Transmembrane activator and calcium modulator and cyclophilin ligand interactor enhances CD40-driven plasma cell differentiation

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

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

Potential Utility of A Proliferation-Inducing Ligand (APRIL) in Colorectal Cancer

APRIL (A proliferation-inducing ligand) is a member of the tumor necrosis factor superfamily that is overexpressed in a variety of malignant tumors, including colorectal cancer (CRC). Its key physiological roles include inducing the immunoglobulin switch and ensuring plasmocyte survival. In terms of pathological roles, APRIL antagonism has been identified as a key target in autoimmune diseases and immunoglobulin disorders. As previously demonstrated, several inflammatory processes occur at the site of neoplastic initial stages, and their local symptoms are difficult to detect, particularly in the early stages. That is why we chose to study the current literature on APRIL's role in the development of colorectal cancer. The main objective of our research was to investigate the role of APRIL in cancer initiation and its usefulness in the detection and therapy of CRC. Interestingly, the findings conducted so far show that the selected protein has a significant potential as a CRC biomarker and treatment target. Importantly, based on its concentration, it is possible to identify CRC patients, but whether the lesion has a benign or malignant nature, indicating the possibility of rapid detection of an ongoing disease process.

New Insights and Future Perspectives of APRIL in IgA Nephropathy

IgA nephropathy (IgAN) is characterized by immune-mediated glomerulonephritis, with the accumulation of galactose-deficient IgA1 (Gd-IgA1) in the glomeruli and increased levels of circulating Gd-IgA1 and Gd-IgA1-containing immune complexes. An incomplete understanding of the underlying mechanisms and differences in clinical and pathological features between individuals and ethnicities has contributed to the lack of established treatments for IgAN. A tumor necrosis factor (TNF) family member, a proliferation-inducing ligand (APRIL), is a crucial cytokine essential for the generation and survival of plasma cells. Recent studies demonstrated that APRIL is a pivotal mediator in the production of Gd-IgA1 in IgAN. As our understanding of the autoimmune pathogenesis underlying IgAN has improved, various pharmacological therapeutic targets, including APRIL antagonists, have emerged. Preliminary results showed that APRIL-targeting agents effectively reduced proteinuria and Gd-IgA1 levels without significantly increasing adverse events, indicating their potential as novel therapeutic agents for IgAN. In the present review, we discuss the current understanding of the role of APRIL in the pathogenesis of IgAN and novel therapeutic strategies focusing on APRIL-targeting agents for IgAN. APRIL inhibitors may offer new hope to patients with IgAN.

Mapping of the gene network that regulates glycan clock of ageing

Glycans are an essential structural component of immunoglobulin G (IgG) that modulate its structure and function. However, regulatory mechanisms behind this complex posttranslational modification are not well known. Previous genome-wide association studies (GWAS) identified 29 genomic regions involved in regulation of IgG glycosylation, but only a few were functionally validated. One of the key functional features of IgG glycosylation is the addition of galactose (galactosylation), a trait which was shown to be associated with ageing. We performed GWAS of IgG galactosylation (N=13,705) and identified 16 significantly associated loci, indicating that IgG galactosylation is regulated by a complex network of genes that extends beyond the galactosyltransferase enzyme that adds galactose to IgG glycans. Gene prioritization identified 37 candidate genes. Using a recently developed CRISPR/dCas9 system we manipulated gene expression of candidate genes in the in vitro IgG expression system. Upregulation of three genes, EEF1A1, MANBA and TNFRSF13B, changed the IgG glycome composition, which confirmed that these three genes are involved in IgG galactosylation in this in vitro expression system.

Refractory Status Epilepticus Associated With a Pathogenic Variant in TNFRSF13B

Febrile infection-related epilepsy syndrome (FIRES) is a rare epileptic syndrome characterized by new-onset refractory status epilepticus preceded by a febrile illness. Limited literature exists regarding the relationship between primary immunodeficiencies and immune-mediated epilepsy, and the relationship between new-onset refractory status epilepticus and common variable immunodeficiency (CVID) is not well-understood. We present a case of a 21-year-old female with a history of recurrent sinus infections, asthma, thrombocytopenia, atrioventricular nodal reentrant tachycardia, and neonatal seizures who presented with fever and new-onset status epilepticus. She was ultimately diagnosed with a heterozygous variant in TNFRSF13B c.311G>A (p.Cys104Tyr), which encodes for a tumor necrosis factor receptor implicated in CVID.

The BAFF-APRIL System in Cancer

B cell-activating factor (BAFF; also known as CD257, TNFSF13B, BLyS) and a proliferation-inducing ligand (APRIL; also known as CD256, TNFSF13) belong to the tumor necrosis factor (TNF) family. BAFF was initially discovered as a B-cell survival factor, whereas APRIL was first identified as a protein highly expressed in various cancers. These discoveries were followed by over two decades of extensive research effort, which identified overlapping signaling cascades between BAFF and APRIL, controlling immune homeostasis in health and driving pathogenesis in autoimmunity and cancer, the latter being the focus of this review. High levels of BAFF, APRIL, and their receptors have been detected in different cancers and found to be associated with disease severity and treatment response. Here, we have summarized the role of the BAFF-APRIL system in immune cell differentiation and immune tolerance and detailed its pathogenic functions in hematological and solid cancers. We also highlight the emerging therapeutics targeting the BAFF-APRIL system in different cancer types.

Evaluating the Link between BAFF System Gene Expression and Acute Rejection Development in Kidney Transplantation

B-cell activating factor (BAFF) system signaling is critical for B-cell homeostasis, effector functions, and tolerance maintenance in transplants, but it has not been studied in kidney transplant recipients (KTRs). The aim was to analyze the changes in BAFF system expression in KTRs with/without acute rejection (AR/NAR). The BAFF system expression was analyzed by qPCR in 40 KTRs. A meta-analysis of BAFF system expression and histological renal damage was identified by the Chronic Allograft Damage Index (CADI) and performed from the GEO database. Proliferation-inducing ligand (APRIL) expression increased at three- and six-months post-KT (p = 0.014 and p < 0.001). B-cell maturation antigen (BCMA) expression increased at six-months post-KT (p = 0.038). BAFF expression remained stable in NAR-KTRs, but was increased in CADI concerning the No-CADI group at one year (p = 0.008). BCMA expression increased in the CADI group at one- (p = 0.001) and six-years post-KT (p = 0.024). At three months, the transmembrane activator and calcium modulator interactor (TACI) gene significantly elevated KTRs with DSAs (donor-specific antibody; p = 0.034). KTRs with DSAs significantly increase the B-cell activating factor receptor (R-BAFF; p = 0.021) and TACI (p = 0.018) between pre- and three-month post-KT. Changes in the expression of the BAFF system increase during post-KTR in the development of AR and chronic allograft damage, and could be an important pathological tool to detect and prevent kidney graft outcomes.

Role of the APRIL molecule in solid tumors

The APRIL molecule, produced by immune cells, their precursors, and cancer cells, is one of the important factors that influences the process of survival and proliferation of cancer cells. In the present review, we summarize the current knowledge on the effects of APRIL on human cancer development and develop a scheme demonstrating the mechanism of the action of APRIL on solid tumors. Understanding the effects of APRIL, including the intracellular signal transduction pathway, may be key for the use of this protein as a biomarker of the cancer process. The correlations observed between APRIL levels and cancer parameters (e.g., disease stage and presence of malignant phenotypes) indicate that APRIL may play an important role, not only in the diagnostic process, but also as a therapeutic target in various cancers.

The number 13 of the family: a proliferation inducing ligand

The TNF superfamily member a proliferation inducing ligand (APRIL, TNFSF13) plays a late role in humoral immunity at the level of antibody-producing plasmocytes. The recent characterization of the first immunodeficient patient with an inactivating mutation in the APRIL gene provided the last piece of functional data lacking in the human system. Based on this function, APRIL has been considered as a valuable target to dampen unwanted antibody production. After reviewing the late data acquired on the physiological function of APRIL in humoral immunity, we will here review the state of the art regarding APRIL targeting in autoimmune diseases.

TACI deficiency - a complex system out of balance

TACI promotes T-cell independent antibody responses and plasma cell differentiation and counteracts BAFF driven B-cell activation. Mutations in TNFRSF13B (encoding TACI) are associated with common variable immunodeficiency (CVID) but are also found in 1-2% of the general population. Although not diseases causing, certain TNFRSF13B mutations predispose CVID patients to autoimmunity and lymphoproliferation. Recently, studies of TACI-deficient humans and murine models revealed novel aspects of TACI, especially its crosstalk with the TLR pathways, differential expression of TACI isoforms, and its role in the generation of autoreactive B-cells. Vice versa, these studies are instrumental for a better understanding of TACI deficiency in humans and suggest that gene dosage, mutation type, and additional clinical or laboratory abnormalities influence the relevance of TNFRSF13B variants in individual CVID patients. TACI is embedded in a complex and well-balanced system, which is vulnerable to genetic and possibly also environmental hits.

Treatment-Induced BAFF Expression and B Cell Biology in Multiple Sclerosis

Although fingolimod and interferon-β are two mechanistically different multiple sclerosis (MS) treatments, they both induce B cell activating factor (BAFF) and shift the B cell pool towards a regulatory phenotype. However, whether there is a shared mechanism between both treatments in how they influence the B cell compartment remains elusive. In this study, we collected a cross-sectional study population of 112 MS patients (41 untreated, 42 interferon-β, 29 fingolimod) and determined B cell subsets, cell-surface and RNA expression of BAFF-receptor (BAFF-R) and transmembrane activator and cyclophilin ligand interactor (TACI) as well as plasma and/or RNA levels of BAFF, BAFF splice forms and interleukin-10 (IL-10) and -35 (IL-35). We added an in vitro B cell culture with four stimulus conditions (Medium, CpG, BAFF and CpG+BAFF) for untreated and interferon-β treated patients including measurement of intracellular IL-10 levels. Our flow experiments showed that interferon-β and fingolimod induced BAFF protein and mRNA expression (P ≤ 3.15 x 10^-4) without disproportional change in the antagonizing splice form. Protein BAFF correlated with an increase in transitional B cells (P = 5.70 x 10^-6), decrease in switched B cells (P = 3.29 x 10^-4), and reduction in B cell-surface BAFF-R expression (P = 2.70 x 10^-10), both on TACI-positive and -negative cells. TACI and BAFF-R RNA levels remained unaltered. RNA, plasma and in vitro experiments demonstrated that BAFF was not associated with increased IL-10 and IL-35 levels. In conclusion, treatment-induced BAFF correlates with a shift towards transitional B cells which are enriched for cells with an immunoregulatory function. However, BAFF does not directly influence the expression of the immunoregulatory cytokines IL-10 and IL-35. Furthermore, the post-translational mechanism of BAFF-induced BAFF-R cell surface loss was TACI-independent. These observations put the failure of pharmaceutical anti-BAFF strategies in perspective and provide insights for targeted B cell therapies.

Multivariate genome-wide analysis of immunoglobulin G N-glycosylation identifies new loci pleiotropic with immune function

The N-glycosylation of immunoglobulin G (IgG) affects its structure and function. It has been demonstrated that IgG N-glycosylation patterns are inherited as complex quantitative traits. Genome-wide association studies identified loci harboring genes encoding enzymes directly involved in protein glycosylation as well as loci likely to be involved in regulation of glycosylation biochemical pathways. Many of these loci could be linked to immune functions and risk of inflammatory and autoimmune diseases. The aim of the present study was to discover and replicate new loci associated with IgG N-glycosylation and to investigate possible pleiotropic effects of these loci onto immune function and the risk of inflammatory and autoimmune diseases. We conducted a multivariate genome-wide association analysis of 23 IgG N-glycosylation traits measured in 8090 individuals of European ancestry. The discovery stage was followed up by replication in 3147 people and in silico functional analysis. Our study increased the total number of replicated loci from 22 to 29. For the discovered loci, we suggest a number of genes potentially involved in the control of IgG N-glycosylation. Among the new loci, two (near RNF168 and TNFRSF13B) were previously implicated in rare immune deficiencies and were associated with levels of circulating immunoglobulins. For one new locus (near AP5B1/OVOL1), we demonstrated a potential pleiotropic effect on the risk of asthma. Our findings underline an important link between IgG N-glycosylation and immune function and provide new clues to understanding their interplay.

Gut T cell-independent IgA responses to commensal bacteria require engagement of the TACI receptor on B cells

The gut mounts secretory immunoglobulin A (SIgA) responses to commensal bacteria through nonredundant T cell-dependent (TD) and T cell-independent (TI) pathways that promote the establishment of mutualistic host-microbiota interactions. SIgAs from the TD pathway target penetrant bacteria, and their induction requires engagement of CD40 on B cells by CD40 ligand on T follicular helper cells. In contrast, SIgAs from the TI pathway bind a larger spectrum of bacteria, but the mechanism underpinning their production remains elusive. Here, we show that the intestinal TI pathway required CD40-independent B cell-activating signals from TACI, a receptor for the innate CD40 ligand-like factors BAFF and APRIL. TACI-induced SIgA responses targeted a fraction of the gut microbiota without shaping its overall composition. Of note, TACI was dispensable for TD induction of IgA in gut-associated lymphoid organs. Thus, BAFF/APRIL signals acting on TACI orchestrate commensal bacteria-specific SIgA responses through an intestinal TI program.

Transmembrane Activator and CAML Interactor (TACI): Another Potential Target for Immunotherapy of Multiple Myeloma?

Multiple myeloma (MM) has emerged as the next most likely oncological or hematological disease indication amenable for cellular immunotherapy. Much of the attention has been focused on B cell maturation antigen (BCMA) as a unique cell surface protein on myeloma cells that is available for monoclonal antibodies, antibody drug conjugates (ADCs), T-cell redirecting bispecific molecules, and chimeric antigen receptor (CAR) T cell targeting. BCMA is a member of the tumor necrosis factor receptor (TNFR) superfamily that binds two ligands B-cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL) and mediates the growth and survival of plasma and MM cells. Interestingly, transmembrane activator and CAML interactor (TACI), another TNFR superfamily member, also binds the same ligands and plays largely overlapping roles as BCMA in normal plasma and malignant MM cells. In this article, we review the biology of TACI, focusing on its role in normal B and plasma cells and malignant MM cells, and also discuss various ways to incorporate TACI as a potential target for immunotherapies against MM.

Unraveling the mysteries of plasma cells

Antibody-secreting plasma cells are the central pillars of humoral immunity. They are generated in a fundamental cellular restructuring process from naive B cells upon contact with antigen. This outstanding process is guided and controlled by a complex transcriptional network accompanied by a fascinating morphological metamorphosis, governed by the combined action of Blimp-1, Xbp-1 and IRF-4. The survival of plasma cells requires the intimate interaction with a specific microenvironment, consisting of stromal cells and cells of hematopoietic origin. Cell-cell contacts, cytokines and availability of metabolites such as glucose and amino acids modulate the survival abilities of plasma cells in their niches. Moreover, plasma cells have been shown to regulate immune responses by releasing cytokines. Furthermore, plasma cells are central players in autoimmune diseases and malignant transformation of plasma cells can result in the generation of multiple myeloma. Hence, the development of sophisticated strategies to deplete autoreactive plasma cells and myeloma cells represents a challenge for current and future research.

Rare TACI Mutation in a 3-Year-Old Boy With CVID Phenotype

Common variable immunodeficiency (CVID) is the most common and clinically relevant primary immunodeficiency (PID). Genetic basis of CVID remains largely unknown. However, in a minority of CVID patients, a number of distinct genetic defects affecting the normal processes of B cell maturation and differentiation into memory B cells have now been identified, resulting in markedly reduced serum levels of immunoglobulin G (IgG) and low immunoglobulin A (IgA) or immunoglobulin M (IgM), with impaired antibody responses, despite the presence of normal levels of B cells. Patients with CVID develop recurrent and chronic infections of respiratory and gastrointestinal tracts, autoimmune diseases, lymphoproliferative complications, malignancies, and granulomatous disease. We report the case of a boy admitted to our unit for the first time at the age of three for reduced gamma globulin levels and a clinical history positive for two episodes of pneumonia. Our patient incompletely met ESID diagnostic criteria for CVID, but molecular genetic analysis, a NGS panel including 47 PID-associated genes was performed in the proband and in his parents, revealing the presence of a heterozygous nucleotide substitution in exon 4 (c.579C>A) of TNFRSF13B encoding TACI. This mutation has been described only in two CVID adult patients and in a child with selective IgA deficiency (sIgAD). We highlighted the same mutation in the asymptomatic mother and detected two extra heterozygous mutations of RIG1 and LIG1. We promptly started intravenous immunoglobulin (IVIG) therapy with good tolerance. Despite the diagnosis of CVID remains clinical, in this case report we underline the importance of considering and planning genetic workup in all subjects with unclear diagnosis and of reporting new molecular diagnosis especially in case of rare mutations.

TACI Isoforms Regulate Ligand Binding and Receptor Function

TACI signals activate B cell proliferation, isotype switch and antibody production in both normal immunity and autoimmune states. In contrast to murine TACI, the human TACI gene undergoes alternative splicing to produce short and long isoforms (TACI-S and TACI-L). In previous studies, we showed that transduction of the short, but not long isoform, into murine B cells or human pre-B cells lacking TACI, caused them to become transcriptional and morphologically identical to plasma cells. These data suggest that the expression of different isoforms in humans provides unique controls on B cell maturation. In these studies we show that TACI-S and TACI-L form complexes in a ligand-independent manner, not dependent on a single extracellular domain. Both TACI isoforms are detectable in the endosomal cellular compartment where they co-localize with MyD88, TRAF6, and the activated 65 kDa form of TLR9, depending on a conserved intracellular TACI sequence. In contrast to TACI-L expressing cells, or cells bearing both isoforms, TACI-S binds ligands BAFF and APRIL with substantially greater affinity and promotes enhanced NF-kB activation. Using isoform-specific monoclonal antibodies, we show that while TACI-L is predominant as a surface receptor surface on human B cells, significantly more TACI-S is noted in the intracellular compartment and also in marginal zone, isotype switched and plasmablast in resting B cells. TACI-S is increased in tonsillar B cells and also in the intracellular compartment of activated peripheral B cells. These data shows that alternative splicing of the human TACI gene leads to two isoforms both of which intersect with MyD88 and TRAF6 and form complexes with TLR9, but the two isoforms have different ligand binding capacities, subcellular locations and activation capabilities.

Novel Insights on the Regulation of B Cell Functionality by Members of the Tumor Necrosis Factor Superfamily in Jawed Fish

Most ligands and receptors from the tumor necrosis factor (TNF) superfamily play very important roles in the immune system. In particular, many of these molecules are essential in the regulation of B cell biology and B cell-mediated immune responses. Hence, in mammals, it is known that many TNF family members play a key role on B cell development, maturation, homeostasis, activation, and differentiation, also influencing the ability of B cells to present antigens or act as regulators of immune responses. Evolutionarily, jawed fish (including cartilaginous and bony fish) constitute the first animal group in which an adaptive immune response based on B cells and immunoglobulins is present. However, until recently, not much was known about the expression of TNF ligands and receptors in these species. The sequences of many members of the TNF superfamily have been recently identified in different species of jawed fish, thus allowing posterior analysis on the role that these ligands and receptors have on B cell functionality. In this review, we summarize the current knowledge on the impact that the TNF family members have in different aspects of B cell functionality in fish, also providing an in depth comparison with functional aspects of TNF members in mammals, that will permit a further understanding of how B cell functionality is regulated in these distant animal groups.

B-cell phenotype and IgD-CD27- memory B cells are affected by TNF-inhibitors and tocilizumab treatment in rheumatoid arthritis

Background

The use of TNF-inhibitors and/or the IL-6 receptor antagonist, tocilizumab, in rheumatoid arthritis (RA) have pleiotropic effects that also involve circulating B-cells. The main goal of this study was to assess the effect of TNF-inhibitors and tocilizumab on B-cell phenotype and gene expression in RA.

Methods

Blood samples were collected from untreated early RA (ERA) patients, established RA patients under methotrexate treatment, established RA patients before and after treatment with TNF-inhibitors and tocilizumab, and healthy donors. B-cell subpopulations were characterized by flow cytometry and B-cell gene expression was analyzed by real-time PCR on isolated B-cells. Serum levels of BAFF, CXCL13 and sCD23 were determined by ELISA.

Results

The frequency of total CD19+ B cells in circulation was similar between controls and all RA groups, irrespective of treatment, but double negative (DN) IgD-CD27- memory B cells were significantly increased in ERA and established RA when compared to controls. Treatment with TNF-inhibitors and tocilizumab restored the frequency of IgD-CD27- B-cells to normal levels, but did not affect other B cell subpopulations. TACI, CD95, CD5, HLA-DR and TLR9 expression on B-cells significantly increased after treatment with either TNF-inhibitors and/ or tocilizumab, but no significant changes were observed in BAFF-R, BCMA, CD69, CD86, CXCR5, CD23, CD38 and IgM expression on B-cells when comparing baseline with post-treatment follow-ups. Alterations in B-cell gene expression of BAFF-R, TACI, TLR9, FcγRIIB, BCL-2, BLIMP-1 and β2M were found in ERA and established RA patients, but no significant differences were observed after TNF-inhibitors and tocilizumab treatment when comparing baseline and follow-ups. Serum levels of CXCL13, sCD23 and BAFF were not significantly affected by treatment with TNF-inhibitors and tocilizumab.

Conclusions

In RA patients, the use of TNF-inhibitors and/ or tocilizumab treatment affects B-cell phenotype and IgD-CD27- memory B cells in circulation, but not B-cell gene expression levels.

Smith-Magenis Syndrome Patients Often Display Antibody Deficiency but Not Other Immune Pathologies

BACKGROUND

Smith-Magenis syndrome (SMS) is a complex neurobehavioral disorder associated with recurrent otitis. Most SMS cases result from heterozygous interstitial chromosome 17p11.2 deletions that encompass not only the intellectual disability gene retinoic acid-induced 1 but also other genes associated with immunodeficiency, autoimmunity, and/or malignancy.

OBJECTIVES

The goals of this study were to describe the immunological consequence of 17p11.2 deletions by determining the prevalence of immunological diseases in subjects with SMS and by assessing their immune systems via laboratory methods.

METHODS

We assessed clinical histories of 76 subjects with SMS with heterozygous 17p11.2 deletions and performed in-depth immunological testing on 25 representative cohort members. Laboratory testing included determination of serum antibody concentrations, vaccine titers, and lymphocyte subset frequencies. Detailed reactivity profiles of SMS serum antibodies were performed using custom-made antigen microarrays.

RESULTS

Of 76 subjects with SMS, 74 reported recurrent infections including otitis (88%), pneumonia (47%), sinusitis (42%), and gastroenteritis (34%). Infections were associated with worsening SMS-related neurobehavioral symptoms. The prevalence of autoimmune and atopic diseases was not increased. Malignancy was not reported. Laboratory evaluation revealed most subjects with SMS to be deficient of isotype-switched memory B cells and many to lack protective antipneumococcal antibodies. SMS antibodies were not more reactive than control antibodies to self-antigens.

CONCLUSIONS

Patients with SMS with heterozygous 17p.11.2 deletions display an increased susceptibility to sinopulmonary infections, but not to autoimmune, allergic, or malignant diseases. SMS sera display an antibody reactivity profile favoring neither recognition of pathogen-associated antigens nor self-antigens. Prophylactic strategies to prevent infections may also provide neurobehavioral benefits to selected patients with SMS.

Increased expression of TACI on NOD B cells results in germinal centre reaction anomalies, enhanced plasma cell differentiation and immunoglobulin production

B cells have an important pathogenic role in the development of type 1 diabetes in the non-obese diabetic (NOD) mouse. We have previously reported that NOD mice display an increased percentage of TACI^high -expressing B cells compared with C57BL/6 mice and this trait is linked to chromosomes 1 and 8. In this paper the genetic association of the transmembrane activator, calcium modulator and cyclophilin ligand interactor (TACI) trait was confirmed using double congenic NOD.B6C1/Idd22 mice. TACI ligation by a proliferation-inducing ligand (APRIL) has been shown to influence plasma cell differentiation, immunoglobulin production and isotype switch. Hence, the functional consequence of the up-regulation of TACI on NOD B cells was analysed both in vitro and in vivo. NOD B cells stimulated with APRIL showed an enhanced plasma cell differentiation and class switch to IgG and IgA compared with B cells from C57BL/6 mice. Moreover, flow cytometry analyses revealed that germinal centre B cells in NOD failed to down-regulate TACI. Availability of the TACI ligand B-cell activating factor (BAFF) has been shown to be a limiting factor in the germinal centre reaction. In line with this, upon immunization with 4-hydroxy-3-nitrophenylacetyl hapten-conjugated hen egg lysozyme, NOD mice produced higher titres of low-affinity antibodies compared with C57BL/6 mice. This observation was supported by the detection of increased levels of BAFF in NOD germinal centres after immunization compared with C57BL/6 by immunofluorescence. Our results support the hypothesis that increased TACI expression on NOD B cells contributes to the pathogenesis of type 1 diabetes in the NOD mouse.

TNF receptor superfamily member 13b (TNFRSF13B) hemizygosity reveals transmembrane activator and CAML interactor haploinsufficiency at later stages of B-cell development

BACKGROUND

Heterozygous C104R or A181E TNF receptor superfamily member 13b (TNFRSF13B) mutations impair removal of autoreactive B cells, weaken B-cell activation, and convey to patients with common variable immune deficiency (CVID) an increased risk for autoimmunity. How mutant transmembrane activator and CAML interactor (TACI) influences wild-type TACI function is unclear; different models suggest either a dominant negative effect or haploinsufficiency.

OBJECTIVE

We investigated potential TACI haploinsufficiency by analyzing patients with antibody-deficient Smith-Magenis syndrome (SMS) who possess only 1 TNFRSF13B allele and antibody-deficient patients carrying one c.204insA TNFRSF13B null mutation.

METHODS

We tested the reactivity of antibodies isolated from single B cells from patients with SMS and patients with a c.204insA TNFRSF13B mutation and compared them with counterparts from patients with CVID with heterozygous C104R or A181E TNFRSF13B missense mutations. We also assessed whether loss of a TNFRSF13B allele induced haploinsufficiency in naive and memory B cells and recapitulated abnormal immunologic features typical of patients with CVID with heterozygous TNFRSF13B missense mutations.

RESULTS

We found that loss of a TNFRSF13B allele does not affect TACI expression, activation responses, or establishment of central B-cell tolerance in naive B cells. Additionally, patients with SMS and those with a c.204insA TNFRSF13B mutation display normal regulatory T-cell function and peripheral B-cell tolerance. The lack of a TNFRSF13B allele did result in decreased TACI expression on memory B cells, resulting in impaired activation and antibody secretion.

CONCLUSION

TNFRSF13B hemizygosity does not recapitulate autoimmune features of CVID-associated C104R and A181E TNFRSF13B mutations, which likely encode dominant negative products, but instead reveals selective TACI haploinsufficiency at later stages of B-cell development.

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.

Differential induction of plasma cells by isoforms of human TACI

Subjects with common variable immune deficiency may have mutations in transmembrane activator calcium modulator and cyclophilin ligand interactor (TACI). Unlike the murine gene, human TACI undergoes alternative messenger (m)RNA splicing to produce isoforms with 1 or 2 ligand-binding domains. Because both isoforms are found in human B cells, we compared their functions in transduced murine B and human pre-B cells. Although murine cells and pre-B cells transduced with the long TACI isoform retained surface CD19 and immunoglobulin G, cells transduced with the short TACI isoform completely lost these B-cell characteristics. Expression of the short TACI isoform produced intense nuclear factor κB activation, nuclear p65 translocation, and colocalization with myeloid differentiation factor 88 and calcium-modulating cyclophilin ligand. The short TACI-transduced cells became larger and CD138 positive, demonstrated upregulated BLIMP1 and XBP1 mRNA, and acquired the morphology of plasma cells. In contrast, cells bearing the long isoform had significantly less BLIMP1 and XBP1 mRNA and, for human pre-B cells, remained CD138 negative. Although human B cells express both isoforms, the short isoform predominates in CD27(+) B cells, toll-like receptor 9-activated peripheral B cells, and splenic marginal zone B cells. Although the transcriptional controls for alternative splicing of isoforms remain unknown, differential signals via isoforms may control plasma-cell generation in humans.

Roles of ligands from the TNF superfamily in B cell development, function, and regulation

Most ligands from the tumour necrosis factor (TNF) superfamily play very important roles in the immune system, and particularly so in B lymphocyte biology. TNF ligands are essential to many aspects of normal B cell biology from development in the bone marrow to maturation in the periphery as well as for activation and differentiation into germinal centre, memory or plasma cells. TNF ligands also influence other aspects of B cell biology such as their ability to present antigens or regulate immune responses. Importantly, inadequate regulation of many TNF ligands is associated with B cell disorders including autoimmunity and cancers. As a result, inhibitors of a number of TNF ligands have been tested in the clinic, with some becoming very successful approved treatments alleviating B cell-mediated pathologies.

Common variable immunodeficiency is associated with a functional deficiency of invariant natural killer T cells

BACKGROUND

Common variable immunodeficiency (CVID) is the commonest symptomatic primary antibody disorder, with monogenic causes identified in less than 10% of all cases. X-linked proliferative disease is a monogenic disorder that is associated with hypogammaglobulinemia and characterized by a deficiency of invariant NKT (iNKT) cells. We sought to evaluate whether a defect in iNKT cell number or function was associated with CVID.

OBJECTIVE

An evaluation of the function and number of iNKT cells in CVID.

METHODS

Six-color flow cytometry enumerated iNKT cells in 36 patients with CVID and 50 healthy controls. Their proliferative capacity and cytokine production (IFN-γ, IL-13, IL-17) was then investigated following activation with CD1d ligand alpha-galactosylceramide.

RESULTS

A reduction in the number of iNKT cells (31 iNKT cells/10(5) T cells) in patients with CVID compared with healthy controls (100 iNKT cells/10(5) T cells) was observed (P < .0001). Two cohorts could be discerned within the CVID group: group 1 with an abnormal number of iNKT cells (n = 28) and group 2 with a normal number of iNKT cells (n = 8). This segregation coassociated with the proliferative capacity of iNKT cells between the 2 groups. However, differences in the function of iNKT cells were noted in group 2, in which an increase in IFN-γ (P = .0016) and a decrease in IL-17 (P = .0002) production was observed between patients with CVID and controls. Finally, a significant association was seen between the number of iNKT cells and the percentage of class-switched memory B cells and propensity to lymphoproliferation (P = .002) in patients with CVID.

CONCLUSION

iNKT cells are deficient and/or functionally impaired in most of the patients with CVID.

APRIL mediates peritoneal B-1 cell homeostasis

BLyS (B lymphocyte stimulator) family cytokines and receptors play key roles in B-2 cell maturation and survival, but their importance for B-1 cells remains less clear. Here we use knockout mice to show that APRIL (A proliferation-inducing ligand), but not BLyS, plays a role in peritoneal B-1 cell maintenance. APRIL likely exerts its effects on peritoneal B-1 cells through binding to HSPG (heparan sulfate proteoglycans) rather than to the TACI (transmembrane activator and cyclophilin ligand interactor) receptor. Finally, we show that peritoneal macrophages express high levels of APRIL message, and are a likely local source of the cytokine in this anatomic locale.

B cell activating factor inhibition impairs bacterial immunity by reducing T cell-independent IgM secretion

B cell activating factor of the tumor necrosis factor family (BAFF) is an essential survival factor for B cells and has been shown to regulate T cell-independent (TI) IgM production. During Ehrlichia muris infection, TI IgM secretion in the spleen was BAFF dependent, and antibody-mediated BAFF neutralization led to an impairment of IgM-mediated host defense. The failure of TI plasmablasts to secrete IgM was not a consequence of alterations in their generation, survival, or early differentiation, since all occurred normally in infected mice following BAFF neutralization. Gene expression characteristic of plasma cell differentiation was also unaffected by BAFF neutralization in vivo, and except for CD138, plasmablast cell surface marker expression was unaffected. IgM was produced, since it was detected intracellularly, and impaired secretion was not due to a failure to express the IgM secretory exon. Addition of BAFF to plasmablasts in vitro rescued IgM secretion, suggesting that BAFF signaling can directly regulate secretory processes. Our findings indicate that BAFF signaling can modulate TI host defense by acting at a late stage in B cell differentiation, via its regulation of terminal plasmablast differentiation and/or IgM secretion.

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.

BAFF and TACI gene expression are increased in patients with untreated very early rheumatoid arthritis

OBJECTIVE

B cells play important roles in rheumatoid arthritis (RA). Given the beneficial effect of B cell depletion therapy in RA as well as the observed alterations in B cell subpopulations in this disease, we evaluated whether changes in the expression of genes related to B cell survival and activation were already present in patients with untreated very early RA (VERA; < 6 weeks of disease duration).

METHODS

The expression of a group of B cell-related activation and survival genes was quantified in peripheral blood mononuclear cells from patients with VERA by real-time PCR and compared with untreated early RA (< 1 year), established treated RA, and other untreated early arthritis conditions. Serum B cell-activating factor belonging to the tumor necrosis factor family (BAFF) was quantified by ELISA.

RESULTS

BAFF gene expression and serum levels were highest in patients with VERA. The expression of BAFF receptor (BAFF-R) increased with disease progression, while transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) was elevated since the first weeks of RA onset. Paired box 5 gene expression was also increased at all RA stages. Chemokine (C-X-C motif) receptor 5 was elevated only in established RA. No differences were observed in B cell maturation antigen, activation-induced cytidine deaminase, B lymphocyte-induced maturation protein, and B cell lymphoma 2 expression.

CONCLUSION

Disturbances in the expression of B cell-related activation and survival genes, particularly BAFF and TACI, occur from the onset of RA and precede changes in BAFF-R. These alterations can lead to the development of autoreactive B cells from the first weeks of RA onset.

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.

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.

Naturally occurring mutation affecting the MyD88-binding site of TNFRSF13B impairs triggering of class switch recombination

Mutations in the transmembrane activator and calcium-modulating cyclophilin ligand interactor (TACI) were previously found to be associated with hypogammaglobulinemia in humans. It has been shown that proliferation inducing ligand (APRIL) elicits class switch recombination (CSR) by inducing recruitment of MyD88 to a TACI highly conserved cytoplasmic domain (THC). We have identified a patient with hypogammaglobulinemia carrying a missense mutation (S231R) predicted to affect the THC. Aiming to evaluate the relevance of this novel mutation of TACI in CSR induction, we tested the ability of TACI, TLR9, or/and CD40 ligands to trigger CSR in naive B cells and B-cell lines carrying S231R. IgG secretion was impaired when triggered by TACI or/and TLR9 ligands on S231R-naive B cells. Likewise, these stimuli induced less expression of activation-induced cytidine deaminase, I(γ)1-C(μ), and I(γ)1-C(μ), while induction by optimal CD40 stimulation was indistinguishable from controls. These cells also showed an impaired cooperation between TACI and TLR9 pathways, as well as a lack of APRIL-mediated enhancement of CD40 activation in suboptimal conditions. Finally, after APRIL ligation, S231R-mutated TACI failed to colocalize with MyD88. Collectively, these results highlight the requirement of an intact MyD88-binding site in TACI to trigger CSR.

Expression of CD40 and growth-inhibitory activity of CD40 agonist in ovarian carcinoma cells

The CD40 receptor is a member of the tumour necrosis factor receptor family and is widely expressed on various cell types. The antitumour activity of CD40 agonist antibody has been observed in B-cell-derived malignancies, but its activity on ovarian cancer remains unclear. However, in this paper, we first confirmed that the anti-CD40 agonist antibody could inhibit the growth of ovarian cancer cells and induce apoptosis. This study investigated the expression of CD40 by ovarian carcinoma tissues and cell lines, at the same time, we evaluated the effect of a recombinant soluble human CD40L (rshCD40L) and an anti-CD40 agonist antibody on cell growth and apoptosis. Flow cytometry and immunohistochemistry assay demonstrated that CD40 was expressed on ovarian carcinoma cell lines and primary ovarian carcinoma cells derived from ascites, as well as on ovarian carcinoma tissues. The growth inhibition of rshCD40L and the anti-CD40 agonist antibody on ovarian carcinoma cells was examined by MTT assay, and the proportion of apoptotic tumour cells was analysed by flow cytometry and Hoechst staining. Our study showed that CD40 was expressed on all ovarian carcinoma cell lines and was examined in 86.2% (162/188) of ovarian cancer tissue samples, but not in normal ovarian tissues (n = 20). Treatment with rshCD40L or anti-CD40 agonist antibody significantly inhibited ovarian carcinoma cell growth and induced apoptosis. Theses results suggest that CD40 is expressed on ovarian carcinoma cells, moreover, that rshCD40L and anti-CD40 agonist antibody have therapeutic potential to inhibit human ovarian cancer growth.

TACI deficiency impairs sustained Blimp-1 expression in B cells decreasing long-lived plasma cells in the bone marrow

Deficiencies in transmembrane activator and CAML interactor (TACI) result in common variable immune deficiency, a syndrome marked by recurrent infections with encapsulated microorganisms, impaired production of antibodies, and lymphoproliferation. How TACI promotes antibody production and inhibits lymphoproliferation is not understood. To answer this question, we studied the generation of immunity to protein antigens in both TACI-deficient and TACI-proficient mice. We show that TACI promotes sustained Blimp-1 expression by B cells responding to antigen, which in turn limits B-cell clonal expansion and facilitates differentiation of long-lived antibody-secreting cells. Short-term IgG secretion occurs independently of TACI as DNA double-strand breaks associated with isotype class switching induce Blimp-1 transiently, independently of TACI. Our results showing that TACI induces and maintains Blimp-1 provide, for the first time, a unified molecular and cellular mechanism explaining the primary features of common variable immune deficiency, exquisite vulnerability to infection with encapsulated organisms, lymphoproliferation, and hypogammaglobulinemia.

The plasma concentration of the B cell activating factor is increased in children with acute malaria

Malaria-specific antibody responses in children often appear to be short-lived but the mechanisms underlying this phenomenon are not well understood. In this study, we investigated the relationship between the B-cell activating factor (BAFF) and its receptors expressed on B cells with antibody responses during and after acute malaria in children. Our results demonstrate that BAFF plasma levels increased during acute malarial disease and reflected disease severity. The expression profiles for BAFF receptors on B cells agreed with rapid activation and differentiation of a proportion of B cells to plasma cells. However, BAFF receptor (BAFF-R) expression was reduced on all peripheral blood B cells during acute infection, but those children with the highest level of BAFF-R expression on B cells maintained schizont-specific immunoglobin G (IgG) over a period of 4 months, indicating that dysregulation of BAFF-R expression on B cells may contribute to short-lived antibody responses to malarial antigens in children. In summary, this study suggests a potential role for BAFF during malaria disease, both as a marker for disease severity and in shaping the differentiation pattern of antigen-specific B cells.

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.

Innate immune defects correlate with failure of antibody responses to H1N1/09 vaccine in HIV-infected patients

BACKGROUND

Mechanisms underlying the failure of influenza vaccine-induced antibody responses in HIV-infected persons are poorly understood.

OBJECTIVE

To investigate innate immune factors regulating B-cell function in HIV-infected persons and to correlate them with serologic responses to H1N1/09 vaccine.

METHODS

We evaluated immunologic characteristics of 17 HIV-infected patients and 8 healthy controls (HCs) at 0, 7, and 28 days (designated T0, T1, and T2) following a single 15-μg dose of nonadjuvanted H1N1/09 influenza vaccine by using flow cytometry, ELISpot, and ELISA. All HCs and 9 patients (53%) seroconverted with >1:40 hemagglutination inhibition antibody titer at T2.

RESULTS

In vaccine responders and HCs, serum levels of BAFF (B cell-activating factor) and APRIL (a proliferation-inducing ligand) increased from T0 to T2 in conjunction with increases in frequencies of memory B cells. Concurrently, receptors for these factors showed changes, with increases in expression of TACI (transmembrane activator and calcium modulator and cyclophilin ligand interactor) and decreases in BAFF receptor in memory B cells. IL-2 secreting cells and IgG antibody-secreting cells increased at T2 in vaccine responders and HCs in ex vivo H1N1 antigen-stimulated cultures. These immunologic responses were not evident at T1 and were deficient in vaccine nonresponder patients at T2. At T0, vaccine nonresponders had lower frequencies of BAFF receptor and TACI-expressing memory B cells than did responders.

CONCLUSION

Impaired memory B-cell responses, deficiencies in serum BAFF and APRIL, and alterations in their receptors on B cells were associated with failure of H1N1/09 influenza vaccine responses among virologically controlled HIV-infected patients.

Toll-like receptor 9, transmembrane activator and calcium-modulating cyclophilin ligand interactor, and CD40 synergize in causing B-cell activation

BACKGROUND

B cells receive activating signals from T cells through CD40, from microbial DNA through Toll-like receptor (TLR) 9, and from dendritic cells through transmembrane activator and calcium-modulating cyclophilin ligand interactor (TACI). TLR9 and CD40 ligation augment TACI-driven B-cell activation, but only the mechanism of synergy between CD40 and TACI has been explored. Synergy between CD40 and TLR9 in B-cell activation is controversial.

OBJECTIVE

We sought to examine the mechanisms by which TLR9 modulates CD40- and TACI-mediated activation of B cells and to determine whether all 3 receptors synergize to activate B cells.

METHODS

Naive murine B cells and human PBMCs were stimulated with combinations of anti-CD40, CpG, and a proliferation inducing ligand in the presence of IL-4. Proliferation was measured by means of tritiated thymidine incorporation. Immunoglobulin production was measured by means of ELISA. Class-switch recombination (CSR) was examined by measuring mRNA for germline transcripts, activation-induced cytidine deaminase (AICDA), and mature immunoglobulin transcripts. Plasma cell differentiation was examined by using syndecan-1/CD138 staining and mRNA expression of B lymphocyte-induced maturation protein 1 (Blimp-1).

RESULTS

TLR9 synergized with CD40 and TACI in driving CSR and inducing IgG(1) and IgE secretion by naive murine B cells and synergized with TACI in driving B-cell proliferation and plasma cell differentiation. All 3 receptors synergized together in driving murine B-cell proliferation, CSR, plasma cell differentiation, and IgG(1) and IgE secretion. TLR9 synergized with CD40 and TACI in driving IgG secretion in IL-4-stimulated human B cells.

CONCLUSION

Signals from TLR9, TACI, and CD40 are integrated to promote B-cell activation and differentiation.

Transmembrane activator and CAML interactor (TACI) haploinsufficiency results in B-cell dysfunction in patients with Smith-Magenis syndrome

BACKGROUND

Heterozygous deleterious mutations in the gene encoding the tumor necrosis factor receptor superfamily member 13b (TNFRSF13B), or transmembrane activator and CAML interactor (TACI), have been associated with the development of common variable immunodeficiency. Smith-Magenis syndrome (SMS) is a genetic disorder characterized by developmental delay, behavioral disturbances, craniofacial anomalies, and recurrent respiratory tract infections. Eighty percent of subjects have a chromosome 17p11.2 microdeletion, which includes TACI. The remaining subjects have mutations sparing this gene.

OBJECTIVE

We examined TACI protein expression and function in patients with SMS to define the role of TACI haploinsufficiency in B-cell function.

METHODS

We studied TACI expression and function in a cohort of 29 patients with SMS.

RESULTS

In patients with SMS with only 1 TACI allele, we found decreased B-cell extracellular and intracellular expression of TACI, reduced binding of a proliferation-inducing ligand, and decreased TACI-induced expression of activation-induced cytidine deaminase mRNA, but these were normal for cells from patients with SMS and 2 TACI alleles. Impaired upregulation of B-cell surface TACI expression by a Toll-like receptor 9 agonist was also observed in cells from patients with 1 TACI allele. Gene sequence analysis of the remaining TACI allele revealed common polymorphisms, with the exception of 1 patient with an amino acid change of uncertain significance. Patients with SMS with the lowest TACI expression had significantly reduced antibody responses to pneumococcal vaccine serotypes.

DISCUSSION

Our findings suggest that haploinsufficiency of the TACI gene results in humoral immune dysfunction, highlighting the role of genomic copy number variants in complex traits.

The C104R mutant impairs the function of transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) through haploinsufficiency

BACKGROUND

TNFRSF13B, which encodes transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), is mutated in 10% of patients with common variable immunodeficiency. One of the 2 most common TACI mutations in common variable immunodeficiency, C104R, abolishes ligand binding and is found predominantly in the heterozygous state. The murine TACI mutant C76R is the equivalent of the human TACI mutant C104R.

OBJECTIVE

We sought to define the consequence of the C76R mutation on TACI function in mice that express both wild-type TACI and the murine C76R mutant.

METHODS

Transgenic mice that express murine TACI C76R, the counterpart of human TACI C104R, on the TACI(+/-) B6/129 background (C76R/TACI(+/-) mice) were constructed. Serum immunoglobulins and antibody responses to the type II T-independent antigen trinitrophenylated (TNP)-Ficoll were determined by means of ELISA. B-cell proliferation in response to a proliferation-inducing ligand was determined based on tritiated thymidine incorporation into DNA. IgG1 secretion by B cells in response to a proliferation-inducing ligand plus IL-4 was determined by means of ELISA.

RESULTS

C76R/TACI(+/-) mice had significantly impaired antibody responses to the type II T-independent antigen TNP-Ficoll compared with TACI(+/+) B6/129 control animals, and their B cells were impaired in their capacity to proliferate and secrete IgG1 in response to TACI ligation. Unexpectedly, TACI(+/-) mice had similarly impaired B-cell function as C76R/TACI(+/-) littermates. Impaired TACI function caused by haploinsufficiency was confirmed in TACI(+/-) mice on the C57BL/6 background.

CONCLUSION

These results suggest that the human TACI mutant C104R might impair TACI function in heterozygotes through haploinsufficiency.

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.

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.

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.

The transmembrane activator TACI triggers immunoglobulin class switching by activating B cells through the adaptor MyD88

BAFF and APRIL are innate immune mediators that trigger immunoglobulin (Ig) G and IgA class switch recombination (CSR) in B cells by engaging the receptor TACI. The mechanism underlying CSR signaling by TACI remains unknown. Here, we found that the cytoplasmic domain of TACI encompasses a conserved motif that bound MyD88, an adaptor protein that activates NF-κB signaling pathways via a Toll-interleukin-1 receptor (TIR) domain. TACI lacks a TIR domain, yet triggered CSR via the DNA-editing enzyme AID by activating NF-κB through a TLR-like MyD88–IRAK-1-IRAK-4–TRAF6–TAK1 pathway. TACI-induced CSR was impaired in mice and humans lacking MyD88 or IRAK-4, indicating that MyD88 controls a B cell-intrinsic, TIR-independent, TACI-dependent pathway for Ig diversification.