stimulator (BLyS/BAFF) may enhance the stringency of TR-->FO selection by increasing TR B cell competition for follicular entry in NOD mice. This study demonstrated that in vivo BLyS neutralization therapy leads to the depletion of follicular and marginal zone B lymphocytes. Long-term in vivo BLyS neutralization caused an increased TR:FO B cell ratio in the periphery indicating a relative resistance to follicular entry. Moreover, in vivo BLyS neutralization: 1) restored negative selection at the TR-->FO checkpoint, 2) abrogated serum insulin autoantibodies, 3) reduced the severity of islet inflammation, 4) significantly reduced the incidence of spontaneous diabetes, 5) arrested the terminal stages of islet cell destruction, and 6) disrupted CD4 T cell activation in NOD mice. Overall, this study demonstrates the efficacy of B lymphocyte-directed therapy via in vivo BLyS neutralization for the prevention of autoimmune diabetes.

The BLyS family: toward a molecular understanding of B cell homeostasis

The B Lymphocyte Stimulator (BLyS) family of ligands and receptors regulates humoral immunity by controlling B lymphocyte survival and differentiation. Herein, we review the ligands and receptors of this family, their biological functions, and the biochemical processes through which they operate. Pre-immune B lymphocytes rely on BLyS signaling for their survival, whereas antigen experienced B lymphocytes generally interact more avidly with a homologous cytokine, A Proliferation Inducing Ligand (APRIL). The molecular basis for signaling via the three BLyS family receptors reveals complex interplay with other B lymphocyte signaling systems, affording the integration of selective and homeostatic processes. As our understanding of this system advances, molecular targets for manipulating humoral immunity in both health and disease should be revealed.

Manipulating B cell homeostasis: a key component in the advancement of targeted strategies

Understanding the homeostatic mechanisms governing lymphocyte pools achieves critical importance as lymphocyte-targeted therapies expand in use and scope. The primacy of B lymphocyte stimulator (BLyS) family ligands and receptors in governing B lymphocyte homeostasis has become increasingly clear in recent years, affording insight into novel opportunities and potential pitfalls for targeted B cell therapeutics. Interclonal competition for BLyS-BR3 interactions determines the size of naïve B cell pools and can regulate the stringency of selection applied as cells complete maturation. Thus one of the predicted consequences of ablative therapies targeting primary pools is relaxed negative selection. This suggests that BLyS levels and B cell reconstitution rates may serve useful prognostic roles and that BLyS itself might be targeted to circumvent relapse. Alternatively, manipulations that allow rare, minimally autoreactive specificities to survive and mature may lead to opportunities in cases where antibody-based vaccine development has heretofore been unsuccessful. BLyS family ligands and receptors also play a role in activated and memory B cell pools, suggesting they might likewise be targeted to promote or delete particular antigen-experienced subpopulations in a similar way.

Characterization of a late transitional B cell population highly sensitive to BAFF-mediated homeostatic proliferation

We have characterized a distinct, late transitional B cell subset, CD21^int transitional 2 (T2) B cells. In contrast to early transitional B cells, CD21^int T2 B cells exhibit augmented responses to a range of potential microenvironmental stimuli. Adoptive transfer studies demonstrate that this subset is an immediate precursor of both follicular mature and marginal zone (MZ) B cells. In vivo, a large percentage of CD21^int T2 B cells has entered the cell cycle, and the cycling subpopulation exhibits further augmentation in mitogenic responses and B cell-activating factor of the TNF family (BAFF) receptor expression. Consistent with these features, CD21^int T2 cells exhibit preferential responses to BAFF-facilitated homeostatic signals in vivo. In addition, we demonstrate that M167 B cell receptor (BCR) idiotypic-specific B cells are first selected within the cycling CD21^int T2 population, ultimately leading to preferential enrichment of these cells within the MZ B cell compartment. These data, in association with the coordinate role for BAFF and microenvironmental cues in determining the mature BCR repertoire, imply that this subset functions as a unique selection point in peripheral B cell development.

Does the BAFF dysregulation play a major role in the pathogenesis of systemic lupus erythematosus?

Given the prominent role currently assigned to B lymphocytes in systemic lupus erythematosus, it is not surprising that the B cell activity factor belonging to the tumor necrosis factor family (BAFF) is involved in its pathogenesis. This cytokine is produced in excess, and inserted into its receptors on the surface of circulating B cells. Up-regulation of BAFF is most likely to lead to breach of tolerance by aberrant survival of B cells directed to the self. Trials aimed at blocking BAFF have thus been set out. Yet the results are awaited.

Cell proliferation and death: forgotten features of chronic lymphocytic leukemia B cells

Chronic lymphocytic leukemia (CLL) results from an accumulation of abnormal B cells due to an imbalance between birth and death rates such that the former exceeds the latter. This imbalance can occur as a result of increased birth, decreased death, or a combination of the two. CLL has long been considered a disease in which cell accumulation results from decreased death, due to a genetic defect, with minimal birth of the leukemic clone. This view was promulgated when experimental options were limited and observations in vivo and in vitro were less precise--e.g. CLL cells appeared as resting lymphocytes by light microscopy and responded poorly to mitogens (primarily T-cell mitogens)--at a time when T- and B-cell discrimination was not well appreciated. However, recent studies using more sophisticated measures suggest that the initial characterization of CLL biology needs re-evaluation. Using a safe, non-radioactive in-vivo labeling method that permits the determination of CLL-cell birth rates, we have directly documented that a small fraction of the clone (approximately 0.1-1.75%), i.e., between approximately 1x10(9) and 1x10(12) cells are born each day in all patients studied. With this value, we calculated death rates of between 0 and 1x10(12) per day of leukemic cells from individual patients. Thus the dynamic interplay between birth and death that characterizes other leukemias and lymphomas applies to CLL. Therefore, CLL is a disease of both proliferation and accumulation in which a homeostatic balance exists in patients with stable lymphocyte counts or an imbalance exists in patients with rising lymphocyte counts.

The BLyS/BAFF family of ligands and receptors: key targets in the therapy and understanding of autoimmunity

The B lymphocyte stimulator (BLyS; also termed BAFF) family of ligands and receptors plays a central role in B lymphocyte development, selection, and homoeostasis. Members of this family can independently influence different B cell subsets, because the interactions between the two ligands and three receptors vary, and the receptors themselves are differentially expressed among developing, naive, and antigen experienced B cell subsets. These properties prompt careful assessment of how ablative therapies may influence the behaviour of upstream or downstream B lineage populations, as well as how the implementation and expectations of therapeutics targeting BLyS family members must be guided by knowledge of the B cell subsets contributing to pathogenesis.

B cells and aging: balancing the homeostatic equation

The interplay of selective and homeostatic processes dominates the behavior of B lineage subsets following B cell antigen receptor (BCR) expression, and extends to determinants of immune response quality and the persistence of immunologic memory. A key concept emerging from these considerations is that primary events acting upstream of mature B lymphocyte pools can profoundly impact downstream populations as the system attempts homeostatic adjustments. Since, advancing age is accompanied by profound changes in B cell generation and homeostasis, establishing the relative contributions of primary lesions versus compensatory homeostatic processes is critical to understanding these perturbations. Exploration of this problem requires an understanding of: (1) the identity, dynamics, and progenitor/successor relationships of marrow and peripheral B cell subsets; (2) the nature and interactions of selective and homeostatic processes acting in these subsets; (3) how these change with age. Our data show that BLyS and its receptors mediate peripheral B cell homeostasis, and that the size, dynamics and behavior of all B cell subsets influenced by B Lymphocyte Stimulator change with age. These findings suggest that homeostatic processes mediated through B Lymphocyte Stimulator are altered with age, and that these perturbations may primarily reflect compensatory homeostatic adjustments to upstream reductions in B cell generation.

Targeting BAFF: Immunomodulation for autoimmune diseases and lymphomas

In an effort to develop more effective treatments for inflammatory diseases, immunologists have targeted numerous molecular pathways, but with limited success. Notable exceptions are anti-TNF agents, which have proved efficacious in a proportion of rheumatoid arthritis (RA) patients. Another TNF family member, termed BAFF ("B cell-activating factor belonging to the TNF family"), plays a central role in autoimmune diseases, as well as in B cell maturation, survival, and T cell activation. Agents that block BAFF have proven to be highly effective in the treatment of certain autoimmune conditions in mice. In addition, phase II data in human clinical trials for RA appear very promising. BAFF is also a survival factor for certain B cell lymphomas. Despite the relatively recent identification of BAFF, this molecule has provided considerable new insight into B cell homeostasis and immune function, and represents an important new molecular target for treatment of autoimmune diseases and lymphomas.

Analysis of the regulatory role of BAFF in controlling the expression of CD21 and CD23

The TNF family member BAFF serves to promote the survival and differentiation of maturing splenic B cells. The major receptor for BAFF (BAFF-R) is expressed by the transition 2, marginal zone and follicular, mature conventional B-2 cell populations; functional BAFF/BAFF-R signaling is required for T1 to T2 cell B cell maturation. Induced expression of CD23 and CD21 is also coincident with the T1 to T2 maturation stage. A key question we address in this report is if BAFF signaling directly induces CD21 and CD23 gene transcription and expression at this B cell transition point, or if their expression is simply coincident with B cell maturation and differentiation. We present data that supports the contention that BAFF does not preferentially induce the expression of CD23 or CD21 at the T1 to T2 transition, nor does exogenous BAFF lead to preferential increased expression of these proteins/genes in mature B cell populations. The analysis of LPS-induced splenic B cells from BAFF-R defective (A/WySnJ) mice did not show the preferential induction of expression of CD21 or CD23 that might have been expected if NF-kappaB-p52 protein was lacking due to insufficient BAFF-R signaling in cells bearing this mutation. Indeed, chromatin immunoprecipitation analysis demonstrated stable NF-kappaB-p52 complexes on CD21 and CD23 genes obtained from both wild type and A/WySnJ B cells. FACS analysis of splenic B cells from 1-, 2-, 3- and 6-week-old A/WySnJ mice demonstrated a block in differentiation (thus reducing overall B cell numbers) resulting in a failure of such cells to express CD21 but allowing for the expression level of CD23 per cell to reach levels approaching wild type. We have dubbed this CD23(HI)CD21(LO) subset as the T1b transition B cell. These data support the recognized role of BAFF as promoting the survival and differentiation of splenic B cells but do not support a model of BAFF signaling directly inducing the expression of the CD21 and CD23 proteins via translocation of NF-kappaB-p52 species.

Analysis of the Individual Contributions of Igα (CD79a)- and Igβ (CD79b)-Mediated Tonic Signaling for Bone Marrow B Cell Development and Peripheral B Cell Maturation

The individual contribution of Igalpha and Igbeta for BCR-triggered fates is unclear. Prior evidence supports conflicting ideas concerning unique as well as redundant functions for these proteins in the context of BCR/pre-BCR signaling. Part of this ambiguity may reflect the recent appreciation that Igalpha and Igbeta participate in both Ag-independent (tonic) and Ag-dependent signaling. The present study undertook defining the individual requirement for Igalpha and Igbeta under conditions where only ligand-independent tonic signaling was operative. In this regard, we have constructed chimeric proteins containing one or two copies of the cytoplasmic domains of either Igalpha or Igbeta and Igalpha/Igbeta heterodimers with targeted Tyr-->Phe modifications. The ability of these proteins to act as surrogate receptors and trigger early bone marrow and peripheral B cell maturation was tested in RAG2(-/-) primary pro-B cell lines and in gene transfer experiments in the muMT mouse model. We considered that the threshold for a functional activity mediated by the pre-BCR/BCR might only be reached when two functional copies of the Igalpha/Igbeta ITAM domain are expressed together, and therefore the specificity conferred by these proteins can only be observed in these conditions. We found that the ligand-independent tonic signal is sufficient to drive development into mature follicular B cells and both Igalpha and Igbeta chains supported formation of this population. In contrast, neither marginal zone nor B1 mature B cell subsets develop from bone marrow precursors under conditions where only tonic signals are generated.

Aging murine B cells have decreased class switch induced by anti-CD40 or BAFF

We previously demonstrated that in vitro stimulated splenic B cells from senescent mice are deficient in production of multiple class switch isotypes, class switch recombination (CSR), induction of the E2A-encoded transcription factor E47, and activation-induced cytidine deaminase (AID) which is necessary for CSR and somatic hypermutation. Both anti-CD40 as well as BAFF have been shown to be able to induce CSR. We have investigated the ability of BAFF/IL-4, as compared to anti-CD40/IL-4, to induce CSR to gamma(1) in splenic B cells from young and old mice. We found that anti-CD40/IL-4 is a better CSR stimulus than BAFF/IL-4 in young B cells, as measured by RT-PCR of post-switch transcripts and flow cytometry. CSR is reduced in old B cells and this is independent of the stimulus. AID and gamma(1)PSTs are significantly reduced in old B cells stimulated with anti-CD40/IL-4, but only slightly reduced with BAFF/IL-4. BAFF receptor mRNA expression (BAFF-R, TACI, and BCMA) is not affected by aging. The age-related decrease in CSR induced by anti-CD40/IL-4 is primarily associated with a decrease in E47, whereas the less affected response to BAFF/IL-4 is associated with decreases in both E47 and NF-kappaB. Therefore, NF-kappaB is not involved in the decreased response of old B cells to anti-CD40/IL-4. These differences in B cell responses to CD40/IL-4 and BAFF/IL-4 may help to explain the maintenance of TI vs TD responses in senescent mice.

BLyS receptor signatures resolve homeostatically independent compartments among naïve and antigen-experienced B cells

The BLyS family of receptors includes two cytokines, BLyS and APRIL; and three receptors, BR3, BCMA and TACI. Together, these regulate the size and composition of peripheral B cell pools. The multiplicity of ligand-receptor sets, in conjunction with differential receptor expression, alternative binding partners and disparate downstream signaling characteristics, affords the potential to establish independently regulated homeostatic niches among primary and antigen-experienced B cell subsets. Thus, BLyS signaling via BR3 is the dominant homeostatic regulator of primary B cell pools, whereas APRIL interactions with BCMA likely govern memory B cell populations. Short-lived antibody forming cell populations and their proliferating progenitors express a TACI-predominant signature. Further, within each niche, relative fitness to compete for available cytokine is determined by exogenous inputs via adaptive and innate receptor systems, affording intramural hierarchies that determine clonotype composition.

Space, selection, and surveillance: setting boundaries with BLyS

The BLyS family of ligands and receptors governs B cell homeostasis by controlling survival, differentiation, and lifespan. This family consists of multiple receptors and ligands, allowing independent regulation of different B cell subsets by varying the combination and levels of receptors expressed. Multiple downstream signaling pathways are implicated in these activities, reflecting this receptor complexity as well as cross-talk with other B cell signaling systems. BLyS levels are associated with multiple forms of humoral autoimmunity and can modulate tolerogenic elimination at the transitional checkpoint. BLyS responsiveness thus balances peripheral selection against cell numbers, providing an elastic system that varies selective stringency based on homeostatic demands.

Expression of the adaptor protein hematopoietic Src homology 2 is up-regulated in response to stimuli that promote survival and differentiation of B cells

Analysis of hematopoietic Src homology 2 (HSH2) protein expression in mouse immune cells demonstrated that it is expressed at low levels in resting B cells but not T cells or macrophages. However, HSH2 expression is up-regulated within 6-12 h in response to multiple stimuli that promote activation, differentiation, and survival of splenic B cells. HSH2 expression is increased in response to anti-CD40 mAb, the TLR ligands LPS and CpG DNA, and B lymphocyte stimulator (BLyS), a key regulator of peripheral B cell survival and homeostasis. Stimulation of B cells with anti-CD40 mAb, LPS, CpG DNA, or BLyS has previously been shown to induce activation of NF-kappaB. In agreement with this finding, up-regulation of HSH2 expression in response to these stimuli is blocked by inhibitors of NF-kappaB activation and is potentiated by stimulation with PMA, suggesting that HSH2 expression is dependent on NF-kappaB activation. In contrast to CD40, BAFF receptor, TLR4, and TLR9 mediated signaling, stimulation of splenic B cells via the BCR was not observed to induce expression of HSH2 unless the cells had been stimulated previously through CD40. Finally, HSH2 expression is down-regulated in splenic B cells in response to stimulation with IL-21, which has been shown to induce apoptosis, even in the presence of anti-CD40 mAb, LPS, or CpG DNA. IL-21 stimulation also results in down-regulation of antiapoptotic proteins such as Bcl-x(L) and up-regulation of proapoptotic proteins like Bim. Therefore, HSH2 expression is coordinately up-regulated with known antiapoptotic molecules and directly correlates with B cell survival.

Similarities and differences between selective and nonselective BAFF blockade in murine SLE

B cells have multiple roles in immune activation and inflammation separate from their capacity to produce antibodies. B cell depletion is currently under intense investigation as a therapeutic strategy for autoimmune diseases. The TNF family members B cell-activating factor of the TNF family (BAFF) and its homolog A proliferation-inducing ligand (APRIL) are B cell survival and differentiation factors and are therefore rational therapeutic targets. We compared the effects of BAFF receptor-Ig, which blocks only BAFF, with those of transmembrane activator and calcium modulator ligand interactor-Ig, which blocks both BAFF and APRIL, in a murine SLE model. Both reagents prolonged the life of NZB/W F1 mice when given either before or after disease onset. Many immunologic effects of the 2 reagents were similar, including B cell and B cell subset depletion and prevention of the progressive T cell activation and dendritic cell accumulation that occurs with age in NZB/W mice without substantial effects on the emergence of the IgG anti-double-stranded DNA response. Furthermore, both reagents inhibited the T cell-independent marginal zone B cell response to particulate antigen delivered i.v., but not the B1 B cell response to the same antigen delivered i.p. In contrast, blockade of both BAFF and APRIL, but not blockade of BAFF alone, reduced the serum levels of IgM antibodies, decreased the frequency of plasma cells in the spleen, and inhibited the IgM response to a T cell-dependent antigen. The differences between selective and nonselective BAFF blockade are relevant to the choice of a BAFF blocking agent for the treatment of autoimmune and malignant diseases.

BAFF regulates B cell survival by downregulating the BH3-only family member Bim via the ERK pathway

The B cell activating factor belonging to the tumor necrosis factor family (BAFF) is required for B cell survival and maturation. The mechanisms by which BAFF mediates B cell survival are less understood. We found that BAFF and a proliferation-inducing ligand (APRIL), which are related, block B cell antigen receptor (BCR)–induced apoptosis upstream of mitochondrial damage, which is consistent with a role for Bcl-2 family proteins. BCR ligation strongly increased expression of the proapoptotic Bcl-2 homology 3–only Bcl-2 protein Bim in both WEHI-231 and splenic B cells, and increases in Bim were reversed by BAFF or APRIL. Small interfering RNA vector–mediated suppression of Bim blocked BCR-induced apoptosis. BAFF also induced Bim phosphorylation and inhibited BCR-induced association of Bim with Bcl-2. BAFF induced delayed but sustained stimulation of extracellular signal–regulated kinase (ERK) and its activators, mitogen-activated protein kinase/ERK activating kinase (MEK) and c-Raf, and MEK inhibitors promoted accumulation and dephosphorylation of Bim. These results suggest that BAFF inhibits BCR-induced death by down-regulating Bim via sustained ERK activation, demonstrating that BAFF directly regulates Bim function. Although transitional immature type 1 (T1) B cell numbers are normal in Bim^−/− mice, T2 and follicular mature B cells are elevated and marginal zone B cells are reduced. Our results suggest that mature B cell homeostasis is maintained by BAFF-mediated regulation of Bim.

FcRY, an Fc receptor related gene differentially expressed during B lymphocyte development and activation

A bioinformatics approach has lead to the identification of FcRY, a new Fc receptor related gene. FcRY is predicted to encode a protein with three immunoglobulin (Ig) domains followed by a mucin-like domain containing a proline-rich stalk and a C-terminal leucine rich region. The predicted protein lacks a hydrophobic domain for insertion into the plasma membrane, suggesting that FcRY is an intracellular or secreted protein. This feature is shared with the product of the FcRX/FCRL/FREB gene that is closely linked to FcRY in both human and mouse genomes. Fcry transcripts are first detectable among mouse B lineage cells at the pre-B cell stage. Splenic B cells of the newly formed, follicular, and marginal zone subsets express Fcry, as do germinal center B cells to a lesser extent. FcRY is also expressed in subpopulations of human B cells. A consistent characteristic of FcRY in both species is low level gene expression, which can be further downregulated in normal mouse B cells by signaling through the B cell receptor (BCR) or CD40, thereby suggesting a correlation between cell cycle entrance and diminished FcRY expression. Fcry is upregulated by short-term treatment with BAFF/BLyS, which promotes B cell survival rather than proliferation. LPS induces very rapid but transient enhancement. We observed a pronounced upregulation of Fcry expression in WEHI 231 cells induced by BCR crosslinking to undergo cell cycle arrest prior to apoptosis, consistent with the possible regulation of Fcry expression by cell cycle status.

Dwindling competition with constant demand: Can homeostatic adjustments explain age-associated changes in peripheral B cell selection?

The close relationship between specificity-based selection and homeostatic processes in maintaining peripheral B cell pools has become increasingly evident. Thus, age-associated changes observed within these pools may reflect homeostatic responses to proximal primary lesions. Marked shifts in the size and dynamics of most B lymphocyte subsets and their progenitors occur with age: perturbations in B lineage precursors result in reduced production of immature B lymphocytes in the bone marrow and transitional pools in the periphery, but these effects appear to be offset by compensatory homeostatic processes at the marrow-periphery interface. We propose a model whereby these "distal" homeostatic adjustments relax the stringency of specificity based selection, affording a potential explanation for the increased frequency of autoreactive specificities with age.