TLR stimulation modifies BLyS receptor expression in follicular and marginal zone B cells

N-3 polyunsaturated fatty acids modulate B cell activity in pre-clinical models: Implications for the immune response to infections

B cell antigen presentation, cytokine production, and antibody production are targets of pharmacological intervention in inflammatory and infectious diseases. Here we review recent pre-clinical evidence demonstrating that pharmacologically relevant levels of n-3 polyunsaturated fatty acids (PUFA) derived from marine fish oils influence key aspects of B cell function through multiple mechanisms. N-3 PUFAs modestly diminish B cell mediated stimulation of classically defined naïve CD4(+) Th1 cells through the major histocompatibility complex (MHC) class II pathway. This is consistent with existing data showing that n-3 PUFAs suppress the activation of Th1/Th17 cells through direct effects on helper T cells and indirect effects on antigen presenting cells. Mechanistically, n-3 PUFAs lower antigen presentation and T cell signaling by disrupting the formation of lipid microdomains within the immunological synapse. We then review data to show that n-3 PUFAs boost B cell activation and antibody production in the absence and presence of antigen stimulation. This has potential benefits for several clinical populations such as the aged and obese that have poor humoral immunity. The mode of action by which n-3 PUFA boost B cell activation and antibody production remains unclear, but may involve Th2 cytokines, enhanced production of specialized proresolving lipid mediators, and targeting of protein lateral organization in lipid microdomains. Finally, we highlight evidence to show that different n-3 PUFAs are not biologically equivalent, which has implications for the development of future interventions to target B cell activity.

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.

Function and regulation of self-reactive marginal zone B cells in autoimmune arthritis

Polyreactive innate-type B cells account for many B cells expressing self-reactivity in the periphery. Improper regulation of these B cells may be an important factor that underlies autoimmune disease. Here we have explored the influence of self-reactive innate B cells in the development of collagen-induced arthritis (CIA), a mouse model of rheumatoid arthritis. We show that splenic marginal zone (MZ), but not B-1 B cells exhibit spontaneous IgM reactivity to autologous collagen II in naı¨ve mice. Upon immunization with heterologous collagen II in complete Freund's adjuvant the collagen-reactive MZ B cells expanded rapidly, while the B-1 B cells showed a modest anti-collagen response. The MZ B cells were easily activated by toll-like receptor (TLR) 4 and 9-ligands in vitro, inducing proliferation and cytokine secretion, implying that dual engagement of the B-cell receptor and TLRs may promote the immune response to self-antigen. Furthermore, collagen-primed MZ B cells showed significant antigen-presenting capacity as reflected by cognate T-cell proliferation in vitro and induction of IgG anti-collagen antibodies in vivo. MZ B cells that were deficient in complement receptors 1 and 2 demonstrated increased proliferation and cytokine production, while Fcγ receptor IIb deficiency of the cells lead to increased cytokine production and antigen presentation. In conclusion, our data highlight self-reactive MZ B cells as initiators of the autoimmune response in CIA, where complement and Fc receptors are relevant in controlling the self-reactivity in the cells.

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.

Effect of TACI signaling on humoral immunity and autoimmune diseases

Transmembrane activator and calcium-modulating cyclophilin ligand interactor (TACI) is one of the receptors of B cell activating factor of the tumor necrosis factor family (BAFF) and a proliferation-inducing ligand (APRIL). TACI is a regulator in the immune responses. TACI inhibits B cell expansion and promotes the differentiation and survival of plasma cells. The mechanisms underlying these effects probably involve changed expressions of some crucial molecules, such as B lymphocyte induced maturation protein-1 (Blimp-1) and inducible T-cell costimulator ligand (ICOSL) in B cells and/or plasma cells. However, abnormal TACI signaling may relate to autoimmune disorders. Common variable immune deficiency (CVID) patients with heterozygous mutations in TACI alleles increase susceptibility to autoimmune diseases. Taci^−/− mice and BAFF transgenic mice both develop signs of human SLE. These findings that indicate inappropriate levels of TACI signaling may disrupt immune system balance, thereby promoting the development of autoimmune diseases. In this review, we summarize the basic characteristics of the TACI ligands BAFF and APRIL, and detail the research findings on the role of TACI in humoral immunity. We also discuss the possible mechanisms underlying the susceptibility of CVID patients with TACI mutations to autoimmune diseases and the role of TACI in the pathogenesis of SLE.

Distinct Transcriptomic Features are Associated with Transitional and Mature B-Cell Populations in the Mouse Spleen

Splenic transitional B-cells (T1 and T2) are selected to avoid self-reactivity and to safeguard against autoimmunity, then differentiate into mature follicular (FO-I and FO-II) and marginal zone (MZ) subsets. Transcriptomic analysis by RNA-seq of the five B-cell subsets revealed T1 cell signature genes included RAG suggesting a potential for receptor revision. T1 to T2 B-cell differentiation was marked by a switch from Myb to Myc, increased expression of the PI3K adapter DAP10 and MHC class II. FO-II may be an intermediate in FO-I differentiation and may also become MZ B-cells as suggested by principle component analysis. MZ B-cells possessed the most distinct transcriptome including down-regulation of CD45 phosphatase-associated protein (CD45-AP/PTPRC-AP), as well as upregulation of IL-9R and innate molecules TLR3, TLR7, and bactericidal Perforin-2 (MPEG1). Among the endosomal TLRs, stimulation via TLR3 further enhanced Perforin-2 expression exclusively in MZ B-cells. Using gene-deleted and overexpressing transgenic mice we show that IL-9/IL-9R interaction resulted in rapid activation of STAT1, 3, and 5, primarily in MZ B-cells. Importantly, CD45-AP mutant mice had reduced transitional and increased mature MZ and FO B-cells, suggesting that it prevents premature entry of transitional B-cells to the mature B-cell pool or their survival and proliferation. Together, these findings suggest, developmental plasticity among splenic B-cell subsets, potential for receptor revision in peripheral tolerance whereas enhanced metabolism coincides with T2 to mature B-cell differentiation. Further, unique core transcriptional signatures in MZ B-cells may control their innate features.

DNA vaccine molecular adjuvants SP-D-BAFF and SP-D-APRIL enhance anti-gp120 immune response and increase HIV-1 neutralizing antibody titers

Broadly neutralizing antibodies (bNAbs) specific for conserved epitopes on the HIV-1 envelope (Env) are believed to be essential for protection against multiple HIV-1 clades. However, vaccines capable of stimulating the production of bNAbs remain a major challenge. Given that polyreactivity and autoreactivity are considered important characteristics of anti-HIV bNAbs, we designed an HIV vaccine incorporating the molecular adjuvants BAFF (B cell activating factor) and APRIL (a proliferation-inducing ligand) with the potential to facilitate the maturation of polyreactive and autoreactive B cells as well as to enhance the affinity and/or avidity of Env-specific antibodies. We designed recombinant DNA plasmids encoding soluble multitrimers of BAFF and APRIL using surfactant protein D as a scaffold, and we vaccinated mice with these molecular adjuvants using DNA and DNA-protein vaccination strategies. We found that immunization of mice with a DNA vaccine encoding BAFF or APRIL multitrimers, together with interleukin 12 (IL-12) and membrane-bound HIV-1 Env gp140, induced neutralizing antibodies against tier 1 and tier 2 (vaccine strain) viruses. The APRIL-containing vaccine was particularly effective at generating tier 2 neutralizing antibodies following a protein boost. These BAFF and APRIL effects coincided with an enhanced germinal center (GC) reaction, increased anti-gp120 antibody-secreting cells, and increased anti-gp120 functional avidity. Notably, BAFF and APRIL did not cause indiscriminate B cell expansion or an increase in total IgG. We propose that BAFF and APRIL multitrimers are promising molecular adjuvants for vaccines designed to induce bNAbs against HIV-1.

IMPORTANCE

Recent identification of antibodies that neutralize most HIV-1 strains has revived hopes and efforts to create novel vaccines that can effectively stimulate HIV-1 neutralizing antibodies. However, the multiple immune evasion properties of HIV have hampered these efforts. These include the instability of the gp120 trimer, the inaccessibility of the conserved sequences, highly variable protein sequences, and the loss of HIV-1-specific antibody-producing cells during development. We have shown previously that tumor necrosis factor (TNF) superfamily ligands, including BAFF and APRIL, can be multitrimerized using the lung protein SP-D (surfactant protein D), enhancing immune responses. Here we show that DNA or DNA-protein vaccines encoding BAFF or APRIL multitrimers, IL-12p70, and membrane-bound HIV-1 Env gp140 induced tier 1 and tier 2 neutralizing antibodies in a mouse model. BAFF and APRIL enhanced the immune reaction, improved antibody binding, and increased the numbers of anti-HIV-1 antibody-secreting cells. Adaptation of this vaccine design may prove useful in designing preventive HIV-1 vaccines for humans.

The effects of diet-induced obesity on B cell function

B-1 and B-2 B cell subsets carry out a diverse array of functions that range broadly from responding to innate stimuli, antigen presentation, cytokine secretion and antibody production. In this review, we first cover the functional roles of the major murine B cell subsets. We then highlight emerging evidence, primarily in preclinical rodent studies, to show that select B cell subsets are a therapeutic target in obesity and its associated co-morbidities. High fat diets promote accumulation of select murine B cell phenotypes in visceral adipose tissue. As a consequence, B cells exacerbate inflammation and thereby insulin sensitivity through the production of autoantibodies and via cross-talk with select adipose resident macrophages, CD4(+) and CD8(+) T cells. In contrast, interleukin (IL)-10-secreting regulatory B cells counteract the proinflammatory profile and improve glucose sensitivity. We subsequently review data from rodent studies that show pharmacological supplementation of obesogenic diets with long chain n-3 polyunsaturated fatty acids or specialized pro-resolving lipid mediators synthesized from endogenous n-3 polyunsaturated fatty acids boost B cell activation and antibody production. This may have potential benefits for improving inflammation in addition to combating the increased risk of viral infection that is an associated complication of obesity and type II diabetes. Finally, we propose potential underlying mechanisms throughout the review by which B cell activity could be differentially regulated in response to high fat diets.

Restoring the balance: immunotherapeutic combinations for autoimmune disease

Autoimmunity occurs when T cells, B cells or both are inappropriately activated, resulting in damage to one or more organ systems. Normally, high-affinity self-reactive T and B cells are eliminated in the thymus and bone marrow through a process known as central immune tolerance. However, low-affinity self-reactive T and B cells escape central tolerance and enter the blood and tissues, where they are kept in check by complex and non-redundant peripheral tolerance mechanisms. Dysfunction or imbalance of the immune system can lead to autoimmunity, and thus elucidation of normal tolerance mechanisms has led to identification of therapeutic targets for treating autoimmune disease. In the past 15 years, a number of disease-modifying monoclonal antibodies and genetically engineered biologic agents targeting the immune system have been approved, notably for the treatment of rheumatoid arthritis, inflammatory bowel disease and psoriasis. Although these agents represent a major advance, effective therapy for other autoimmune conditions, such as type 1 diabetes, remain elusive and will likely require intervention aimed at multiple components of the immune system. To this end, approaches that manipulate cells ex vivo and harness their complex behaviors are being tested in preclinical and clinical settings. In addition, approved biologic agents are being examined in combination with one another and with cell-based therapies. Substantial development and regulatory hurdles must be overcome in order to successfully combine immunotherapeutic biologic agents. Nevertheless, such combinations might ultimately be necessary to control autoimmune disease manifestations and restore the tolerant state.

B-cell activating factor-receptor specific activation of tumor necrosis factor receptor associated factor 6 and the phosphatidyl inositol 3-kinase pathway in lymphoma B cells

B-cell activating factor-receptor (BAFF-R) is the primary BAFF receptor that is responsible for promoting B-cell development and survival. Malignant B-cells exploit the BAFF/BAFF-R system, and high serum BAFF levels or genetic alterations in BAFF receptors have been found in B-cell cancers. BAFF signaling impacts pro-survival pathways. However, other than nuclear factor-κB2 (NF-κB2), little is known about the specific pathways activated by individual BAFF receptors. Using a novel BAFF-R expression model we have demonstrated that activation of BAFF-R, independent of transmembrane activator and cytophilin ligand interactor (TACI) and B-cell maturation antigen (BCMA), can induce phosphorylation of Akt and glycogen synthase kinase 3β (GSK3β). Expression of an activated form of BAFF-R also enhanced a pro-survival gene expression pattern, including the novel BAFF-regulated gene Pin1, whose expression was phosphatidyl inositol 3-kinase (PI3K)-dependent. Additionally, we showed that TRAF6 is essential for mediating BAFF-R dependent activation of Akt. Together these data describe a novel role for TRAF6 in BAFF-R-specific activation of the PI3K pathway and provide evidence suggesting a new role for Pin1 in BAFF-R signaling.

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.

B cell activation through CD40 and IL4R ligation modulates the response of chronic lymphocytic leukaemia cells to BAFF and APRIL

The two tumour necrosis factor family proteins BAFF (TNFSF13B) and APRIL (TNFSF13) and their receptors [BAFF-R (TNFRSF13C), TACI (TNFRSF13B), BCMA (TNFRSF17)] play a critical role in the survival of normal B cells. The sensitivity of normal B cells to BAFF and APRIL can be modulated by signals regulated by their receptors. This modulation, however, has not been extensively investigated in chronic lymphocytic leukaemia (CLL) cells. We evaluated the expression, regulation and signalling of BAFF and APRIL receptors in normal and in CLL cells upon stimulation through CD40+IL4R and BCR. We further analysed the prognostic value of BAFF and APRIL receptors expression in patients with CLL. BCMA expression was significantly higher on CLL cells than on normal B cells. BCR and CD40+IL4R stimulation promoted an increase in TACI and BCMA expression, cell viability and activation in normal B cells. A similar effect was observed in CLL cells after CD40+IL4R but not BCR stimulation. BCMA expression correlated with unmutated IGHV genes, poor-risk cytogenetics, and short progression-free survival. These findings further characterize the link between CD40+IL4R regulatory signals, BAFF, APRIL and their receptors and the survival of leukaemic cells and clinical features of CLL.

Efficient B cell responses to Borrelia hermsii infection depend on BAFF and BAFFR but not TACI

T cell-independent antibody responses develop rapidly, within 3 to 4 days, and are critical for preventing blood-borne pathogens from evolving into life-threatening infections. The interaction of BAFF, also known as BLyS, with its receptors BAFFR and TACI on B cells is critical for B cell homeostasis and function. Using a synthetic polysaccharide antigen, it has previously been shown that TACI is critical for T cell-independent antibody responses. To examine the role of BAFFR and TACI in T cell-independent antibody responses to an active infection, we utilized the Borrelia hermsii infection system. In this infection system, T cell-independent responses mediated by the B1b cell subset are critical for controlling bacteremia. We found that B1b cells express BAFFR and TACI and that the surface expression of both receptors is upregulated on B1b cells following exposure to whole B. hermsii cells. Surprisingly, we found that TACI(-/-) mice are not impaired either in specific antibody responses to B. hermsii or in controlling B. hermsii bacteremia. In contrast, TACI-deficient mice immunized with heat-killed type 3 serotype pneumococcus cells are impaired in generating pneumococcal polysaccharide-specific responses and succumb to challenge with live type 3 serotype pneumococcus, indicating that TACI is required for T cell-independent antibody responses to bacterial-associated polysaccharides. Although we have found that TACI is dispensable for controlling B. hermsii infection, mice deficient in BAFFR or BAFF exhibit impairment in B. hermsii-specific IgM responses and clearance of bacteremia. Collectively, these data indicate a disparity in the roles for TACI and BAFFR in primary T cell-independent antibody responses to bacterial pathogens.

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.

The TACI receptor regulates T-cell-independent marginal zone B cell responses through innate activation-induced cell death

Activation-induced cell death (AICD) plays a critical role in immune homeostasis and tolerance. In T-cell-dependent humoral responses, AICD of B cells is initiated by Fas ligand (FasL) on T cells, stimulating the Fas receptor on B cells. In contrast, T-cell-independent B cell responses involve innate-type B lymphocytes, such as marginal zone (MZ) B cells, and little is known about the mechanisms that control AICD during innate B cell responses to Toll-like receptor (TLR) activation. Here, we show that MZ B cells undergo AICD in response to TLR4 activation in vivo. The transmembrane activator, calcium modulator, and cyclophilin ligand interactor (TACI) receptor and TLR4 cooperate to upregulate expression of both FasL and Fas on MZ B cells and also to repress inhibitors of Fas-induced apoptosis signaling. These findings demonstrate an unappreciated role for TACI and its ligands in the regulation of AICD during T-cell-independent B cell responses.

Marginal zone B cells: virtues of innate-like antibody-producing lymphocytes

Protective responses to microorganisms involve the nonspecific but rapid defence mechanisms of the innate immune system, followed by the specific but slow defence mechanisms of the adaptive immune system. Located as sentinels at the interface between the circulation and lymphoid tissue, splenic marginal zone B cells rapidly respond to blood-borne antigens by adopting 'crossover' defensive strategies that blur the conventional boundaries of innate and adaptive immunity. This Review discusses how marginal zone B cells function as innate-like lymphocytes that mount rapid antibody responses to both T cell-dependent and T cell-independent antigens. These responses require the integration of activation signals from germline-encoded and somatically recombined receptors for microorganisms with helper signals from effector cells of the innate and adaptive immune systems.

Toll-like receptor 9 activation induces expression of membrane-bound B-cell activating factor (BAFF) on human B cells and leads to increased proliferation in response to both soluble and membrane-bound BAFF

OBJECTIVES

Activation of TLR7 and TLR9 and high serum levels of BAFF have been implicated in the pathogenesis of SLE. However, little is known about the effects of TLR9 activation on BAFF expression by human B cells. We investigated the effect of the TLR9 agonist, CpG-ODN 2006, on the expression of BAFF and its receptors BAFF-R, TACI and BCMA, in isolated B cells from healthy donors.

METHODS

We used RT-PCR, flow cytometry and ELISA to investigate the expression of BAFF, and flow cytometry for BAFF-R, TACI and BCMA. Functional assays assessed the responses of resting and CpG-ODN-activated B cells to exogenous soluble and membrane-bound BAFF.

RESULTS

CpG-ODN did not induce BAFF secretion, but increased expression of membrane-bound BAFF on B cells. CpG-ODN also induced the expression of TACI and BCMA, but did not up-regulate BAFF-R expression. In functional studies, CpG-ODN sensitized human B cells to proliferate in response to exogenous BAFF. This effect was inhibited by a blocking antibody against BAFF-R, but was not inhibited by anti-TACI or anti-BCMA antibodies. Membrane-bound BAFF, induced by CpG-ODN, co-stimulated the proliferation of B cells stimulated with anti-IgM in a manner that was dependent on the expression of surface BAFF on the CpG-ODN-treated B cells.

CONCLUSION

TLR9 activation induces expression of membrane-bound BAFF on human B cells and leads to increased proliferation in response to both soluble and membrane-bound BAFF. These data extend our understanding of the role of TLR9 activation on human B cells and provide insights into the mechanisms by which TLR9 may participate in the pathogenesis of SLE.

B cell-activating factor enhances interleukin-6 and interleukin-10 production by ODN-activated human B cells

We aim to investigate the additive value of B cell-activating factor (BAFF) when added to oligodeoxynucleotides (ODN)-activated B cells with respect to TLR-9, CD69, MHC-II expression, IL-6 and IL-10 secretion and B cell cycling. Therefore, B cells from healthy individuals were incubated under the following conditions: (1) B cells with medium, (2) B cells with ODN 0.5 μm, (3) B cells with BAFF 20 μm and (4) B cells with both ODN 0.5 μm and BAFF 20 μm. We found that addition of BAFF did not enhance the expression of TLR-9, CD69 and MHC-II in ODN-activated B cells. Incubation of B cells with BAFF and ODN together leads to a marked elevation of IL-6 and IL-10 levels compared to ODN alone. Synthesis and mitosis were higher in B cells stimulated by BAFF than in B cells stimulated by ODN. These findings suggest that both BAFF and TLR-9 contribute independently to B cell function.

The rationale for BAFF inhibition in systemic lupus erythematosus

BAFF (B-cell-activating factor) is a critical survival factor for transitional and mature B cells and is a promising therapeutic target for systemic lupus erythematosus (SLE). In 2010-2011, two phase 3 clinical trials showed that the addition of the anti-BAFF antibody belimumab to standard-of-care therapy in patients with moderately active SLE results in a better outcome at 52 weeks than standard-of-care therapy alone. Belimumab has been US Food and Drug Administration approved for the treatment of SLE, and other drugs that target BAFF are now in various stages of clinical testing. This review describes the function of BAFF and its homolog APRIL (a proliferation-inducing ligand) and addresses the rationale for the treatment of SLE with BAFF/APRIL inhibitors.

Common variable immunodeficiency: an update

Common variable immunodeficiency (CVID) describes a heterogeneous subset of hypogammaglobulinemias of unknown etiology. Typically, patients present with recurrent bacterial infections of the respiratory and gastrointestinal tract. A significant proportion of CVID patients develops additional autoimmune, inflammatory or lymphoproliferative complications. CVID is the most frequent symptomatic primary immunodeficiency encountered in adults. Informative monogenetic defects have been found in single patients and families but in most cases the pathogenesis is still elusive. Numerous immunological studies have demonstrated phenotypic and functional abnormalities of T cells, B cells and antigen-presenting cells. A hallmark is the impaired memory B-cell formation that has been taken advantage of for classifying CVID patients. Clinical multi-center studies have demonstrated a correlation between immunological markers and clinical presentation. Long-term outcome is significantly influenced by delay of diagnosis and treatment and the presence of chronic inflammatory complications. While immunoglobulin replacement therapy plus antibiotics can control infections in most cases, patients with non-infectious inflammatory complications such as granulomatous inflammation, interstitial lung disease, inflammatory bowel disease, lymphoproliferation and developing malignancies still represent a therapeutic challenge. In this review we provide a systematic overview of the immunological, clinical, diagnostic and therapeutic aspects of CVID and highlight recent developments in these fields.

Integration of B cell responses through Toll-like receptors and antigen receptors

Unlike other immune cells, B cells express both an antigen-specific B cell receptor (BCR) and Toll-like receptors (TLRs). Dual BCR and TLR engagement can fine-tune functional B cell responses, directly linking cell-intrinsic innate and adaptive immune programmes. Although most data regarding B cell-specific functions of the TLR signalling pathway have been obtained in mice, the discovery of patients with a deficiency in this pathway has recently provided an insight into human B cell responses. Here, we highlight the importance of the integration of signalling pathways downstream of BCRs and TLRs in modulating B cell function, focusing when possible on B cell-intrinsic roles.

Roles of pro- and anti-inflammatory cytokines in the pathogenesis of SLE

SLE is an autoimmune inflammatory disease in which various pro- and anti-inflammatory cytokines, including TGF-β, IL-10, BAFF, IL-6, IFN-α, IFN-γ, IL-17, and IL-23, play crucial pathogenic roles. Virtually, all these cytokines can be generated by both innate and adaptive immune cells and exert different effects depending on specific local microenvironment. They can also interact with each other, forming a complex network to maintain delicate immune homeostasis. In this paper, we elaborate on the abnormal secretion and functions of these cytokines in SLE, analyze their potential pathogenic roles, and probe into the possibility of them being utilized as targets for therapy.

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.

Differential impact of Toll-like receptor signaling on distinct B cell subpopulations

B cells exhibit a range of functional responses following TLR engagement including immunoglobulin and cytokine production, proliferation, antigen presentation and migration. However, B cell intrinsic TLR responses appear to be precisely programmed based upon the developmental stage of the cell. B cell subpopulations classified as innate immune cells including marginal zone and B-1 B cells exhibit robust responses to TLR stimulation. In contrast, activation of other B cell subsets is constrained via a variety of developmentally regulated events. In this review we provide an overview of TLR responses in murine and human B cells and specifically highlight patterns of TLR expression and developmentally regulated functional responses.

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.

Cannabinoid receptor 2 is critical for the homing and retention of marginal zone B lineage cells and for efficient T-independent immune responses

The endocannabinoid system has emerged as an important regulator of immune responses, with the cannabinoid receptor 2 (CB2) and its principle ligand 2-archidonoylglycerol playing a major role. How CB2 regulates B cell functions is not clear, even though they express the highest levels of CB2 among immune cell subsets. In this study, we show that CB2-deficient mice have a significant reduction in the absolute number of marginal zone (MZ) B cells and their immediate precursor, transitional-2 MZ precursor. The loss of MZ lineage cells in CB2(-/-) mice was shown to be B cell intrinsic using bone marrow chimeras and was not due to a developmental or functional defect as determined by B cell phenotype, proliferation, and Ig production. Furthermore, CB2(-/-) B cells were similar to wild type in their apoptosis, cell turnover, and BCR and Notch-2 signaling. We then demonstrated that CB2(-/-) MZ lineage B cells were less efficient at homing to the MZ and that their subsequent retention was also regulated by CB2. CB2(-/-) mice immunized with T-independent Ags produced significantly less Ag-specific IgM. This study demonstrates that CB2 positively regulates T-independent immune responses by controlling the localization and positioning of MZ lineage cells to the MZ.

Are Toll-like receptors and decoy receptors involved in the immunopathogenesis of systemic lupus erythematosus and lupus-like syndromes?

In this paper we focus our attention on the role of two families of receptors, Toll-like receptors (TLR) and decoy receptors (DcR) involved in the generation of systemic lupus erythematosus (SLE) and lupus-like syndromes in human and mouse models. To date, these molecules were described in several autoimmune disorders such as rheumatoid arthritis, antiphospholipids syndrome, bowel inflammation, and SLE. Here, we summarize the findings of recent investigations on TLR and DcR and their role in the immunopathogenesis of the SLE.

Pathway of Toll-like receptor 7/B cell activating factor/B cell activating factor receptor plays a role in immune thrombocytopenia in vivo

Immune thrombocytopenia (ITP) is an autoimmune disorder characterized by anti-platelet autoantibody-mediated platelet destruction. Antigen-presenting cell (APC) dysfunction is considered to play crucial roles in ITP. However, how APC affects autoreactive B cells in ITP is still unknown. Using a mouse model of immune thrombocytopenia, we demonstrated an increase in levels of TLR7 in splenic mononuclear cells (SMCs). Using both TLR7 agonist and TLR7 silencing lentivirus, we found stimulation of TLR7 decreased platelet counts and increased levels of platelet-associated IgG (PAIgG) in ITP mice, which correlates TLR7 with platelet destruction by autoantibodies. Levels of serum BAFF increased significantly in ITP mice and stimulation of TLR7 promoted secretion of BAFF. Among the three BAFF receptors, only BAFF receptor (BAFF-R) increased in ITP mice. However, activation of TLR7 showed no effect on the expression of BAFF receptors. These findings indicate that upregulation of TLR7 may augment BAFF secretion by APC and through ligation of BAFF-R promote autoreactive B cell survival and thus anti-platelet autoantibody production. The pathway of TLR7/BAFF/BAFF-R provides us with an explanation of how activation of APC affects autoantibody production by B cells in ITP and thus might provide a reasonable therapeutic strategy for ITP.

BAFF and selection of autoreactive B cells

B cell activating factor (BAFF) is a crucial survival factor for transitional and mature B cells, and is a promising therapeutic target for systemic lupus erythematosus (SLE). A BAFF inhibitor, belimumab, is the first new drug in 50 years to be approved for the treatment of SLE. However, the mechanism of action of this drug is not entirely clear. In this review we will focus on the role of the BAFF-APRIL signaling pathway in the selection of autoreactive B cells, and discuss whether altered selection is the mechanism for the therapeutic efficacy of BAFF inhibition in SLE.

Toll-like receptor driven B cell activation in the induction of systemic autoimmunity

Studies over the past decade have demonstrated a key role for pattern recognition receptors in the activation of autoreactive B cells. Self reactive B cells that manage to escape negative selection often express relatively low affinity receptors for self antigens (ignorant B cells), and can only be activated by integrating a relatively weak BCR signal with signals from additional receptors. Members of the toll-like receptor (TLR) gene family, and especially the nucleic acid binding receptors TLR 7, 8 and 9, appear to play a key role in this regard and promote the production of autoantibodies reactive with DNA- or RNA-associated autoantigens. These autoantibodies are able to form immune complexes with soluble or cell-bound ligands, and these immune complexes can in turn activate a second round of proinflammatory cells that further contribute to the autoimmune disease process. Recent data have emerged showing a pathogenic role for TLR7, with an opposing, protective role for TLR9. Targeting these disregulated pathways offers a therapeutic opportunity to treat autoimmune diseases without crippling the entire immune system. Further understanding of the role of specific receptors, cell subsets, and inhibitory signals that govern these TLR-associated pathways will enable future therapeutics to be tailored to specific categories of autoimmune disease.

Aberrant expression of functional BAFF-system receptors by malignant B-cell precursors impacts leukemia cell survival

Despite exhibiting oncogenic events, patient's leukemia cells are responsive and dependent on signals from their malignant bone marrow (BM) microenvironment, which modulate their survival, cell cycle progression, trafficking and resistance to chemotherapy. Identification of the signaling pathways mediating this leukemia/microenvironment interplay is critical for the development of novel molecular targeted therapies.

We observed that primary leukemia B-cell precursors aberrantly express receptors of the BAFF-system, BAFF-R, BCMA, and TACI. These receptors are functional as their ligation triggers activation of NF-κB, MAPK/JNK, and Akt signaling. Leukemia cells express surface BAFF and APRIL ligands, and soluble BAFF is significantly higher in leukemia patients in comparison to age-matched controls. Interestingly, leukemia cells also express surface APRIL, which seems to be encoded by APRIL-δ, a novel isoform that lacks the furin convertase domain. Importantly, we observed BM microenvironmental cells express the ligands BAFF and APRIL, including surface and secreted BAFF by BM endothelial cells. Functional studies showed that signals through BAFF-system receptors impact the survival and basal proliferation of leukemia B-cell precursors, and support the involvement of both homotypic and heterotypic mechanisms.

This study shows an unforeseen role for the BAFF-system in the biology of precursor B-cell leukemia, and suggests that the target disruption of BAFF signals may constitute a valid strategy for the treatment of this cancer.

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

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

Functions of Toll-like receptors as an inborn immunity component and their participation in the pathogenesis of dermatoses of different etiologies

In addition to serving as a mechanical barrier protecting our organism from the damaging effect of different factors, our skin also takes part in immune reactions developing in case of microbial intervention. Toll-like receptors (TLR) mediating recognition of molecular structures of pathogens are expressed in skin cells of different types initiating the development of adaptive immune reactions when associated with different ligands. The review presents data on the structure and functions of TLR, their localization in skin compartments and their role in the pathogenesis of skin diseases being of infectious etiology and non-infectious origin.

B Cell and BAFF dependence of IFN-α-exaggerated disease in systemic lupus erythematosus-prone NZM 2328 mice

IFN-α is a potent activator of innate and adaptive immunity, and its administration to preautoimmune (NZB×NZW)F1 mice promotes virulent systemic lupus erythematosus (SLE) disease. Given the known contributions of B cells and BAFF to SLE, we evaluated the ability of IFN-α administration to induce disease in wild-type (WT), B cell-deficient, and BAFF-deficient NZM 2328 mice. Whereas WT mice rapidly developed proliferative glomerulonephritis, marked proteinuria, and increased mortality in response to IFN-α administration, B cell-deficient mice developed neither renal pathology nor clinical disease. Moreover, BAFF-deficient mice, despite developing limited glomerular IgG and C3 deposition, also remained free of histological glomerulonephritis and clinical disease. Strikingly, similar T cell expansion and serum IgG responses were observed in adenovirus (Adv)-IFN-treated WT and BAFF-deficient mice despite their disparate pathological and clinical responses, whereas numbers of activated B cells increased in WT mice but not in BAFF-deficient mice. Nonetheless, B cell, plasma cell, and T cell infiltration of the kidneys in Adv-IFN-treated WT mice was similar to that in WT mice treated with Adv-control. Its ability to promote SLE disease in WT mice notwithstanding, IFN-α administration failed to drive the preferential expansion of CD4(+) memory T cells that occurs during the natural course of disease, and glomerular infiltration of macrophages failed to associate with development of disease. These results collectively suggest that therapeutic targeting in SLE of BAFF and/or B cells in SLE could be successful even in states of IFN-α overexpression. Moreover, our results document important biological differences between IFN-α-driven and spontaneous natural SLE disease.

Innate control of B cell responses

Mature B cells generate protective immunity by undergoing immunoglobulin (Ig) class switching and somatic hypermutation, two Ig gene-diversifying processes that usually require cognate interactions with T cells that express CD40 ligand. This T cell-dependent pathway provides immunological memory but is relatively slow to occur. Thus, it must be integrated with a faster, T cell-independent pathway for B cell activation through CD40 ligand-like molecules that are released by innate immune cells in response to microbial products. Here, we discuss recent advances in our understanding of the interplay between the innate immune system and B cells, particularly at the mucosal interface. We also review the role of innate signals in the regulation of Ig diversification and production.

BAFF inhibition: a new class of drugs for the treatment of autoimmunity

BAFF (BLyS) and APRIL are TNF-like cytokines that support survival and differentiation of B cells. Recent studies have discovered a role for BAFF in augmenting both innate and adaptive immune responses as well as in collaborating with other inflammatory cytokines to promote the activation and differentiation of effector immune cells. BAFF is an important pathogenic factor in lupus mouse models and BAFF inhibition successfully delays disease onset in these mice, although the responsiveness to BAFF inhibition varies among different strains. These results have led to the development of inhibitors targeting BAFF and APRIL in humans. An anti-BAFF antibody has shown significant but modest efficacy in two Phase III clinical trials for moderately active SLE and other inhibitors are being developed or at early stages of clinical testing.

Interferon-α accelerates murine systemic lupus erythematosus in a T cell-dependent manner

OBJECTIVE

To investigate the mechanism by which interferon-α (IFNα) accelerates systemic lupus erythematosus (SLE) in (NZB×NZW)F1 (NZB/NZW) mice.

METHODS

NZB/NZW mice were treated with an adenovirus expressing IFNα. In some mice, T cells were depleted with an anti-CD4 antibody. The production of anti-double-stranded DNA (anti-dsDNA) antibodies was measured by enzyme-linked immunosorbent assay and enzyme-linked immunospot assay. Germinal centers and antibody-secreting cells (ASCs) in spleens and IgG deposition and leukocyte infiltrates in kidneys were visualized by immunofluorescence staining. The phenotype of splenic cells was determined by flow cytometry. Finally, somatic hypermutation and gene usage in VH regions of IgG2a and IgG3 were studied by single-cell polymerase chain reaction.

RESULTS

IFNα-accelerated lupus in NZB/NZW mice was associated with elevated serum levels of IgG2 and IgG3 anti-dsDNA antibodies and accumulation of many IgG ASCs in the spleen, which did not develop into long-lived plasma cells. Furthermore, IgG2a and IgG3 antibodies in the mice were highly somatically mutated and used distinct repertoires of VH genes. The induction of SLE in the mice was associated with an increase in B cell Toll-like receptor 7 expression, increased serum levels of BAFF, interleukin-6 (IL-6), and tumor necrosis factor α, and induction of T cells expressing IL-21. Although IFNα drove a T cell-independent increase in serum levels of IgG, autoantibody induction and the development of nephritis were both completely dependent on CD4+ T cell help.

CONCLUSION

These findings demonstrate that, although IFNα activates both innate and adaptive immune responses in NZB/NZW mice, CD4+ T cells are necessary for IFNα-driven induction of anti-dsDNA antibodies and clinical SLE.

Emerging small molecule and biological therapeutic approaches for the treatment of autoimmunity

IMPORTANCE OF THE FIELD

Biological therapeutics targeting TNF-α, IL-6, CD20 and CD80/86 is proving to be an important weapon in the clinicians' armory to fight autoimmunity alongside long-standing small molecule therapeutics such as methotrexate and glucocorticoids. However, there still remains a high unmet clinical need in the field of autoimmunity and many researchers are continuing to discover and develop new therapeutics to address this.

AREAS COVERED IN THIS REVIEW

A new wave of small molecule and biological therapeutics targeting different pathways is being developed which could generate exciting new options for clinicians. This review aims to highlight those emerging therapies that are most advanced in clinical development.

WHAT THE READER WILL GAIN

The reader will gain an appreciation of new approaches being developed to address the high unmet clinical need in the field of autoimmunity.

TAKE HOME MESSAGE

Despite recent success in the development of therapeutics to treat autoimmunity, new therapeutic strategies are being developed to address the remaining areas of a high unmet clinical need.

Targeting BAFF in autoimmunity

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

T cell-dependent and -independent IgA responses: role of TLR signalling

Immunoglobulin A (IgA) represents the primary line of protection against incoming pathogens since it is the predominant isotype on mucosal surfaces. Mucosal surfaces are constantly exposed to inhaled, digested and sexually transmitted agents and therefore highly susceptible to infection by invading pathogens. Such pathogens typically carry pathogen-associated molecular patterns (PAMPs) which primarily signal through Toll-like receptors (TLRs). TLRs belong to a family of pattern-recognition receptors that link the innate and the acquired immune system. TLR stimulation in professional antigen-presenting cells (APCs) such as dendritic cells (DCs) is crucial for an optimal cellular and humoral immune response to be induced. Moreover TLRs have been shown to improve humoral responses by direct stimulation of B cells. Herein we review recent data, which points to a pivotal role of TLR signalling in controlling T-cell dependent and independent IgA responses both at mucosal and systemic levels. A better understanding of these mechanisms may facilitate the use of TLR agonists as adjuvants and consequently improve the development of effective mucosal vaccines.

Mouse Models of Systemic Lupus Erythematosus Reveal a Complex Pathogenesis

The mammalian immune system is remarkable in that it can respond to an essentially infinite number of foreign antigens. The ability to mount a long-lasting (adaptive) immune response against foreign antigen requires the participation of cells selected from an enormously diverse population of B and T cells. Because the B and T cell receptors expressed by these cells are generated at random, a significant percentage of B and T cells are invariably directed against self-antigen. Under normal circumstances, autoreactive B and T cells are eliminated, reprogrammed, or inactivated in the primary and secondary lymphoid organs. Despite these checks and balances, a small but significant number of people and animals still develop autoimmune disease. One such autoimmune disease—systemic lupus erythematosus—is characterized by the loss of B- and T-cell tolerance to self-antigens (principally nuclear), culminating in multisystemic inflammation. Multiple genetic defects, drug exposure, infectious agents, and environmental factors can contribute to the pathogenesis of the disease. Loss of B- and T-cell tolerance precipitates activation of plasmacytoid and myeloid dendritic cells; collectively, these cells cooperate to form a complex positive feedback loop, continually stimulated by the persistence of self-antigen. Novel treatment strategies now focus on specific inhibition of various aspects of the feedback loop. These specific inhibitors have the potential to be more effective and lack the side effects associated with generalized immunosuppression.

TLR4 promotes B cell maturation: independence and cooperation with B lymphocyte-activating factor

We have previously shown that TLR4 triggering promotes the generation of CD23(+)CD93(+) transitional T2-like cells in vitro from mouse B cell precursors, suggesting a possible role for this receptor in B cell maturation. In this study, we perform an extensive study of cell surface markers and functional properties of B cells matured in vitro with LPS, comparatively with the well-known B cell maturation factor B lymphocyte-activating factor (BAFF). LPS increased generation of CD23(+) transitional B cells in a TLR4-dependent way, upregulating IgD and CD21 and downregulating CD93, without inducing cell proliferation, in a manner essentially equivalent to BAFF. For both BAFF and LPS, functional maturation of the IgM(+)CD23(+)CD93(+) cells was confirmed by their higher proliferative response to anti-CD40 plus IL-4 compared with IgM(+)CD23(neg)CD93(+) cells. BAFF-R-Fc-mediated neutralization experiments showed that TLR4-induced B cell maturation was independent of BAFF. Distinct from BAFF, maturation by LPS relied on the activation of canonical NF-kappaB pathway, and the two factors together had complementary effects, leading to higher numbers of IgM(+)CD23(+)CD93(+) cells with their simultaneous addition. Importantly, BCR cross-linking abrogated the generation of CD23(+) B cells by LPS or BAFF, indicating that signals mimicking central tolerance act on both systems. Addition of cyclosporin A reverted BCR-mediated inhibition, both for BAFF and LPS, suggesting similar regulation of signaling pathways by calcineurin. Finally, LPS-injected mice showed a rapid increase of mature B cells in the bone marrow, suggesting that TLR4 signaling may effectively stimulate B cell maturation in vivo, acting as an accessory stimulus in B cell development, complementary to the BAFF physiological pathway.