Btk levels set the threshold for B-cell activation and negative selection of autoreactive B cells in mice

Secreted IgM deficiency leads to increased BCR signaling that results in abnormal splenic B cell development

Mice lacking secreted IgM (sIgM ^-/-) antibodies display abnormal splenic B cell development, which results in increased marginal zone and decreased follicular B cell numbers. However, the mechanism by which sIgM exhibit this effect is unknown. Here, we demonstrate that B cells in sIgM ^-/- mice display increased B cell receptor (BCR) signaling as judged by increased levels of phosphorylated Bruton's tyrosine kinase (pBtk), phosphorylated Spleen tyrosine kinase (pSyk), and nuclear receptor Nur77. Low dosage treatment with the pBtk inhibitor Ibrutinib reversed the altered B cell development in the spleen of sIgM ^-/- mice, suggesting that sIgM regulate splenic B cell differentiation by decreasing BCR signaling. Mechanistically, we show that B cells, which express BCRs specific to hen egg lysozyme (HEL) display diminished responsiveness to HEL stimulation in presence of soluble anti-HEL IgM antibodies. Our data identify sIgM as negative regulators of BCR signaling and suggest that they can act as decoy receptors for self-antigens that are recognized by membrane bound BCRs.

Cell lines generated from a chronic lymphocytic leukemia mouse model exhibit constitutive Btk and Akt signaling

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of mature CD5⁺ B cells in blood. Spontaneous apoptosis of CLL cells in vitro has hampered in-depth investigation of CLL pathogenesis. Here we describe the generation of three monoclonal mouse cell lines, EMC2, EMC4 and EMC6, from the IgH.TEμ CLL mouse model based on sporadic expression of SV40 large T antigen. The cell lines exhibit a stable CD5⁺CD43⁺IgM⁺CD19⁺ CLL phenotype in culture and can be adoptively transferred into Rag1^−/− mice. RNA-seq analysis revealed only minor differences between the cell lines and their primary tumors and suggested that NF-κB and mTOR signaling pathways were involved in cell line outgrowth. In vitro survival and proliferation was dependent on constitutive phosphorylation of Bruton's tyrosine kinase (Btk) at Y551/Y223, and Akt(S473). Treatment of the cell lines with small molecule inhibitors specific for Btk (ibrutinib) or PI3K (idelalisib), which is upstream of Akt, resulted in reduced viability, proliferation and fibronectin-dependent cell adhesion. Treatment of cell line-engrafted Rag1^−/− mice with ibrutinib was associated with transient lymphocytosis, reduced splenomegaly and increased overall survival. Thus, by generating stable cell lines we established a novel platform for in vitro and in vivo investigation of CLL signal transduction and treatment modalities.

Enhanced Bruton's Tyrosine Kinase Activity in Peripheral Blood B Lymphocytes From Patients With Autoimmune Disease

OBJECTIVE

Bruton's tyrosine kinase (BTK) transmits crucial survival signals from the B cell receptor (BCR) in B cells. Pharmacologic BTK inhibition effectively diminishes disease symptoms in mouse models of autoimmunity; conversely, transgenic BTK overexpression induces systemic autoimmunity in mice. We undertook this study to investigate BTK expression and activity in human B cells in the context of autoimmune disease.

METHODS

Using intracellular flow cytometry, we quantified BTK expression and phosphorylation in subsets of peripheral blood B cells from 30 patients with rheumatoid arthritis (RA), 26 patients with primary Sjögren's syndrome (SS), and matched healthy controls.

RESULTS

In circulating B cells, BTK protein expression levels correlated with BTK phosphorylation. BTK expression was up-regulated upon BCR stimulation in vitro and was significantly higher in CD27+ memory B cells than in CD27-IgD+ naive B cells. Importantly, BTK protein and phospho-BTK were significantly increased in B cells from anti-citrullinated protein antibody (ACPA)-positive RA patients but not in B cells from ACPA-negative RA patients. BTK was increased both in naive B cells and in memory B cells and correlated with frequencies of circulating CCR6+ Th17 cells. Likewise, BTK protein was increased in B cells from a major fraction of patients with primary SS and correlated with serum rheumatoid factor levels and parotid gland T cell infiltration. Interestingly, targeting T cell activation in patients with primary SS using the CTLA-4Ig fusion protein abatacept restored BTK protein expression in B cells to normal levels.

CONCLUSION

These data indicate that autoimmune disease in humans is characterized by enhanced BTK activity, which is linked not only to autoantibody formation but also to T cell activity.

Prospects for combining targeted and conventional cancer therapy with immunotherapy

Over the past 25 years, research in cancer therapeutics has largely focused on two distinct lines of enquiry. In one approach, efforts to understand the underlying cell-autonomous, genetic drivers of tumorigenesis have led to the development of clinically important targeted agents that result in profound, but often not durable, tumour responses in genetically defined patient populations. In the second parallel approach, exploration of the mechanisms of protective tumour immunity has provided several therapeutic strategies - most notably the 'immune checkpoint' antibodies that reverse the negative regulators of T cell function - that accomplish durable clinical responses in subsets of patients with various tumour types. The integration of these potentially complementary research fields provides new opportunities to improve cancer treatments. Targeted and immune-based therapies have already transformed the standard-of-care for several malignancies. However, additional insights into the effects of targeted therapies, along with conventional chemotherapy and radiation therapy, on the induction of antitumour immunity will help to advance the design of combination strategies that increase the rate of complete and durable clinical response in patients.

Btk-specific inhibition blocks pathogenic plasma cell signatures and myeloid cell-associated damage in IFN-driven lupus nephritis

Systemic lupus erythematosus (SLE) is often associated with exaggerated B cell activation promoting plasma cell generation, immune-complex deposition in the kidney, renal infiltration of myeloid cells, and glomerular nephritis. Type-I IFNs amplify these autoimmune processes and promote severe disease. Bruton's tyrosine kinase (Btk) inhibitors are considered novel therapies for SLE. We describe the characterization of a highly selective reversible Btk inhibitor, G-744. G-744 is efficacious, and superior to blocking BAFF and Syk, in ameliorating severe lupus nephritis in both spontaneous and IFNα-accelerated lupus in NZB/W_F1 mice in therapeutic regimens. Selective Btk inhibition ablated plasmablast generation, reduced autoantibodies, and - similar to cyclophosphamide - improved renal pathology in IFNα-accelerated lupus. Employing global transcriptional profiling of spleen and kidney coupled with cross-species human modular repertoire analyses, we identify similarities in the inflammatory process between mice and humans, and we demonstrate that G-744 reduced gene expression signatures essential for splenic B cell terminal differentiation, particularly the secretory pathway, as well as renal transcriptional profiles coupled with myeloid cell-mediated pathology and glomerular plus tubulointerstitial disease in human glomerulonephritis patients. These findings reveal the mechanism through which a selective Btk inhibitor blocks murine autoimmune kidney disease, highlighting pathway activity that may translate to human SLE.

Distinct and Overlapping Functions of TEC Kinase and BTK in B Cell Receptor Signaling

The Tec tyrosine kinase is expressed in many cell types, including hematopoietic cells, and is a member of the Tec kinase family that also includes Btk. Although the role of Btk in B cells has been extensively studied, the role of Tec kinase in B cells remains largely unclear. It was previously shown that Tec kinase has the ability to partly compensate for loss of Btk activity in B cell differentiation, although the underlying mechanism is unknown. In this study, we confirm that Tec kinase is not essential for normal B cell development when Btk is present, but we also found that Tec-deficient mature B cells showed increased activation, proliferation, and survival upon BCR stimulation, even in the presence of Btk. Whereas Tec deficiency did not affect phosphorylation of phospholipase Cγ or Ca²⁺ influx, it was associated with significantly increased activation of the intracellular Akt/S6 kinase signaling pathway upon BCR and CD40 stimulation. The increased S6 kinase phosphorylation in Tec-deficient B cells was dependent on Btk kinase activity, as ibrutinib treatment restored pS6 to wild-type levels, although Btk protein and phosphorylation levels were comparable to controls. In Tec-deficient mice in vivo, B cell responses to model Ags and humoral immunity upon influenza infection were enhanced. Moreover, aged mice lacking Tec kinase developed a mild autoimmune phenotype. Taken together, these data indicate that in mature B cells, Tec and Btk may compete for activation of the Akt signaling pathway, whereby the activating capacity of Btk is limited by the presence of Tec kinase.

Potential influences of complement factor H in autoimmune inflammatory and thrombotic disorders

Complement system homeostasis is important for host self-protection and anti-microbial immune surveillance, and recent research indicates roles in tissue development and remodelling. Complement also appears to have several points of interaction with the blood coagulation system. Deficiency and altered function due to gene mutations and polymorphisms in complement effectors and regulators, including Factor H, have been associated with familial and sporadic autoimmune inflammatory - thrombotic disorders, in which autoantibodies play a part. These include systemic lupus erythematosus, rheumatoid arthritis, atypical haemolytic uremic syndrome, anti-phospholipid syndrome and age-related macular degeneration. Such diseases are generally complex - multigenic and heterogeneous in their symptoms and predisposition/susceptibility. They usually need to be triggered by vascular trauma, drugs or infection and non-complement genetic factors also play a part. Underlying events seem to include decline in peripheral regulatory T cells, dendritic cell, and B cell tolerance, associated with alterations in lymphoid organ microenvironment. Factor H is an abundant protein, synthesised in many cell types, and its reported binding to many different ligands, even if not of high affinity, may influence a large number of molecular interactions, together with the accepted role of Factor H within the complement system. Factor H is involved in mesenchymal stem cell mediated tolerance and also contributes to self-tolerance by augmenting iC3b production and opsonisation of apoptotic cells for their silent dendritic cell engulfment via complement receptor CR3, which mediates anti-inflammatory-tolerogenic effects in the apoptotic cell context. There may be co-operation with other phagocytic receptors, such as complement C1q receptors, and the Tim glycoprotein family, which specifically bind phosphatidylserine expressed on the apoptotic cell surface. Factor H is able to discriminate between self and nonself surfaces for self-protection and anti-microbe defence. Factor H, particularly as an abundant platelet protein, may also modulate blood coagulation, having an anti-thrombotic role. Here, we review a number of interaction pathways in coagulation and in immunity, together with associated diseases, and indicate where Factor H may be expected to exert an influence, based on reports of the diversity of ligands for Factor H.

Bruton's Tyrosine Kinase Deficiency Inhibits Autoimmune Arthritis in Mice but Fails to Block Immune Complex-Mediated Inflammatory Arthritis

OBJECTIVE

Bruton's tyrosine kinase (BTK) is a B cell signaling protein that also contributes to innate immunity. BTK inhibitors prevent autoimmune arthritis but have off-target effects, and the mechanisms of protection remain unknown. We undertook these studies using genetic deletion to investigate the role of BTK in adaptive and innate immune responses that drive inflammatory arthritis.

METHODS

BTK-deficient K/BxN mice were generated to study the role of BTK in a spontaneous model that requires both adaptive and innate immunity. The K/BxN serum-transfer model was used to bypass the adaptive system and elucidate the role of BTK in innate immune contributions to arthritis.

RESULTS

BTK deficiency conferred disease protection to K/BxN mice, confirming outcomes of BTK inhibitors. B lymphocytes were profoundly reduced, more than in other models of BTK deficiency. Subset analysis revealed loss of B cells at all developmental stages. Germinal center B cells were also decreased, with downstream effects on numbers of follicular helper T cells and greatly reduced autoantibodies. In contrast, total IgG was only mildly decreased. Strikingly, and in contrast to small molecule inhibitors, BTK deficiency had no effect in the serum-transfer model of arthritis.

CONCLUSION

BTK contributes to autoimmune arthritis primarily through its role in B cell signaling and not through innate immune components.

B-cell subsets, signaling and their roles in secretion of autoantibodies

B cells play a pivotal role in the pathogenesis of autoimmune diseases. In patients with systemic lupus erythematosus (SLE), the percentages of plasmablasts and IgD−CD27− double-negative memory B cells in peripheral blood are significantly increased, while IgD+CD27+ IgM memory B cells are significantly decreased compared to healthy donors. The phenotypic change is significantly associated with disease activity and concentration of autoantibodies. Treatment of B-cell depletion using rituximab results in the reconstitution of peripheral B cells in SLE patients with subsequent improvement in disease activity. Numerous studies have described abnormalities in B-cell receptor (BCR)-mediated signaling in B cells of SLE patients. Since differences in BCR signaling are considered to dictate the survival or death of naïve and memory B cells, aberrant BCR signal can lead to abnormality of B-cell subsets in SLE patients. Although Syk and Btk function as key molecules in BCR signaling, their pathological role in SLE remains unclear. We found that Syk and Btk do not only transduce activation signal through BCR, but also mediate crosstalk between BCR and Toll-like receptor (TLR) as well as BCR and JAK-STAT pathways in human B cells in vitro. In addition, pronounced Syk and Btk phosphorylation was observed in B cells of patients with active SLE compared to those of healthy individuals. The results suggest the involvement of Syk and Btk activation in abnormalities of BCR-mediated signaling and B-cell phenotypes during the pathological process of SLE and that Syk, Btk and JAK are potential therapeutic targets in SLE.

Use of Biologics in Sjögren's Syndrome

The management of patients suffering from primary Sjögren syndrome (pSS) has long been mainly symptomatic and demonstration of effectiveness of systemic drugs was lacking. However, progress made in the understanding of pSS pathogenesis has allowed moving into a more targeted approach to therapeutic intervention. Given the key role of chronic B-cell activation, B-cell target therapies were the first candidates. New pathways are currently being investigated, including costimulation and ectopic germinal center. In this review, we summarize the current evidence regarding B-cell targeted and anti-TNF therapies and provide an overview of promising drugs in the pipeline.

Systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease that can affect many organs, including the skin, joints, the central nervous system and the kidneys. Women of childbearing age and certain racial groups are typically predisposed to developing the condition. Rare, inherited, single-gene complement deficiencies are strongly associated with SLE, but the disease is inherited in a polygenic manner in most patients. Genetic interactions with environmental factors, particularly UV light exposure, Epstein-Barr virus infection and hormonal factors, might initiate the disease, resulting in immune dysregulation at the level of cytokines, T cells, B cells and macrophages. Diagnosis is primarily clinical and remains challenging because of the heterogeneity of SLE. Classification criteria have aided clinical trials, but, despite this, only one drug (that is, belimumab) has been approved for use in SLE in the past 60 years. The 10-year mortality has improved and toxic adverse effects of older medications such as cyclophosphamide and glucocorticoids have been partially offset by newer drugs such as mycophenolate mofetil and glucocorticoid-sparing regimes. However, further improvements have been hampered by the adverse effects of renal and neuropsychiatric involvement and late diagnosis. Adding to this burden is the increased risk of premature cardiovascular disease in SLE together with the risk of infection made worse by immunosuppressive therapy. Challenges remain with treatment-resistant disease and symptoms such as fatigue. Newer therapies may bring hope of better outcomes, and the refinement to stem cell and genetic techniques might offer a cure in the future.

Enhanced Expression of Bruton's Tyrosine Kinase in B Cells Drives Systemic Autoimmunity by Disrupting T Cell Homeostasis

Upon BCR stimulation, naive B cells increase protein levels of the key downstream signaling molecule Bruton's tyrosine kinase (BTK). Transgenic CD19-hBtk mice with B cell-specific BTK overexpression show spontaneous germinal center formation, anti-nuclear autoantibodies, and systemic autoimmunity resembling lupus and Sjögren syndrome. However, it remains unknown how T cells are engaged in this pathology. In this study, we found that CD19-hBtk B cells were high in IL-6 and IL-10 and disrupted T cell homeostasis in vivo. CD19-hBtk B cells promoted IFN-γ production by T cells and expression of the immune-checkpoint protein ICOS on T cells and induced follicular Th cell differentiation. Crosses with CD40L-deficient mice revealed that increased IL-6 production and autoimmune pathology in CD19-hBtk mice was dependent on B-T cell interaction, whereas IL-10 production and IgM autoantibody formation were CD40L independent. Surprisingly, in Btk-overexpressing mice, naive B cells manifested increased CD86 expression, which was dependent on CD40L, suggesting that T cells interact with B cells in a very early stage of immune pathology. These findings indicate that increased BTK-mediated signaling in B cells involves a positive-feedback loop that establishes T cell-propagated autoimmune pathology, making BTK an attractive therapeutic target in autoimmune disease.

New biological therapies in Sjögren's syndrome

Primary Sjögren's syndrome (pSS) is a systemic autoimmune disease characterized by dryness and systemic involvement in more than a third of the patients. Patient management has suffered from the lack of effective treatments. However, progresses made in the understanding of pSS pathogenesis have allowed a move to a more targeted approach to therapeutic intervention. Given the key role of chronic B cell activation, B cell-targeted therapies were the first candidate. New pathways are currently investigated including costimulation and ectopic germinal centre formation. In this review, we have summarized the new tools available in clinical research in the field of pSS, the current evidence regarding B cell-targeted therapies and an overview of the promising drugs in the pipeline.

Interfering with baffled B cells at the lupus tollway: Promises, successes, and failed expectations

B cells play an important role in systemic lupus erythematosus by acting not only as precursors of autoantibody-producing cells but also as antigen-presenting, cytokine-secreting, and regulatory cells. Unopposed activation of B cells through their B-cell receptor for antigen, as seen in B cells lacking Lyn kinase, results in systemic autoimmunity. The B-cell activating factor of the TNF family (BAFF), nucleic acid-sensing Toll-like receptors (TLRs), and type I interferon can affect B-cell survival and decrease their threshold for activation. Herein we discuss both direct and indirect strategies aimed at targeting B cells in patients with lupus by blocking BAFF, type I interferon, or TLR7 to TLR9. Although BAFF-depleting therapy with belimumab achieved approval for lupus, other BAFF inhibitors were much less beneficial in clinical trials. Inhibitors of the B-cell receptor for antigen signaling and antibodies against type I interferon are in the pipeline. The TLR7 to TLR9 blocker hydroxychloroquine has been in use in patients with lupus for more than 50 years, but oligonucleotide-based inhibitors of TLR7 to TLR9, despite showing promise in animal models of lupus, have not reached the primary end point in a recent phase 1 trial. These data point toward possible redundancies in B-cell signaling/survival pathways, which must be better understood before future clinical trials are executed.

The role of Bruton's tyrosine kinase in autoimmunity and implications for therapy

Bruton's tyrosine kinase (BTK) mediates B cell signaling and is also present in innate immune cells but not T cells. BTK propagates B cell receptor (BCR) responses to antigen-engagement as well as to stimulation via CD40, toll-like receptors (TLRs), Fc receptors (FCRs) and chemokine receptors. Importantly, BTK can modulate signaling, acting as a "rheostat" rather than an "on-off" switch; thus, overexpression leads to autoimmunity while decreased levels improve autoimmune disease outcomes. Autoreactive B cells depend upon BTK for survival to a greater degree than normal B cells, reflected as loss of autoantibodies with maintenance of total antibody levels when BTK is absent. This review describes contributions of BTK to immune tolerance, including studies testing BTK-inhibitors for treatment of autoimmune diseases.

The Role of Aggregates of Therapeutic Protein Products in Immunogenicity: An Evaluation by Mathematical Modeling

Therapeutic protein products (TPP) have been widely used to treat a variety of human diseases, including cancer, hemophilia, and autoimmune diseases. However, TPP can induce unwanted immune responses that can impact both drug efficacy and patient safety. The presence of aggregates is of particular concern as they have been implicated in inducing both T cell-independent and T cell-dependent immune responses. We used mathematical modeling to evaluate several mechanisms through which aggregates of TPP could contribute to the development of immunogenicity. Modeling interactions between aggregates and B cell receptors demonstrated that aggregates are unlikely to induce T cell-independent immune responses by cross-linking B cell receptors because the amount of signal transducing complex that can form under physiologically relevant conditions is limited. We systematically evaluate the role of aggregates in inducing T cell-dependent immune responses using a recently developed multiscale mechanistic mathematical model. Our analysis indicates that aggregates could contribute to T cell-dependent immune response by inducing high affinity epitopes which may not be present in the nonaggregated TPP and/or by enhancing danger signals to break tolerance. In summary, our computational analysis is suggestive of novel insights into the mechanisms underlying aggregate-induced immunogenicity, which could be used to develop mitigation strategies.

Long-Range Regulation of V(D)J Recombination

Given their essential role in adaptive immunity, antigen receptor loci have been the focus of analysis for many years and are among a handful of the most well-studied genes in the genome. Their investigation led initially to a detailed knowledge of linear structure and characterization of regulatory elements that confer control of their rearrangement and expression. However, advances in DNA FISH and imaging combined with new molecular approaches that interrogate chromosome conformation have led to a growing appreciation that linear structure is only one aspect of gene regulation and in more recent years, the focus has switched to analyzing the impact of locus conformation and nuclear organization on control of recombination. Despite decades of work and intense effort from numerous labs, we are still left with an incomplete picture of how the assembly of antigen receptor loci is regulated. This chapter summarizes our advances to date and points to areas that need further investigation.

Emerging and Novel Functions of Complement Protein C1q

Complement protein C1q, the recognition molecule of the classical pathway, performs a diverse range of complement and non-complement functions. It can bind various ligands derived from self, non-self, and altered self and modulate the functions of immune and non-immune cells including dendritic cells and microglia. C1q involvement in the clearance of apoptotic cells and subsequent B cell tolerance is more established now. Recent evidence appears to suggest that C1q plays an important role in pregnancy where its deficiency and dysregulation can have adverse effects, leading to preeclampsia, missed abortion, miscarriage or spontaneous loss, and various infections. C1q is also produced locally in the central nervous system, and has a protective role against pathogens and possible inflammatory functions while interacting with aggregated proteins leading to neurodegenerative diseases. C1q role in synaptic pruning, and thus CNS development, its anti-cancer effects as an immune surveillance molecule, and possibly in aging are currently areas of extensive research.

Animal Models of Interferon Signature Positive Lupus

Human lupus is strongly associated with a gene expression signature characterized by over-expression of Type I interferon-regulated genes. A strong interferon signature generally is not seen in the standard mouse models of lupus, despite considerable evidence for the involvement of toll-like receptor-driven interferon production. In contrast, pristane-induced lupus exhibits a prominent TLR7-dependent interferon signature. Importantly, genetic disorders with dysregulated interferon production in both human beings and mice cause severe autoinflammatory diseases but not the typical manifestations of lupus, suggesting that interferon over-production is insufficient to cause systemic lupus erythematosus itself. Single-gene models in mice suggest that lupus-like disease may result from abnormalities in B-cell activation and the clearance of dead cells. Pristane may mimic human systemic lupus erythematosus by causing synergistic abnormalities in interferon production along with defective clearance of apoptotic cells and over-active B-cell signaling.

Effect of Leflunomide on the Abnormal Expression of Lipid Rafts and F-Actin in B Lymphocytes from Patients with Systemic Lupus Erythematosus

Purposes. To investigate the possible changes in B cell subsets and in B cell expression patterns of lipid rafts (LRs) and F-actin in patients with SLE and whether leflunomide treatment may have effect on these changes. Methods. The B cell subsets and LRs expression were determined by flow cytometry and confocal microscopy, and F-actin expression was examined by confocal microscopy. Results. CD27⁺IgD⁺ B cell subsets were significantly decreased while CD38⁺CD95⁺ B cell subsets increased in SLE patients. The LRs levels of B cells were remarkably increased and positively correlated with SLEDAI and anti-dsDNA titer in SLE patients. The expression level of LRs was significantly higher in CD38⁺ B cells than CD38⁻ B cells and negatively correlated with C3 levels. The increased expression of LRs was associated with reduced expression of F-actin in the B cells from active SLE patients. Furthermore, in vitro treatment of the cells with A771726 reduced the expression level of LRs, attenuated the overaggregation of LRs, and normalized the distribution of F-actin. Conclusions. There were abnormalities in B cell subsets and LRs and F-actin expression of B cell from SLE patients. Modulation of B cell expression of LRs and F-actin by LEF could be a potential therapeutic target for SLE.

Mechanisms of autoantibody production in systemic lupus erythematosus

Autoantibodies against a panoply of self-antigens are seen in systemic lupus erythematosus, but only a few (anti-Sm/RNP, anti-Ro/La, anti-dsDNA) are common. The common lupus autoantigens are nucleic acid complexes and levels of autoantibodies can be extraordinarily high. We explore why that is the case. Lupus is associated with impaired central or peripheral B-cell tolerance and increased circulating autoreactive B cells. However, terminal differentiation is necessary for autoantibody production. Nucleic acid components of the major lupus autoantigens are immunostimulatory ligands for toll-like receptor (TLR)7 or TLR9 that promote plasma cell differentiation. We show that the levels of autoantibodies against the U1A protein (part of a ribonucleoprotein) are markedly higher than autoantibodies against other antigens, including dsDNA and the non-nucleic acid-associated autoantigens insulin and thyroglobulin. In addition to driving autoantibody production, TLR7 engagement is likely to contribute to the pathogenesis of inflammatory disease in lupus.

Increased frequency of a unique spleen tyrosine kinase bright memory B cell population in systemic lupus erythematosus

OBJECTIVE

Systemic lupus erythematosus (SLE) is characterized by B cell hyperactivity and autoantibody production. As spleen tyrosine kinase (Syk) is pivotal in B cell activation, these experiments aimed to examine the extent to which Syk was abnormally expressed in SLE B cells and the nature of the B cell subset that differently expressed Syk.

METHODS

B cells from healthy donors and SLE patients were analyzed by flow cytometry to assess basal expression of Syk and phosphorylated Syk. B cell subsets expressing higher levels of Syk were found, and their detailed phenotype, in vitro differentiation into plasmablasts/plasma cells, and Syk induction by cytokines were determined.

RESULTS

Syk expression was higher in CD27+ memory B cells than in naive B cells from SLE patients. However, a significantly increased frequency of CD27- B cells with bright expression of Syk (Syk++) was found in SLE patients. CD27-Syk++ B cells showed enhanced basal expression of p-Syk and stronger Syk phosphorylation upon B cell receptor (BCR) engagement as compared to CD27-Syk+ B cells. CD27-Syk++ B cells were CD38- as well as CD19++, CD20++, and mainly CD21-, with decreased ABCB1 transporter activity. In contrast to CD27-Syk+ B cells, CD27-Syk++ B cells exhibited enhanced differentiation into CD27++ IgG-secreting cells and expressed somatically mutated BCR gene rearrangements. Syk++ B cells were inducible in vitro by stimulation with interferon-γ, lipopolysaccharide, or tumor necrosis factor α.

CONCLUSION

SLE patients exhibit an increased frequency of hitherto unknown CD27-Syk++ memory-like B cells, indicating that intracellular Syk density could distinguish CD27- memory B cells from truly naive B cell subsets. Furthermore, the CD27-Syk++ subset is a candidate for a source of increased plasma cells in SLE.

An essential role for RGS protein/Gαi2 interactions in B lymphocyte-directed cell migration and trafficking

Chemokines engage B lymphocyte surface receptors, triggering heterotrimeric G protein Gαi subunit guanine nucleotide exchange. RGS proteins limit the duration that Gαi subunits remain GTP bound, and the loss of an individual RGS protein typically enhances chemokine receptor signaling. In this study, we show that B cells carrying a Gαi2 (G184S/G184S) mutation that disables all RGS protein/Gαi2 interactions exhibit an unexpectedly severe reduction in chemokine receptor signaling. The Gαi2 (G184S/G184S) B cells have markedly elevated basal calcium levels, but poor chemokine-induced increases, enhanced nonspecific migration, but extremely poor chemotaxis. In striking contrast, the Gαi2 (G184S/G184S) B cells exhibited enhanced sensitivity to sphingosine 1-phosphate (S1P). S1P elicited heightened intracellular calcium responses and enhanced S1P-triggered cell migration. Mice with the Gαi2 (G184S/G184S) mutation displayed excessive numbers of germinal center-like structures; abnormal serum Ig profiles; and aberrant B lymphocyte trafficking. These findings establish an essential role for RGS proteins in B cell chemoattractant signaling and for the proper position of B lymphocytes in lymphoid organs.

BTK Signaling in B Cell Differentiation and Autoimmunity

Since the original identification of Bruton's tyrosine kinase (BTK) as the gene defective in the primary immunodeficiency X-linked agammaglobulinemia (XLA) in 1993, our knowledge on the physiological function of BTK has expanded impressively. In this review, we focus on the role of BTK during B cell differentiation in vivo, both in the regulation of expansion and in the developmental progression of pre-B cells in the bone marrow and as a crucial signal transducer of signals downstream of the IgM or IgG B cell antigen receptor (BCR) in mature B cells governing proliferation, survival, and differentiation. In particular, we highlight BTK function in B cells in the context of host defense and autoimmunity. Small-molecule inhibitors of BTK have very recently shown impressive anti-tumor activity in clinical studies in patients with various B cell malignancies. Since promising effects of BTK inhibition were also seen in experimental animal models for lupus and rheumatoid arthritis, BTK may be a good target for controlling autoreactive B cells in patients with systemic autoimmune disease.

BTK inhibitors in chronic lymphocytic leukemia: a glimpse to the future

The treatment of chronic lymphocytic leukemia (CLL) with inhibitors targeting B cell receptor signaling and other survival mechanisms holds great promise. Especially the early clinical success of Ibrutinib, an irreversible inhibitor of Bruton's tyrosine kinase (BTK), has received widespread attention. In this review we will focus on the fundamental and clinical aspects of BTK inhibitors in CLL, with emphasis on Ibrutinib as the best studied of this class of drugs. Furthermore, we summarize recent laboratory as well as clinical findings relating to the first cases of Ibrutinib resistance. Finally, we address combination strategies with Ibrutinib, and attempt to extrapolate its current status to the near future in the clinic.

Bruton's tyrosine kinase: from X-linked agammaglobulinemia toward targeted therapy for B-cell malignancies

Discovery of Bruton's tyrosine kinase (BTK) mutations as the cause for X-linked agammaglobulinemia was a milestone in understanding the genetic basis of primary immunodeficiencies. Since then, studies have highlighted the critical role of this enzyme in B-cell development and function, and particularly in B-cell receptor signaling. Because its deletion affects mostly B cells, BTK has become an attractive therapeutic target in autoimmune disorders and B-cell malignancies. Ibrutinib (PCI-32765) is the most advanced BTK inhibitor in clinical testing, with ongoing phase III clinical trials in patients with chronic lymphocytic leukemia and mantle-cell lymphoma. In this article, we discuss key discoveries related to BTK and clinically relevant aspects of BTK inhibitors, and we provide an outlook into clinical development and open questions regarding BTK inhibitor therapy.

Small chemicals with inhibitory effects on PtdIns(3,4,5)P3 binding of Btk PH domain

Phosphatidylinositol-3,4-5-triphosphates (PtdIns(3,4,5)P3) formed by phosphoinositide-3-kinase (PI3K) had been known as a signaling molecule that plays important roles in diverse cellular processes such as cell signaling, metabolism, cell differentiation, and apoptosis. PtdIns(3,4,5)P3 regulates diverse cellular processes by recruiting effector proteins to the specific cellular locations for correct functions. In this study, we reported the inhibitory effect of small chemicals on the interaction between PtdIns(3,4,5)P3-Btk PH domain. Small chemicals were synthesized based on structural similarity of PtdInsP head-groups, and tested the inhibitory effects in vitro via surface plasmon resonance (SPR). As a result, the chemical 8 showed highest inhibitory effect with 17μM of IC50 value. To elucidate diverse inhibitory effects of different small chemicals we employed in silico docking experiment using molecular modeling and simulation. The result of docking experiments showed chemical 8 has more hydrogen bonding with the residues in PtdIns(3,4,5)P3 binding site of Btk PH domain than others. Overall, our studies demonstrate the efficient approach to develop lipid binding inhibitors, and further we can use these chemicals to regulate effector proteins. In addition, our study would provide new insight that lipid binding domain may be the attractive therapeutic targets to treat severe human diseases.

Targeting Bruton's tyrosine kinase in B cell malignancies

Bruton's tyrosine kinase (BTK) is a key component of B cell receptor (BCR) signalling and functions as an important regulator of cell proliferation and cell survival in various B cell malignancies. Small-molecule inhibitors of BTK have shown antitumour activity in animal models and, recently, in clinical studies. High response rates were reported in patients with chronic lymphocytic leukaemia and mantle cell lymphoma. Remarkably, BTK inhibitors have molecular effects that cannot be explained by the classic role of BTK in BCR signalling. In this Review, we highlight the importance of BTK in various signalling pathways in the context of its therapeutic inhibition.

Targeting spleen tyrosine kinase-Bruton's tyrosine kinase axis for immunologically mediated glomerulonephritis

The importance of B-cell activation and immune complex-mediated Fc-receptor activation in the pathogenesis of immunologically mediated glomerulonephritis has long been recognized. The two nonreceptor tyrosine kinases, spleen tyrosine kinase (Syk) and Bruton's tyrosine kinase (Btk), are primarily expressed by hematopoietic cells, and participate in B-cell-receptor- and Fc-receptor-mediated activation. Pharmacological inhibitors of Syk or Btk are undergoing preclinical development and clinical trials for several immune diseases; and Syk inhibitors have been shown to reduce disease activity in rheumatoid arthritis patients. However, the clinical therapeutic efficacies of these inhibitors in glomerulonephritis have not been evaluated. Herein, we review recent studies of Syk and Btk inhibitors in several experimental primary and secondary glomerulonephritis models. These inhibitors suppressed development of glomerular injury, and also ameliorated established kidney disease. Thus, targeting Syk and Btk signaling pathways is a potential therapeutic strategy for glomerulonephritis, and further evaluation is recommended.

The role of tyrosine kinases in systemic lupus erythematosus and their potential as therapeutic targets

The autoimmune disease systemic lupus erythematosus is characterized by loss of tolerance to nuclear antigens. Breakdown of tolerance is associated with alterations in T-cell and B-cell receptor signal transduction, including increased protein phosphorylation that may underlie pathogenesis and explain the characteristic hyperactivity of T and B cells and other immune cells in active disease. Tyrosine kinases play a central role in signaling processes in cells known to be important in the pathogenesis of autoimmune diseases. Considerable progress has been made in understanding the function of tyrosine kinases in immune cell signaling pathways. In this review, we will summarize the function of tyrosine kinases and their novel inhibitors from studies made in animal lupus models and systemic lupus erythematosus patients.

Molecular abnormalities of the B cell in systemic lupus erythematosus are candidates for functional inhibition treatments

INTRODUCTION

The B cell is a key player in the pathogenesis of systemic lupus erythematosus (SLE). Loss of B cell tolerance resulting in autoantibody production and immune complex formation and deposition are central features of the disease. B cell overactivity is a hallmark of SLE and molecular abnormalities in B cell signaling cascade have been described.

AREAS COVERED

In this review, we will focus on the aberrant phenotype of B cell signaling in patients with lupus. We will also discuss data stemming from the use of small molecules that have recently been recognized to target important steps of the B cell signal transduction pathways with therapeutic implications for SLE.

EXPERT OPINION

Attempts to target the B cell in SLE have been made through depletion, blocking of survival factors and co-receptor inhibition. However, the still unmet need for effective therapy of refractory disease makes the necessity for new drugs impelling.

B-cell hyperactivity in primary Sjögren's syndrome

Primary Sjögren's syndrome (pSS) is characterized by mononuclear inflammatory infiltrates and IgG plasma cells in salivary and lacrimal glands which lead to irreversible destruction of the glandular tissue and is accompanied by sensation of dryness of mouth and eyes. B cells play a central role in the immunopathogenesis and exhibit signs of hyperactivity. Hyperactivity of B cells is the consequence of the coordinated and integrated action of stimulation of the B-cell receptor, CD40 and toll-like receptors in the presence of appropriate cytokines. As discussed, overexpression of type I IFN and BAFF on one hand and IL-6 and IL-21 on the other hand are critically involved in the enhanced plasma cell formation in pSS patients. Hyperactivity of B cells results in secretion of autoantibodies and production of various cytokines. These insights in the role of B cells in the pathogenetic process of pSS offer ample targets for successful therapeutical intervention in pSS.

Bruton's tyrosine kinase promotes persistence of mature anti-insulin B cells

Autoreactive B lymphocytes are essential for the development of T cell-mediated type 1 diabetes (T1D). Cytoplasmic Bruton's tyrosine kinase (BTK) is a key component of B cell signaling, and its deletion in T1D-prone NOD mice significantly reduces diabetes. However, the role of BTK in the survival and function of autoreactive B cells is not clear. To evaluate the contributions of BTK, we used mice in which B cells express an anti-insulin BCR (125Tg) and promote T1D, despite being anergic. Crossing Btk deficiency onto 125Tg mice reveals that, in contrast to immature B cells, mature anti-insulin B cells are exquisitely dependent upon BTK, because their numbers are reduced by 95%. BTK kinase domain inhibition reproduces this effect in mature anti-insulin B cells, with less impact at transitional stages. The increased dependence of anti-insulin B cells on BTK became particularly evident in an Igκ locus site-directed model, in which 50% of B cells edit their BCRs to noninsulin specificities; Btk deficiency preferentially depletes insulin binders from the follicular and marginal zone B cell subsets. The persistent few Btk-deficient anti-insulin B cells remain competent to internalize Ag and invade pancreatic islets. As such, loss of BTK does not significantly reduce diabetes incidence in 125Tg/NOD mice as it does in NOD mice with a normal B cell repertoire. Thus, BTK targeting may not impair autoreactive anti-insulin B cell function, yet it may provide protection in an endogenous repertoire by decreasing the relative availability of mature autoreactive B cells.

Suppression of glomerulonephritis in lupus-prone NZB × NZW mice by RN486, a selective inhibitor of Bruton's tyrosine kinase

OBJECTIVE

Bruton's tyrosine kinase (BTK) plays a critical role in B cell development and function. We recently described a selective BTK inhibitor, RN486, that blocks B cell receptor (BCR) and Fcγ receptor signaling and is efficacious in animal models of arthritis. The aim of this study was to examine the potential efficacy of BTK in systemic lupus erythematosus (SLE), using an NZB × NZW mouse model of spontaneous SLE.

METHODS

Mice received RN486 or its vehicle (administered in chow) at a final concentration of 30 mg/kg for 8 weeks, starting at 32 weeks of age.

RESULTS

The administration of RN486 completely stopped disease progression, as determined by histologic and functional analyses of glomerular nephritis. The efficacy was associated with striking inhibition of B cell activation, as demonstrated by a significant reduction in CD69 expression in response to BCR crosslinking. RN486 markedly reduced the secretion of IgG anti-double-stranded DNA (anti-dsDNA) secretion, as determined by enzyme-linked immunosorbent and enzyme-linked immunospot assays. Flow cytometric analysis demonstrated depletion of CD138(high) B220(low) plasma cells in the spleen. RN486 inhibited secretion of IgG anti-dsDNA but not IgM anti-dsDNA, suggesting that pharmacologic blockade of BTK resembles the reported transgenic expression of low levels of endogenous BTK in B cells. In addition, RN486 may also impact the effector function of autoantibodies, as evidenced by a significant reduction in immune complex-mediated activation of human monocytes in vitro and down-regulation of the expression of macrophage-related and interferon-inducible genes in both the kidneys and spleens of treated mice.

CONCLUSION

Collectively, our data suggest that BTK inhibitors may simultaneously target autoantibody-producing and effector cells in SLE, thus constituting a promising therapeutic alternative for this disease.

Bruton tyrosine kinase inhibitor ibrutinib (PCI-32765)

Over the past 3 years, ibrutinib (PCI-32765) has emerged as a breakthrough in targeted therapy for patients with certain types of B cell malignancies. Early stage clinical trials found ibrutinib to be particularly active in chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL), providing the rationale for ongoing phase 3 trials. In contrast to conventional chemo-immunotherapy, ibrutinib is not myelosuppressive, and responses are not affected by disease features that predict failure to respond to or short remission durations after chemo-immunotherapy, such as del17p. In CLL, ibrutinib characteristically causes an early redistribution of tissue-resident CLL cells into the blood, with rapid resolution of enlarged lymph nodes, along with a surge in lymphocytosis. Later, after weeks to months of continuous ibrutinib therapy, the growth- and survival-inhibitory activities of ibrutinib result in the normalization of lymphocyte counts and remissions in a majority of patients. This review discusses the discovery, preclinical and clinical development of ibrutinib, its pathophysiological basis, and outlines perspectives for future use of ibrutinib.

Regulation of B-cell development and function by microRNAs

MicroRNAs (miRNAs) have emerged as a new class of gene expression regulators whose functions influence a myriad of biological processes, from developmental decisions through immune responses and numerous pathologies, including cancer and autoimmunity. miRNAs are small RNA molecules that drive post-transcriptional negative regulation of gene expression by promoting the degradation or translational block of their target mRNAs. Here, we review some of the data relating to the role of miRNAs in the regulation of the B-cell lineage, with a special focus on results obtained in vivo. We start by giving a general overview of miRNA activity, including the issue of target specificity and the experimental approaches more widely used to analyze the function of these molecules. We then go on to discuss the function of miRNAs during B-cell differentiation in the bone marrow and in the periphery as well as during the humoral immune response. Finally, we describe a few examples of the contribution of miRNAs, both as oncogenes and tumor suppressors, to the development of B-cell neoplasias.

Bruton's tyrosine kinase mediates the synergistic signalling between TLR9 and the B cell receptor by regulating calcium and calmodulin

B cells signal through both the B cell receptor (BCR) which binds antigens and Toll-like receptors (TLRs) including TLR9 which recognises CpG DNA. Activation of TLR9 synergises with BCR signalling when the BCR and TLR9 co-localise within an auto-phagosome-like compartment. Here we report that Bruton's tyrosine kinase (BTK) is required for synergistic IL6 production and up-regulation of surface expression of MHC-class-II, CD69 and CD86 in primary murine and human B cells. We show that BTK is essential for co-localisation of the BCR and TLR9 within a potential auto-phagosome-like compartment in the Namalwa human B cell line. Downstream of BTK we find that calcium acting via calmodulin is required for this process. These data provide new insights into the role of BTK, an important target for autoimmune diseases, in B cell activation.

X-linked immunodeficient mice exhibit enhanced susceptibility to Cryptococcus neoformans Infection

Bruton’s tyrosine kinase (Btk) is a signaling molecule that plays important roles in B-1 B cell development and innate myeloid cell functions and has recently been identified as a target for therapy of B cell lymphomas. We examined the contribution of B-1 B cells to resistance to Cryptococcus neoformans infection by utilizing X-linked immunodeficient (XID) mice (CBA-CaHN-XID), which possess a mutation in Btk. XID mice had significantly higher brain fungal burdens than the controls 6 weeks after infection with C. neoformans strain 52D (CN52D); however, consistent with the propensity for greater virulence of C. neoformans strain H99 (CNH99), CNH99-infected XID mice had higher lung and brain fungal burdens than the controls 3 weeks after infection. Further studies in a chronic CN52D model revealed markedly lower levels of total and C. neoformans-specific serum IgM in XID mice than in the control mice 1 and 6 weeks after infection. Alveolar macrophage phagocytosis was markedly impaired in CN52D-infected XID mice compared to the controls, with XID mice exhibiting a disorganized lung inflammatory pattern in which Gomori silver staining revealed significantly more enlarged, extracellular C. neoformans cells than the controls. Adoptive transfer of B-1 B cells to XID mice restored peritoneal B-1 B cells but did not restore IgM levels to those of the controls and had no effect on the brain fungal burden at 6 weeks. Taken together, our data support the hypothesis that IgM promotes fungal containment in the lungs by enhancing C. neoformans phagocytosis and restricting C. neoformans enlargement. However, peritoneal B-1 B cells are insufficient to reconstitute a protective effect in the lungs.

Cryptococcus neoformans is a fungal pathogen that causes an estimated 600,000 deaths per year. Most infections occur in individuals who are immunocompromised, with the majority of cases occurring in those with HIV/AIDS, but healthy individuals also develop disease. Immunoglobulin M (IgM) has been linked to resistance to disease in humans and mice. In this article, we found that X-linked immunodeficient (XID) mice, which have markedly reduced levels of IgM, were unable to contain Cryptococcus in the lungs. This was associated with reduced yeast uptake by macrophages, an aberrant tissue inflammatory response, an enlargement of the yeast cells in the lungs, and fungal dissemination to the brain. Since XID mice have a mutation in the Bruton’s tyrosine kinase (Btk) gene, our data suggest that treatments aimed at blocking the function of Btk could pose a higher risk for cryptococcosis.

B-lymphocyte signalling abnormalities and lupus immunopathology

Lupus is a complex autoimmune rheumatic disease of unknown aetiology. The disease is associated with diverse features of immunological abnormality in which B-lymphocytes play a central role. However, the cause of atypical B-lymphocyte responses remains unclear. In this article, we provide a synopsis of current knowledge on intracellular signalling abnormalities in B-lymphocytes in lupus and their potential effects on the response of these cells in mouse models and in patients. There are numerous reported defects in the regulation of intracellular signalling proteins and pathways in B-lymphocytes in lupus that, potentially, affect critical biological responses. Most of the evidence for these defects comes from studies of disease models and genetically engineered mice. However, there is also increasing evidence from studying B-lymphocytes from patients and from genome-wide linkage analyses for parallel defects to those observed in mice. These studies provide molecular and genetic explanations for the key immunological abnormalities associated with lupus. Most of the new information appears to relate to defects in intracellular signalling that impact B-lymphocyte tolerance, cytokine production and responses to infections. Some of these abnormalities will be discussed within the context of disease pathogenesis.