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

Origin and differentiation trajectories of fibroblastic reticular cells in the splenic white pulp

The splenic white pulp is underpinned by poorly characterized stromal cells that demarcate distinct immune cell microenvironments. Here we establish fibroblastic reticular cell (FRC)-specific fate-mapping in mice to define their embryonic origin and differentiation trajectories. Our data show that all reticular cell subsets descend from multipotent progenitors emerging at embryonic day 19.5 from periarterial progenitors. Commitment of FRC progenitors is concluded during the first week of postnatal life through occupation of niches along developing central arterioles. Single cell transcriptomic analysis facilitated deconvolution of FRC differentiation trajectories and indicated that perivascular reticular cells function both as adult lymphoid organizer cells and mural cell progenitors. The lymphotoxin-β receptor-independent sustenance of postnatal progenitor stemness unveils that systemic immune surveillance in the splenic white pulp is governed through subset specification of reticular cells from a multipotent periarterial progenitor cell. In sum, the finding that discrete signaling events in perivascular niches determine the differentiation trajectories of reticular cell networks explains the development of distinct microenvironmental niches in secondary and tertiary lymphoid tissues that are crucial for the induction and regulation of innate and adaptive immune processes.

The white pulp of spleen is an important immune structure dynamically modulated during development and immune responses. Here the authors define, using multi-color lineage tracing and single-cell transcriptome analysis, the subset distribution and differentiation trajectory of fibroblastic reticular cells to serve structural insights for splenic white pulps.

Inflammatory Markers and Immune Response to Pneumococcal Vaccination in HIV-Positive and -Negative Adults

Background

Members of the Tumor Necrosis Factor (TNF)-superfamily have speculated roles in the response against T-independent type II antigens (TI-II) including pneumococcal polysaccharides (PPS). Dysregulation in their expression is associated with an enhanced risk for pneumococcal disease in neonates but their expression in other high-risk populations including HIV-positive individuals remains to be elucidated.

Objective

To investigate signals that contribute towards PPS-response and identify potential anomalies that may account for diminished serological response in HIV-positive individuals post Pneumovax (PPV23) immunization.

Methods

Markers of inflammation, C-reactive protein (CRP), IL-6, sCD27 and sCD30, were assessed in HIV-positive and -negative individuals as potential predictors of PPV23 response. Serum levels of B cell activating factor (BAFF), transmembrane activator and calcium-modulator and cytophilin ligand interactor (TACI), B cell maturation antigen (BCMA) and B cell expression of BAFF-R, TACI, BCMA, CD40 and CD21 were assessed in total (unselected) and PPS23F (antigen)-specific B cells of PPV23 immunized HIV-positive and -negative individuals.

Results

CRP, sCD27, sCD30 and BAFF were significantly elevated in the serum of HIV-positive individuals but did not adversely affect PPV23 response. Assessment of PPS-specific B cells revealed enhanced TACI and reduced BAFF-R expression compared to unselected B cells in HIV-positive and -negative individuals. Surface TACI was similar but soluble TACI was significantly lower in HIV-positive compared to HIV-negative individuals.

Conclusion

Current studies highlight a potential role for TACI in PPV23 response based on its enhanced expression on PPS-specific B cells. Although surface levels of TACI were similar, diminished soluble TACI (sTACI) in HIV-positive compared to HIV-negative individuals could potentially decrease BAFF responsiveness and Ig response. A better understanding of the role of TNF receptors could contribute to the design of improved pneumococcal vaccines.

Trial Registration

ClinicalTrials.gov NCT02515240

Reprint of: B cells in chronic graft-versus-host disease

Chronic graft-versus-host disease (cGVHD) continues to be a common complication of allogeneic hematopoietic stem cell transplantation. Unlike acute graft-versus-host disease, which is mediated almost entirely by donor T cells, the immune pathology of cGVHD is more complex and donor B cells have also been found to play an important role. Recent studies from several laboratories have enhanced our understanding of how donor B cells contribute to this clinical syndrome and this has led to new therapeutic opportunities. Here, Dr Sarantopoulos reviews some of the important mechanisms responsible for persistent B cell activation and loss of B cell tolerance in patients with cGVHD. Dr Blazar describes recent studies in preclinical models that have identified novel B cell-directed agents that may be effective for prevention or treatment of cGVHD. Some B cell-directed therapies have already been tested in patients with cGVHD and Dr Cutler reviews the results of these studies documenting the potential efficacy of this approach. Supported by mechanistic studies in patients and preclinical models, new B cell-directed therapies for cGVHD will now be evaluated in clinical trials.

BAFF receptor and TACI in B-1b cell maintenance and antibacterial responses

Although evidence of the protective immunity conferred by B-1b cells (CD19(+) B220(+) IgM(hi) Mac1(+) CD5(-)) has been established, the mechanisms governing the maintenance and activation of B-1b cells following pathogen encounter remain unclear. B cell-activating factor (BAFF) and a proliferation-inducing ligand (APRIL) mediate their function in mature B cells through the BAFF receptor (BAFFR) and transmembrane activator and CAML interactor (TACI). BAFFR-deficient mice have lower numbers of B-1b cells, and this reduction is directly proportional to BAFFR levels. The generation of B-1b cells is also dependent on the strength of B cell receptor (BCR) signaling. Mice with impaired BCR signaling, such as X-linked immunodeficient (xid) mice, have B-1b cell deficiency, indicating that both BCR- and BAFFR-mediated signaling are critical for B-1b cell homeostasis. Borrelia hermsii induces expansion and persistence of B-1b cells in xid mice, and these B-1b cells provide a heightened protective response. Toll-like receptor (TLR)-mediated stimulation of xid B cells results in a significant increase in TACI expression and restoration of TACI-mediated functions. The activation of TLR signaling by B. hermsii and BCR/TLR costimulation-mediated upregulation of BAFFR and TACI on B-1b cells suggests that B-1b cell maintenance and function following bacterial exposure may depend on BAFFR- and TACI-mediated signaling. In fact, the loss of both BAFFR and TACI results in a greater impairment in anti-B. hermsii responses compared to deficiency of BAFFR or TACI alone.

Aberrant B-cell homeostasis in chronic GVHD

Recent studies have compelled further interest in the potential pathological role of B cells in chronic graft-versus-host disease (cGVHD). In patients with cGVHD, B cells are activated and primed for survival via B-cell activating factor and B-cell receptor-associated pathways. Understanding the signaling pathways that drive immune pathology in cGVHD will facilitate the development of new strategies to selectively target aberrantly activated B cells and restore normal B-cell homeostasis after allogeneic stem cell transplantation.

B cells in chronic graft-versus-host disease

Chronic graft-versus-host disease (cGVHD) continues to be a common complication of allogeneic hematopoietic stem cell transplantation. Unlike acute graft-versus-host disease, which is mediated almost entirely by donor T cells, the immune pathology of cGVHD is more complex and donor B cells have also been found to play an important role. Recent studies from several laboratories have enhanced our understanding of how donor B cells contribute to this clinical syndrome and this has led to new therapeutic opportunities. Here, Dr Sarantopoulos reviews some of the important mechanisms responsible for persistent B cell activation and loss of B cell tolerance in patients with cGVHD. Dr Blazar describes recent studies in preclinical models that have identified novel B cell-directed agents that may be effective for prevention or treatment of cGVHD. Some B cell-directed therapies have already been tested in patients with cGVHD and Dr Cutler reviews the results of these studies documenting the potential efficacy of this approach. Supported by mechanistic studies in patients and preclinical models, new B cell-directed therapies for cGVHD will now be evaluated in clinical trials.

Spinal cord injury impacts B cell production, homeostasis, and activation

Complex interactions govern the interplay of central nervous and immune systems, including the generation, homeostatic maintenance, and activation of B cells. Accordingly, spinal cord injury will likely impact all of these processes. Several laboratories have recently explored this possibility, and their observations in aggregate reveal both acute and chronic consequences that can vary based on the injury location. Acute effects include a transient cessation of bone marrow B lymphopoiesis, with a corresponding drop in the peripheral follicular and transitional B cell subsets, whereas the marginal zone subset is preserved. Despite recovery of B lymphopoiesis by 28 days post injury, follicular B cell numbers remain depressed; this may reflect reduced levels of the homeostatic cytokine BLyS. In general, the ability to mount T dependent antibody responses after injury are intact, as are pre-existing memory B cell pools and antibody levels. In contrast, T-independent responses are chronically compromised. Both glucocorticoid-dependent and -independent processes mediate these effects, but a detailed understanding of the mechanisms involved awaits further study. Nonetheless, these observations in toto strengthen the growing appreciation for bidirectional interactions between the CNS and immune system, highlighting the need for further basic and translational efforts.

Regulatory roles of the tumor necrosis factor receptor BCMA

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

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.

Molecular structure, expression pattern and functional characterisation of APRIL in an aquatic mammal

The Yangtze finless porpoise (Neophocaena phocaenoides asiaorientalis) is listed on the First Order of Protected Animals in China and was identified as an endangered species by the International Union for Conservation of Nature and Natural Resources (IUCN) in 2011. A proliferation inducing ligand (APRIL), belonging to the tumour necrosis factor (TNF) family, is critical for immune regulation. In this study, we identified a finless porpoise APRIL cDNA (fAPRIL) by RACE (rapid amplification of cDNA ends) strategies, from the Yangtze finless porpoise (fAPRIL). This gene encodes 247 amino acids containing a predicted transmembrane domain and a TNF domain, and phylogenetic analysis of the APRIL sequence indicated that finless porpoises are closely related to Artiodactyla. In vitro, soluble fAPRIL (fsAPRIL) not only promoted the survival/proliferation of the mouse spleen lymphocytes, but also bound specifically to the surface of the B cells. The results of this study contribute to our understanding of the immune mechanisms in the finless porpoise and other aquatic mammals.

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.

Evolution of vertebrate immunity

All multicellular organisms protect themselves against pathogens using sophisticated immune defenses. Functionally interconnected humoral and cellular facilities maintain immune homeostasis in the absence of overt infection and regulate the initiation and termination of immune responses directed against pathogens. Immune responses of invertebrates, such as flies, are innate and usually stereotyped; those of vertebrates, encompassing species as diverse as jawless fish and humans, are additionally adaptive, enabling more rapid and efficient immune reactivity upon repeated encounters with a pathogen. Many of the attributes historically defining innate and adaptive immunity are in fact common to both, blurring their functional distinction and emphasizing shared ancestry and co-evolution. These findings provide indications of the evolutionary forces underlying the origin of somatic diversification of antigen receptors and contribute to our understanding of the complex phenotypes of human immune disorders. Moreover, informed by phylogenetic considerations and inspired by improved knowledge of functional networks, new avenues emerge for innovative therapeutic strategies.

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.

Beyond transitional selection: New roles for BLyS in peripheral tolerance

B cell targeted therapies have enjoyed recent success in the treatment of systemic autoimmune diseases. Among these, Belimumab, which blocks the B cell survival cytokine BLyS, was recently approved for the treatment of Systemic Lupus Erythematosus. It is therefore important to consider the roles BLyS plays in B cell tolerance. Herein, we review how BLyS contributes to the negative selection of autoreactive B cell clones from the preimmune repertoire as well as its role in regulating both germinal center and extrafollicular peripheral B cell responses. We further examine the complex role of Toll-like receptors (TLRs) in humoral autoimmunity, pointing out potential crosstalk between BLyS and TLR pathways.

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.

The immortality of humoral immunity

Decades of high-titered antibody are sustained due to the persistence of memory B cells and long-lived plasma cells (PCs). The differentiation of each of these subsets is antigen- and T-cell driven and is dependent on signals acquired and integrated during the germinal center response. Inherent in the primary immune response must be the delivery of signals to B cells to create these populations, which have virtual immortality. Differences in biology and chemotactic behavior disperse memory B cells and long-lived PCs to a spectrum of anatomic sites. Each subset must rely on survival factors that can support their longevity. This review focuses on the generation of each of these subsets, their survival, and renewal, which must occur to sustain serological memory. In this context, we discuss the role of antigen, bystander inflammation, and cellular niches. The contribution of BAFF (B-cell activating factor belonging to the tumor necrosis factor family) and APRIL (a proliferation-inducing ligand) to the persistence of memory B cells and PCs are also detailed. Insights that have been provided over the past few years in the regulation of long-lived B-cell responses will have profound impact on vaccine development, the treatment of pre-sensitized patients for organ transplantation, and therapeutic interventions in both antibody- and T-cell-mediated autoimmunity.

Effect of long-term belimumab treatment on B cells in systemic lupus erythematosus: extension of a phase II, double-blind, placebo-controlled, dose-ranging study

OBJECTIVE

To understand the effects of long-term BLyS inhibition in human systemic lupus erythematosus (SLE).

METHODS

Seventeen patients with SLE who were enrolled in a clinical trial of belimumab, a BLyS-specific inhibitor, plus standard of care therapy were studied. Phenotypic analysis of lymphocytes was performed using flow cytometry. Circulating antibody-secreting cells were enumerated using enzyme-linked immunospot assay. Serum was analyzed by enzyme-linked immunosorbent assay using an antibody that recognizes products of the V(H)4-34 gene. Lymphocyte counts, Ig levels, and anti-double-stranded DNA antibody levels were available as part of the clinical trial analyses.

RESULTS

Samples were collected on days 0, 84, 168, 365, and 532 and after day 730. The total number of B cells started to decrease from baseline between days 84 and 168. This was due to a decrease in naive and transitional B cells. CD27+IgD+ memory B cells and plasmablasts decreased only after 532 days, whereas CD27+IgD- memory B cells were not affected, and there were no changes in T cells. Serum IgM levels began to decline between days 84 and 168, but there were no changes in serum levels of IgG, IgG anti-DNA antibodies, or V(H)4-34 antibodies during the study. SLE patients had more IgM-, IgG-, and autoantibody-producing B cells than did normal controls on day 0. There was only a modest decrease in the frequency of total IgM-producing, but not IgG-producing, cells on days 365 and 532, consistent with the phenotypic and serologic data.

CONCLUSION

Our data confirm the dependence of newly formed B cells on BLyS for survival in humans. In contrast, memory B cells and plasma cells are less susceptible to selective BLyS inhibition.

BAFF overexpression promotes anti-dsDNA B-cell maturation and antibody secretion

Overexpression of BAFF is believed to play an important role in systemic lupus erythematosus and elevated levels of serum BAFF have been found in lupus patients. Excess BAFF also leads to overproduction of anti-dsDNA antibodies and a lupus-like syndrome in mice. In the present study, we use mice transgenic for the R4A-Cmu (IgM) heavy chain of an anti-dsDNA antibody, to study the effects of BAFF overexpression on anti-dsDNA B-cell regulation. We observe that overexpression of BAFF promotes anti-dsDNA B-cell maturation and secretion of antibody and enriches for transgenic anti-dsDNA B cells in the marginal zone and follicular splenic compartments. In addition, our data suggests that BAFF rescues a subset of anti-dsDNA B cells from a regulatory checkpoint in the transitional stage of development.

Gene expression changes in the mesenteric lymph nodes of rats after oral peanut extract exposure

New techniques are needed to broaden the understanding of the food allergic response. The capacity of peanut extract to influence gene expression profiles was investigated in a Brown Norway rat model for food allergy. Brown Norway rats were sensitized to peanut extract (0, 1 and 10 mg/rat/d) by daily oral gavage and were dissected after 3, 7 or 14 days of exposure. RNA extracted from mesenteric lymph nodes of individual rats were hybridized against a common reference pool on Agilent whole rat genome (4*44k) arrays. The raw data were normalized and statistically analyzed using the statistical program R. A False Discovery Rate of 10% and a Fold Ratio of - 1.5 < or = Fold Ratio or Fold Ratio > or = 1.5 between the experimental groups and their respective control groups were applied. Differentially expressed genes were clustered into a heatmap. Functional annotation and GeneOntology term enrichment were examined. Furthermore, the involvement of the differentially expressed genes in specific cellular pathways was investigated with MetaCore. Gene expression changes, which were both dose- and time-dependent, were detected after sensitization to peanut. A total of 64 genes were differentially expressed, of which 60 were up-regulated and four were down-regulated. These changes were related to the regulation of immunological processes, most notably increased cell division. The findings indicate that responses to peanut include proliferation of immunologically relevant tissues, which can be identified by analysis of gene expression profiles. This may lay a basis for further research into possibilities for discrimination of allergenic from non-allergenic proteins.

Cell-autonomous role for NF-kappa B in immature bone marrow B cells

The NF-kappaB transcription factors have many essential functions in B cells, such as during differentiation and proliferation of Ag-challenged mature B cells, but also during final maturation of developing B cells in the spleen. Among the various specific functions NF-kappaB factors carry out in these biologic contexts, their ability to assure the survival of mature and maturing B cells in the periphery stands out. Less clear is what if any roles NF-kappaB factors play during earlier stages of B cell development in the bone marrow. Using mice deficient in both NF-kappaB1 and NF-kappaB2, which are thus partially compromised in both the classical and alternative activation pathways, we demonstrate a B cell-autonomous contribution of NF-kappaB to the survival of immature B cells in the bone marrow. NF-kappaB1 and NF-kappaB2 also play a role during the earlier transition from proB to late preB cells; however, in this context these factors do not act in a B cell-autonomous fashion. Although NF-kappaB1 and NF-kappaB2 are not absolutely required for survival and progression of immature B cells in the bone marrow, they nevertheless make a significant contribution that marks the beginning of the profound cell-autonomous control these factors exert during all subsequent stages of B cell development. Therefore, the lifelong dependency of B cells on NF-kappaB-mediated survival functions is set in motion at the time of first expression of a full BCR.

Homeostatic control of B lymphocyte subsets

Lymphocyte homeostasis poses a multi-faceted biological puzzle, because steady pre-immune populations must be maintained at an acceptable steady state to yield effective protection, despite stringent selective events during their generation. In addition, activated, memory and both short- and long-term effectors must be governed by independent homeostatic mechanisms. Finally, advancing age is accompanied by substantial changes that impact the dynamics and behavior of these pools, leading to cumulative homeostatic perturbations and compensation. Our laboratory has focused on the over-arching role of BLyS family ligands and receptors in these processes. These studies have led to a conceptual framework within which distinct homeostatic niches are specified by BLyS receptor signatures, which define the BLyS family ligands that can afford survival. The cues for establishing these receptor signatures, as well as the downstream survival mechanisms involved, are integrated with cell extrinsic inputs via cross talk among downstream mediators. A refined understanding of these relationships should yield insight into the selection and maintenance of B cell subsets, as well as an appreciation of how homeostatic mechanisms may contribute to immunosenescence.

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.

TACI, an enigmatic BAFF/APRIL receptor, with new unappreciated biochemical and biological properties

BAFF is a B cell survival factor that binds to three receptors BAFF-R, TACI and BCMA. BAFF-R is the receptor triggering naïve B cell survival and maturation while BCMA supports the survival of plasma cells in the bone marrow. Excessive BAFF production leads to autoimmunity, presumably as the consequence of inappropriate survival of self-reactive B cells. The function of TACI has been more elusive with TACI(-/-) mice revealing two sides of this receptor, a positive one driving T cell-independent immune responses and a negative one down-regulating B cell activation and expansion. Recent work has revealed that the regulation of TACI expression is intimately linked to the activation of innate receptors on B cells and that TACI signalling in response to multimeric BAFF and APRIL provides positive signals to plasmablasts. How TACI negatively regulates B cells remains elusive but may involve an indirect control of BAFF levels. The discovery of TACI mutations associated with common variable immunodeficiency (CVID) in humans not only reinforces its important role for humoral responses but also suggests a more complex role than first anticipated from knockout animals. TACI is emerging as an unusual TNF receptor-like molecule with a sophisticated mode of action.

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.

Signaling pathways for B cell-activating factor (BAFF) and a proliferation-inducing ligand (APRIL) in human placenta

The tumor necrosis superfamily (TNFSF) contains two soluble ligands that are involved in B lymphocyte development, BAFF (B cell activating factor, BlyS, TALL-1, CD257, TNFSF13B) and APRIL (a proliferation inducing ligand, CD256, TNFSF13). These two ligands signal through three receptors: the exclusive BAFF receptor (BAFF-R, CD268, TNFRSF17) and two receptors that recognize both BAFF and APRIL, TACI (transmembrane-activator-1 and calcium-modulator- and cyclophilin ligand-interactor CD267, TNFRSF13B) and BCMA (B cell maturation antigen, CD269, TNFRSF13C). All but BAFF-R are known to be synthesized in term placentas. In this study, expression of the ligands and receptors were distinguished in two embryologically discrete subpopulations of placental cells, villous cytotrophoblast (vCTB) cells and mesenchymal cells (MCs). Real-Time PCR showed that vCTB cells contain low levels of BAFF and APRIL transcripts whereas MCs contain high levels. Both Real-Time PCR and immunohistochemistry identified BAFF-R and BCMA mRNA and proteins in vCTB cells but essentially no TACI. By contrast, MCs contained readily detectable levels of all three receptors. These results illustrating potential autocrine and paracrine pathways for BAFF and APRIL signaling in human placentas suggest that lineage-specific regulation of placental cell viability, differentiation and/or other activities may be novel functions of these proteins.

The biology behind the new therapies for SLE

BACKGROUND

Systemic lupus erythematosus (SLE) is a heterogenous disease with complex pathogenesis.

AIM

In this review, we summarise recent progress in the understanding of SLE pathogenesis and discuss implications for the treatment of SLE patients using standard and experimental medications.

CONCLUSIONS

The discovery that Toll-like receptor signalling and interferon-alpha abundance are central elements of the disease process has led to a new appreciation for hydroxychloroquine as an essential baseline medication. Although much needs to be learned, modulation of the immune system via B-cell depletion is entering clinical practice. Mycophenolate mofetil is an effective and safer alternative to cyclophosphamide for many patients with lupus nephritis. Several other therapeutic approaches are at various stages of preclinical and clinical development. These include anticytokine therapies, co-stimulatory blockade, antigen-specific immune modulation and haematopoietic stem cell transplantation.

TACI, unlike BAFF-R, is solely activated by oligomeric BAFF and APRIL to support survival of activated B cells and plasmablasts

The cytokine BAFF binds to the receptors TACI, BCMA, and BAFF-R on B cells, whereas APRIL binds to TACI and BCMA only. The signaling properties of soluble trimeric BAFF (BAFF 3-mer) were compared with those of higher-order BAFF oligomers. All forms of BAFF bound BAFF-R and TACI, and elicited BAFF-R-dependent signals in primary B cells. In contrast, signaling through TACI in mature B cells or plasmablasts was only achieved by higher-order BAFF and APRIL oligomers, all of which were also po-tent activators of a multimerization-dependent reporter signaling pathway. These results indicate that, although BAFF-R and TACI can provide B cells with similar signals, only BAFF-R, but not TACI, can respond to soluble BAFF 3-mer, which is the main form of BAFF found in circulation. BAFF 60-mer, an efficient TACI agonist, was also detected in plasma of BAFF transgenic and nontransgenic mice and was more than 100-fold more active than BAFF 3-mer for the activation of multimerization-dependent signals. TACI supported survival of activated B cells and plasmablasts in vitro, providing a rational basis to explain the immunoglobulin deficiency reported in TACI-deficient persons.

Immunology research: challenges and opportunities in a time of budgetary constraint

There have been enormous advances in the field of immunology over the past 3 decades, and those advances have had a positive effect on many subspecialties of medicine. Opportunities for even more notable advances remain. However, present and projected budget constraints for the National Institutes of Health have created formidable challenges. This commentary addresses the opportunities and challenges for the field of immunology during a period of restricted budgets.