Bruton's tyrosine kinase links the B cell receptor to nuclear factor kappaB activation

Small-molecule BTK inhibitors: From discovery to clinical application

Bruton's tyrosine kinase (BTK) inhibitors constitute a promising category of small molecules for the therapy of diverse B-cell malignancies and autoimmune disorders. This review examines the journey of BTK inhibitors from their discovery to clinical development, highlighting key milestones in their design, mechanism of action, and progression through preclinical and clinical stages. Initially identified through high-throughput screening of compound libraries, early BTK inhibitors were optimized for selectivity and potency. The discovery of ibrutinib, the first Food and Drug Administration (FDA)-approved BTK inhibitor, marked a significant breakthrough, providing a new therapeutic option for patients with chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL). Following this success, numerous second-generation inhibitors have been identified to address resistance mechanisms, improve pharmacokinetics, and target specific patient populations. The challenges faced during the transition from preclinical validation to clinical trials have been discussed. Additionally, ongoing trials and emerging data on novel BTK inhibitors provide insights into their evolving role in oncology and immunology. This review emphasizes the importance of rational drug design and clinical strategy in shaping the future of BTK inhibitors.

From development to clinical success: the journey of established and next-generation BTK inhibitors

Over the past decade, Bruton's tyrosine kinase (BTK) has emerged as a pivotal therapeutic target for B-cell malignancies and autoimmune diseases, given its essential role in B-cell development and function. Dysregulation of BTK signalling is implicated in a range of hematologic cancers, including Waldenström's macroglobulinaemia (WM), mantle cell lymphoma (MCL), and chronic lymphocytic leukaemia (CLL). The development of BTK inhibitors (BTKIs), starting with ibrutinib, has revolutionized the treatment of these malignancies by inhibiting B-cell receptor (BCR) signalling and inducing apoptosis in malignant B-cells. Despite the impressive clinical efficacy of ibrutinib, challenges such as resistance mutations and off-target effects remain. To address these issues, next-generation BTKIs, including acalabrutinib, orelabrutinib, zanubrutinib, and pirtobrutinib, have been developed, offering improved specificity and reduced toxicity profiles. This review highlights the therapeutic potential of BTK-targeted therapies in treating B-cell malignancies, discusses recent advancements with FDA-approved BTKIs, and explores the latest clinical outcomes from ongoing trials of novel inhibitors.

Advances in Targeted Therapy: Addressing Resistance to BTK Inhibition in B-Cell Lymphoid Malignancies

B-cell lymphoid malignancies are a heterogeneous group of hematologic cancers, where Bruton's tyrosine kinase (BTK) inhibitors have received FDA approval for several subtypes. The first-in-class covalent BTK inhibitor, Ibrutinib, binds to the C481 amino acid residue to block the BTK enzyme and prevent the downstream signaling. Resistance to covalent BTK inhibitors (BTKi) can occur through mutations at the BTK binding site (C481S) but also other BTK sites and the phospholipase C gamma 2 (PLCγ2) resulting in downstream signaling. To bypass the C481S mutation, non-covalent BTKi, such as Pirtobrutinib, were developed and are active against both wild-type and the C481S mutation. In this review, we discuss the molecular and genetic mechanisms which contribute to acquisition of resistance to covalent and non-covalent BTKi. In addition, we discuss the new emerging class of BTK degraders, which utilize the evolution of proteolysis-targeting chimeras (PROTACs) to degrade the BTK protein and constitute an important avenue of overcoming resistance. The moving landscape of resistance to BTKi and the development of new therapeutic strategies highlight the ongoing advances being made towards the pursuit of a cure for B-cell lymphoid malignancies.

The function of serine/threonine-specific protein kinases in B cells

The serine/threonine-specific protein kinases (STKs) are important for cell survival, proliferation, differentiation, and apoptosis. In B cells, these kinases play indispensable roles in regulating important cellular functions. Multiple studies on human and other animal cells have shown that multiple STKs are involved in different stages of B cell development and antibody production. However, how STKs affect B cell development and function is still not completely understood. Considering that B cells are clinically important in immunity and diseases, our understanding of STKs' roles in B cells is in great need of investigation with current technologies. Investigating serine/threonine kinases will not only deepen our insight into B cell-related disorders but also facilitate the identification of more effective drug targets for conditions like lymphoma and systemic lupus erythematosus.

Readministration of high-dose adeno-associated virus gene therapy vectors enabled by ImmTOR nanoparticles combined with B cell-targeted agents

Tolerogenic ImmTOR nanoparticles encapsulating rapamycin have been demonstrated to mitigate immunogenicity of adeno-associated virus (AAV) gene therapy vectors, enhance levels of transgene expression, and enable redosing of AAV at moderate vector doses of 2 to 5E12 vg/kg. However, recent clinical trials have often pushed AAV vector doses 10-fold to 50-fold higher, with serious adverse events observed at the upper range. Here, we assessed combination therapy of ImmTOR with B cell-targeting drugs for the ability to increase the efficiency of redosing at high vector doses. The combination of ImmTOR with a monoclonal antibody against B cell activation factor (aBAFF) exhibited strong synergy leading to more than a 5-fold to 10-fold reduction of splenic mature B cells and plasmablasts while increasing the fraction of pre-/pro-B cells. In addition, this combination dramatically reduced anti-AAV IgM and IgG antibodies, thus enabling four successive AAV administrations at doses up to 5E12 vg/kg and at least two AAV doses at 5E13 vg/kg, with the transgene expression level in the latter case being equal to that observed in control animals receiving a single vector dose of 1E14 vg/kg. Similar synergistic effects were seen with a combination of ImmTOR and a Bruton's tyrosine kinase inhibitor, ibrutinib. These results suggest that ImmTOR could be combined with B cell-targeting agents to enable repeated vector administrations as a potential strategy to avoid toxicities associated with vector doses above 1E14 vg/kg.

Identification and Biological Evaluation of a Water-Soluble Fullerene Nanomaterial as BTK Kinase Inhibitor

Introduction

Thanks to recent advances in synthetic methodology, water-soluble fullerene nanomaterials that interfere with biomolecules, especially DNA/RNA and selected proteins, have been found with tremendous potential for applications in nanomedicine. Herein, we describe the synthesis and evaluation of a water-soluble glycine-derived [60]fullerene hexakisadduct (HDGF) with T _h symmetry, which is a first-in-class BTK protein inhibitor.

Methods

We synthesized and characterized glycine derived [60]fullerene using NMR, ESI-MS, and ATR-FT-IR. DLS and zeta potential were measured and high-resolution transmission electron microscopy (HRTEM) observations were performed. The chemical composition of the water-soluble fullerene nanomaterial was examined by X-ray photoelectron spectrometry. To observe aggregate formation, the cryo-TEM analysis was carried out. The docking studies and molecular dynamic simulations were performed to determine interactions between HDGF and BTK. The in vitro cytotoxicity was evaluated on RAJI and K562 blood cancer cell lines. Subsequently, we examined the induction of cell death by autophagy and apoptosis by determining the expression levels of crucial genes and caspases. We investigated the direct association of HDGF on inhibition of the BTK signalling pathway by examining changes in the calcium levels in RAJI cells after treatment. The inhibitory potential of HDGF against non-receptor tyrosine kinases was evaluated. Finally, we assessed the effects of HDGF and ibrutinib on the expression of the BTK protein and downstream signal transduction in RAJI cells following anti-IgM stimulation.

Results

Computational studies revealed that the inhibitory activity of the obtained [60]fullerene derivative is multifaceted: it hampers the BTK active site, interacting directly with the catalytic residues, rendering it inaccessible to phosphorylation, and binds to residues that form the ATP binding pocket. The anticancer activity of produced carbon nanomaterial revealed that it inhibited the BTK protein and its downstream pathways, including PLC and Akt proteins, at the cellular level. The mechanistic studies suggested the formation of autophagosomes (increased gene expression of LC3 and p62) and two caspases (caspase-3 and -9) were responsible for the activation and progression of apoptosis.

Conclusion

These data illustrate the potential of fullerene-based BTK protein inhibitors as nanotherapeutics for blood cancer and provide helpful information to support the future development of fullerene nanomaterials as a novel class of enzyme inhibitors.

Research-based flow cytometry assays for pathogenic assessment in the human B-cell biology of gene variants revealed in the diagnosis of inborn errors of immunity: a Bruton's tyrosine kinase case-study

Introduction

Inborn errors of immunity (IEI) are an expanding group of rare diseases whose field has been boosted by next-generation sequencing (NGS), revealing several new entities, accelerating routine diagnoses, expanding the number of atypical presentations and generating uncertainties regarding the pathogenic relevance of several novel variants.

Methods

Research laboratories that diagnose and provide support for IEI require accurate, reproducible and sustainable phenotypic, cellular and molecular functional assays to explore the pathogenic consequences of human leukocyte gene variants and contribute to their assessment. We have implemented a set of advanced flow cytometry-based assays to better dissect human B-cell biology in a translational research laboratory. We illustrate the utility of these techniques for the in-depth characterization of a novel (c.1685G>A, p.R562Q) de novo gene variant predicted as probably pathogenic but with no previous insights into the protein and cellular effects, located in the tyrosine kinase domain of the Bruton's tyrosine kinase (BTK) gene, in an apparently healthy 14-year-old male patient referred to our clinic for an incidental finding of low immunoglobulin (Ig) M levels with no history of recurrent infections.

Results and discussion

A phenotypic analysis of bone marrow (BM) revealed a slightly high percentage of pre-B-I subset in BM, with no blockage at this stage, as typically observed in classical X-linked agammaglobulinemia (XLA) patients. The phenotypic analysis in peripheral blood also revealed reduced absolute numbers of B cells, all pre-germinal center maturation stages, together with reduced but detectable numbers of different memory and plasma cell isotypes. The R562Q variant allows Btk expression and normal activation of anti-IgM-induced phosphorylation of Y551 but diminished autophosphorylation at Y223 after anti IgM and CXCL12 stimulation. Lastly, we explored the potential impact of the variant protein for downstream Btk signaling in B cells. Within the canonical nuclear factor kappa B (NF-κB) activation pathway, normal IκBα degradation occurs after CD40L stimulation in patient and control cells. In contrast, disturbed IκBα degradation and reduced calcium ion (Ca2⁺) influx occurs on anti-IgM stimulation in the patient's B cells, suggesting an enzymatic impairment of the mutated tyrosine kinase domain.

Resistance to Bruton tyrosine kinase inhibition in chronic lymphocytic leukaemia and non-Hodgkin lymphoma

Bruton tyrosine kinase inhibitors (BTKi) have transformed the therapeutic landscape of chronic lymphocytic leukaemia (CLL) and non-Hodgkin lymphoma. However, primary and acquired resistance to BTKi can be seen due to a variety of mechanisms including tumour intrinsic and extrinsic mechanisms such as gene mutations, activation of bypass signalling pathways and tumour microenvironment. Herein, we provide an updated review of the key clinical data of BTKi treatment in CLL, mantle cell lymphoma, and diffuse large B-cell lymphoma (DLBCL). We incorporate the most recent findings regarding mechanisms of resistance to covalent and non-covalent inhibitors, including ibrutinib, acalabrutinib, zanubrutinib and pirtobrutinib. We also cover the clinical sensitivity of certain molecular subtypes of DLBCL to an ibrutinib-containing regimen. Lastly, we summarise ongoing clinical investigations aimed at overcoming resistance via use of BTKi-containing combined therapies or the novel non-covalent BTKi. The review article targets an audience of clinical practitioners, clinical investigators and translational researchers.

B Cell Activation Results in IKK-Dependent, but Not c-Rel- or RelA-Dependent, Decreases in Transcription of the B Cell Tolerance-Inducing Gene Ets1

Ets1 is a key transcription factor in B cells that is required to prevent premature differentiation into Ab-secreting cells. Previously, we showed that BCR and TLR signaling downregulate Ets1 levels and that the kinases PI3K, Btk, IKK, and JNK are required for this process. PI3K is important in activating Btk by generating the membrane lipid phosphatidylinositol (3,4,5)-trisphosphate, to which Btk binds via its PH domain. Btk in turn is important in activating the IKK kinase pathway, which it does by activating phospholipase Cγ2→protein kinase Cβ signaling. In this study, we have further investigated the pathways regulating Ets1 in mouse B cells. Although IKK is well known for its role in activating the canonical NF-κB pathway, IKK-mediated downregulation of Ets1 does not require either RelA or c-Rel. We also examined the potential roles of two other IKK targets that are not part of the NF-κB signaling pathway, Foxo3a and mTORC2, in regulating Ets1. We find that loss of Foxo3a or inhibition of mTORC2 does not block BCR-induced Ets1 downregulation. Therefore, these two pathways are not key IKK targets, implicating other as yet undefined IKK targets to play a role in this process.

Ibrutinib-Associated Cardiotoxicity: From the Pharmaceutical to the Clinical

Ibrutinib is the first-in-class Bruton tyrosine kinase (BTK) inhibitor that has revolutionized the treatment of B cell malignancies. Unfortunately, increased incidences of cardiotoxicity have limited its use. Despite over a decade of research, the biological mechanisms underlying ibrutinib cardiotoxicity remain unclear. In this review, we discuss the pharmacological properties of ibrutinib, the incidence and mechanisms of ibrutinib-induced cardiotoxicity, and practical management to prevent and treat this condition. We also synopsize and discuss the cardiovascular adverse effects related to other more selective BTK inhibitors, which may guide the selection of appropriate BTK inhibitors.

Kinase-deficient BTK mutants confer ibrutinib resistance through activation of the kinase HCK

The Bruton's tyrosine kinase (BTK) inhibitor ibrutinib irreversibly binds BTK at Cys⁴⁸¹, inhibiting its kinase activity and thus blocking transduction of B cell receptor (BCR) signaling. Although ibrutinib is durably effective in patients with B cell malignancies, many patients still develop ibrutinib-resistant disease. Resistance can arise because of mutations at the ibrutinib-binding site in BTK. Here, we characterized the mechanism by which two BTK mutations, C481F and C481Y, may lead to ibrutinib resistance. Both mutants lacked detectable kinase activity in in vitro kinase assays. Structural modeling suggested that bulky Phe and Tyr side chains at position 481 sterically hinder access to the ATP-binding pocket in BTK, contributing to loss of kinase activity. Nonetheless, BCR signaling still propagated through BTK C481F and C481Y mutants to downstream effectors, the phospholipase PLCγ2 and the transcription factor NF-κB. This maintenance of BCR signaling was partially achieved through the physical recruitment and kinase-independent activation of hematopoietic cell kinase (HCK). Upon BCR activation, BTK C481F or C481Y was phosphorylated by Src family kinases at Tyr⁵⁵¹, which then bound to the SH2 domain of HCK. Modeling suggested that this binding disrupted an intramolecular autoinhibitory interaction in HCK. Activated HCK subsequently phosphorylated PLCγ2, which propagated BCR signaling and promoted clonogenic cell proliferation. This kinase-independent mechanism could inform therapeutic approaches to CLL bearing either the C481F or C481Y BTK mutants.

Chimeric antigen receptor (CAR) T-cell treatment for mantle cell lymphoma (MCL)

Mantle cell lymphoma (MCL) is a rare B-cell malignancy that remains challenging to treat with high rates of relapse. Frontline strategies range from intensive chemotherapy followed by consolidation with autologous stem cell transplant (ASCT), to less-intensive therapies including combination regimens. The treatment landscape for relapsed patients includes Bruton tyrosine kinase (BTK) inhibitors among other targeted treatments. Novel agents such as the selective BCL2 inhibitor venetoclax showed high response rates when used as monotherapy for refractory relapsed MCL. The rituximab, bendamustine, and cytarabine (R-BAC) regimen, while response rates were high, were not durable. Chimeric antigen receptor (CAR) T-cell products targeting CD19 have been efficacious in relapsed and refractory MCL patients. Brexucabtagene autoleucel (brexu-cel, formerly KTE-X19) was approved by US Food and Drug Administration (FDA) in July, 2020, for treatment of refractory and relapsed MCL. This article provides an overview for the available management strategies for relapsed MCL and examines the role of CAR T-cell in the current and future treatment of MCL.

Bruton's Tyrosine Kinase Inhibition as an Emerging Therapy in Systemic Autoimmune Disease

Systemic autoimmune disorders are complex heterogeneous chronic diseases involving many different immune cells. A significant proportion of patients respond poorly to therapy. In addition, the high burden of adverse effects caused by "classical" anti-rheumatic or immune modulatory drugs provides a need to develop more specific therapies that are better tolerated. Bruton's tyrosine kinase (BTK) is a crucial signaling protein that directly links B-cell receptor (BCR) signals to B-cell activation, proliferation, and survival. BTK is not only expressed in B cells but also in myeloid cells, and is involved in many different signaling pathways that drive autoimmunity. This makes BTK an interesting therapeutic target in the treatment of autoimmune diseases. The past decade has seen the emergence of first-line BTK small-molecule inhibitors with great efficacy in the treatment of B-cell malignancies, but with unfavorable safety profiles for use in autoimmunity due to off-target effects. The development of second-generation BTK inhibitors with superior BTK specificity has facilitated the investigation of their efficacy in clinical trials with autoimmune patients. In this review, we discuss the role of BTK in key signaling pathways involved in autoimmunity and provide an overview of the different inhibitors that are currently being investigated in clinical trials of systemic autoimmune diseases, including rheumatoid arthritis and systemic lupus erythematosus, as well as available results from completed trials.

Bruton's tyrosine kinase inhibition induces rewiring of proximal and distal B-cell receptor signaling in mice

Bruton′s tyrosine kinase (Btk) is a crucial signaling molecule in BCR signaling and a key regulator of B‐ cell differentiation and function. Btk inhibition has shown impressive clinical efficacy in various B‐cell malignancies. However, it remains unknown whether inhibition additionally induces changes in BCR signaling due to feedback mechanisms, a phenomenon referred to as BCR rewiring. In this report, we studied the impact of Btk activity on major components of the BCR signaling pathway in mice. As expected, NF‐κB and Akt/S6 signaling was decreased in Btk‐deficient B cells. Unexpectedly, phosphorylation of several proximal signaling molecules, including CD79a, Syk, and PI3K, as well as the key Btk‐effector PLCγ2 and the more downstream kinase Erk, were significantly increased. This pattern of BCR rewiring was essentially opposite in B cells from transgenic mice overexpressing Btk. Importantly, prolonged Btk inhibitor treatment of WT mice or mice engrafted with leukemic B cells also resulted in increased phosho‐CD79a and phospho‐PLCγ2 in B cells. Our findings show that Btk enzymatic function determines phosphorylation of proximal and distal BCR signaling molecules in B cells. We conclude that Btk inhibitor treatment results in rewiring of BCR signaling, which may affect both malignant and healthy B cells.

TRPM5 Negatively Regulates Calcium-Dependent Responses in Lipopolysaccharide-Stimulated B Lymphocytes

B cells produce high amounts of cytokines and immunoglobulins in response to lipopolysaccharide (LPS) stimulation. Calcium signaling cascades are critically involved in cytokine production of T cells, and the cytosolic calcium concentration is regulated by calcium-activated monovalent cation channels (CAMs). Calcium signaling is also implicated in B cell activation; however, its involvement in the cytokine production of LPS-stimulated B cells remains less well characterized. Here, we show that the transient receptor potential melastatin 5 channel (TRPM5), which is one of the CAMs, negatively modulates calcium signaling, thereby regulating LPS-induced proliferative and inflammatory responses by B cells. LPS-stimulated B cells of Trpm5-deficient mice exhibit an increased cytosolic calcium concentration, leading to enhanced proliferation and the production of the inflammatory cytokines interleukin-6 and CXCL10. Furthermore, Trpm5-deficient mice show an exacerbation of endotoxic shock with high mortality. Our findings demonstrate the importance of TRPM5-dependent regulatory mechanisms in LPS-induced calcium signaling of splenic B cells.

Bruton's tyrosine kinase: an emerging targeted therapy in myeloid cells within the tumor microenvironment

Bruton’s tyrosine kinase (BTK) is a non-receptor kinase belonging to the Tec family of kinases. The role of BTK in B cell receptor signaling is well defined and is known to play a key role in the proliferation and survival of malignant B cells. Moreover, BTK has been found to be expressed in cells of the myeloid lineage. BTK has been shown to contribute to a variety of cellular pathways in myeloid cells including signaling in the NLRP3 inflammasome, receptor activation of nuclear factor-κβ and inflammation, chemokine receptor activation affecting migration, and phagocytosis. Myeloid cells are crucial components of the tumor microenvironment and suppressive myeloid cells contribute to cancer progression, highlighting a potential role for BTK inhibition in the treatment of malignancy. The increased interest in BTK inhibition in cancer has resulted in many preclinical studies that are testing the efficacy of using single-agent BTK inhibitors. Moreover, the ability of tumor cells to develop resistance to single-agent checkpoint inhibitors has resulted in clinical studies utilizing BTK inhibitors in combination with these agents to improve clinical responses. Furthermore, BTK regulates the immune response in microbial and viral infections through B cells and myeloid cells such as monocytes and macrophages. In this review, we describe the role that BTK plays in supporting suppressive myeloid cells, including myeloid-derived suppressor cells (MDSC) and tumor-associated macrophages (TAM), while also discussing the anticancer effects of BTK inhibition and briefly describe the role of BTK signaling and BTK inhibition in microbial and viral infections.

A Versatile Protocol to Quantify BCR-mediated Phosphorylation in Human and Murine B Cell Subpopulations

Signal transduction is the process by which molecular signals are transmitted from the cell surface to its interior, resulting in functional changes inside the cell. B cell receptor (BCR) signaling is of crucial importance for B cells, as it regulates their differentiation, selection, survival, cellular activation and proliferation. Upon BCR engagement by antigen several protein kinases, lipases and linker molecules become phosphorylated. Phosphoflow cytometry (phosphoflow) is a flow cytometry-based method allowing for analysis of protein phosphorylation in single cells. Due to recent advances in methodology and antibody availability - together with the relatively easy quantification of phosphorylation - phosphoflow is increasingly and more commonly used, compared to classical western blot analysis. It can however be challenging to set-up a method that works for all targets of interest. Here, we present a step-by-step phosphoflow protocol allowing the evaluation of the phosphorylation status of signaling molecules in conjunction with extensive staining to identify various human and murine B cell subpopulations, as was previously published in the original paper by Rip et al. (2020). Next to a description of phosphoflow targets from the original paper, we provide directions on additional targets that play a pivotal role in BCR signaling. The step-by-step phosphoflow protocol is user-friendly and provides sensitive detection of phosphorylation of various BCR signaling molecules in human and murine B cell subpopulations.

Novel mutations in hyper-IgM syndrome type 2 and X-linked agammaglobulinemia detected in three patients with primary immunodeficiency disease

Background

Ambiguous or atypical phenotypes can make a definite diagnosis of primary immunodeficiency diseases based on biochemical indices alone challenging. Further, mortality in early life because of infections in patients with these conditions supports the use of genetic tests to facilitate rapid and accurate diagnoses.

Methods

Genetic and clinical analyses of three unrelated Chinese children with clinical manifestations of recurrent infections, who were considered to have primary immunodeficiency diseases, were conducted. Patient clinical features and serum immunological indices were recorded. Next‐generation sequencing was used to screen for suspected pathogenic variants. Family co‐segregation and in silico analysis were conducted to evaluate the pathogenicity of identified variants, following the American College of Medical Genetics and Genomics guidance.

Results

All three patients were found to have predominant antibody defects. Sequencing analysis revealed that one had two compound heterozygous variants, c.255C>A and c.295C>T, in the autosomal gene, activation‐induced cytidine deaminase (AICDA). The other two patients were each hemizygous for the variants c.1185G>A and c.82C>T in the Bruton's tyrosine kinase (BTK) gene on the X chromosome. In silico analysis revealed that identified substituted amino acids were highly conserved and predicted to cause structural and functional damage to the proteins.

Conclusion

Four pathogenic variants in AICDA and BTK were confirmed to cause different forms of hyper‐IgM syndrome type 2 (HIGM2) and X‐linked agammaglobulinemia (XLA); two were novel mutations that have never been reported previously. This is the first report of HIGM2 caused by AICDA deficiency in a patient from the Chinese mainland.

BCR-Induced Ca2+ Signals Dynamically Tune Survival, Metabolic Reprogramming, and Proliferation of Naive B Cells

B cell receptor (BCR) engagement induces naive B cells to differentiate and perform critical immune-regulatory functions. Acquisition of functional specificity requires that a cell survive, enter the cell cycle, and proliferate. We establish that quantitatively distinct Ca2+ signals triggered by variations in the extent of BCR engagement dynamically regulate these transitions by controlling nuclear factor κB (NF-κB), NFAT, and mTORC1 activity. Weak BCR engagement induces apoptosis by failing to activate NF-κB-driven anti-apoptotic gene expression. Stronger signals that trigger more robust Ca2+ signals promote NF-κB-dependent survival and NFAT-, mTORC1-, and c-Myc-dependent cell-cycle entry and proliferation. Finally, we establish that CD40 or TLR9 costimulation circumvents these Ca2+-regulated checkpoints of B cell activation and proliferation. As altered BCR signaling is linked to autoimmunity and B cell malignancies, these results have important implications for understanding the pathogenesis of aberrant B cell activation and differentiation and therapeutic approaches to target these responses.

Evaluating Acalabrutinib In The Treatment Of Mantle Cell Lymphoma: Design, Development, And Place In Therapy

Mantle cell lymphoma (MCL) is an incurable intermediate-grade lymphoma representing 5-6% of non-Hodgkin's lymphomas diagnosed in the United States. The introduction of inhibitors of Bruton's tyrosine kinase (BTK) into targeted therapy for MCL has significantly improved outcomes in patients with relapsed/refractory (R/R) disease. Since the initial approval of the first-generation inhibitor, ibrutinib, several second-generation inhibitors have been explored. Acalabrutinib, a second-generation BTK inhibitor, has demonstrated impressive efficacy in clinical trials along with a safety profile that thus far appears improved compared to ibrutinib. The results of a Phase II trial in patients with R/R MCL led to the approval of acalabrutinib in this patient population while fueling further exploration of acalabrutinib in several ongoing clinical trials.

A novel Bruton's tyrosine kinase inhibitor, acalabrutinib, suppresses osteoclast differentiation and Porphyromonas gingivalis lipopolysaccharide-induced alveolar bone resorption

BACKGROUND

Periodontitis is not only one of the most prevalent inflammatory diseases among adults, but also commonly linked to numerous systemic conditions including cardiovascular diseases, stroke, and diabetes. Although osteoclasts are responsible for the alveolar bone resorption during periodontitis pathogenesis, the development of pharmacologic strategies targeting these cells has not been vastly fruitful.

METHODS

Bone marrow macrophages were cultured in the presence of macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor κB ligand (RANKL) to examine the direct effect of acalabrutinib on osteoclastogenesis. Ca²⁺ oscillation and nuclear localization of NFATc1 in osteoclast precursors were examined to determine the precise molecular mechanism. LPS-induced alveolar bone loss model was employed for studying effect in in vivo bone resorption.

RESULTS

Acalabrutinib directly inhibited RANKL and LPS-induced in vitro osteoclast differentiation. In addition, acalabrutinib inhibited RANKL-induced phosphorylation of mitogen-activated protein kinases and reduced the expression of NF-κB. The inhibitory mechanism involved suppression of Ca²⁺ oscillation in osteoclast precursors resulting in the decreased NFATc1 expression and nuclear localization, which is a crucial prerequisite for osteoclastogenesis. The administration of acalabrutinib significantly reduced P. gingivalis lipopolysaccharide-induced alveolar bone erosion in mice.

CONCLUSION

These data indicate that acalabrutinib is an effective inhibitor of osteoclastogenesis both in vitro and in vivo, with a potential for a novel strategy against bone destruction by periodontitis.

Discovery and biological evaluation of N5-substituted 6,7-dioxo-6,7-dihydropteridine derivatives as potent Bruton's tyrosine kinase inhibitors

Bruton's tyrosine kinase (BTK) plays a critical role in B cell receptor (BCR)-mediated signaling pathways responsible for the development and function of B cells, which makes it an attractive target for the treatment of many types of B-cell malignancies. Herein, a series of N5-substituted 6,7-dioxo-6,7-dihydropteridine-based, irreversible BTK inhibitors were reported with IC₅₀ values ranging from 1.9 to 236.6 nM in the enzymatic inhibition assay. Compounds 6 and 7 significantly inhibited the proliferation of Ramos cells which overexpress the BTK enzyme, as well as the autophosphorylation of BTK at Tyr223 and the activation of its downstream signaling molecule PLCγ2. Overall, this series of compounds could provide a promising starting point for further development of potent BTK inhibitors for B-cell malignancy treatment.

Role of Bruton's tyrosine kinase in B cells and malignancies

Bruton’s tyrosine kinase (BTK) is a non-receptor kinase that plays a crucial role in oncogenic signaling that is critical for proliferation and survival of leukemic cells in many B cell malignancies. BTK was initially shown to be defective in the primary immunodeficiency X-linked agammaglobulinemia (XLA) and is essential both for B cell development and function of mature B cells. Shortly after its discovery, BTK was placed in the signal transduction pathway downstream of the B cell antigen receptor (BCR). More recently, small-molecule inhibitors of this kinase have shown excellent anti-tumor activity, first in animal models and subsequently in clinical studies. In particular, the orally administered irreversible BTK inhibitor ibrutinib is associated with high response rates in patients with relapsed/refractory chronic lymphocytic leukemia (CLL) and mantle-cell lymphoma (MCL), including patients with high-risk genetic lesions. Because ibrutinib is generally well tolerated and shows durable single-agent efficacy, it was rapidly approved for first-line treatment of patients with CLL in 2016. To date, evidence is accumulating for efficacy of ibrutinib in various other B cell malignancies. BTK inhibition has molecular effects beyond its classic role in BCR signaling. These involve B cell-intrinsic signaling pathways central to cellular survival, proliferation or retention in supportive lymphoid niches. Moreover, BTK functions in several myeloid cell populations representing important components of the tumor microenvironment. As a result, there is currently a considerable interest in BTK inhibition as an anti-cancer therapy, not only in B cell malignancies but also in solid tumors. Efficacy of BTK inhibition as a single agent therapy is strong, but resistance may develop, fueling the development of combination therapies that improve clinical responses. In this review, we discuss the role of BTK in B cell differentiation and B cell malignancies and highlight the importance of BTK inhibition in cancer therapy.

First-in-human phase 1 study of the BTK inhibitor GDC-0853 in relapsed or refractory B-cell NHL and CLL

GDC-0853 is a selective, reversible, and non-covalent inhibitor of Bruton’s tyrosine kinase (BTK) that does not require interaction with the Cys481 residue for activity. In this first-in-human phase 1 study we evaluated safety, tolerability, pharmacokinetics, and activity of GDC-0853 in patients with relapsed or refractory non-Hodgkin lymphoma (NHL) or chronic lymphocytic leukemia (CLL). Twenty-four patients, enrolled into 3 cohorts, including 6 patients who were positive for the C481S mutation, received GDC-0853 at 100, 200, or 400 mg once daily, orally. There were no dose limiting toxicities. GDC-0853 was well tolerated and the maximum tolerated dose (MTD) was not reached due to premature study closure. Common adverse events (AEs) in ≥ 15% of patients regardless of causality included fatigue (37%), nausea (33%), diarrhea (29%), thrombocytopenia (25%), headache (20%), and abdominal pain, cough, and dizziness (16%, each). Nine serious AEs were reported in 5 patients of whom 2 had fatal outcomes (confirmed H1N1 influenza and influenza pneumonia). A third death was due to progressive disease. Eight of 24 patients responded to GDC-0853: 1 complete response, 4 partial responses, and 3 partial responses with lymphocytosis, including 1 patient with the C481S mutation. Two additional C481S mutation patients had a decrease in size of target tumors (–23% and –44%). These data demonstrate GDC-0853 was generally well-tolerated with antitumor activity.

Bruton's Tyrosine Kinase, a Component of B Cell Signaling Pathways, Has Multiple Roles in the Pathogenesis of Lupus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the loss of adaptive immune tolerance to nucleic acid-containing antigens. The resulting autoantibodies form immune complexes that promote inflammation and tissue damage. Defining the signals that drive pathogenic autoantibody production is an important step in the development of more targeted therapeutic approaches for lupus, which is currently treated primarily with non-specific immunosuppression. Here, we review the contribution of Bruton’s tyrosine kinase (Btk), a component of B and myeloid cell signaling pathways, to disease in murine lupus models. Both gain- and loss-of-function genetic studies have revealed that Btk plays multiple roles in the production of autoantibodies. These include promoting the activation, plasma cell differentiation, and class switching of autoreactive B cells. Small molecule inhibitors of Btk are effective at reducing autoantibody levels, B cell activation, and kidney damage in several lupus models. These studies suggest that Btk may promote end-organ damage both by facilitating the production of autoantibodies and by mediating the inflammatory response of myeloid cells to these immune complexes. While Btk has not been associated with SLE in GWAS studies, SLE B cells display signaling defects in components both upstream and downstream of Btk consistent with enhanced activation of Btk signaling pathways. Taken together, these observations indicate that limiting Btk activity is critical for maintaining B cell tolerance and preventing the development of autoimmune disease. Btk inhibitors, generally well-tolerated and approved to treat B cell malignancy, may thus be a useful therapeutic approach for SLE.

Experience with ibrutinib for first-line use in patients with chronic lymphocytic leukemia

Ibrutinib is the first in-class, orally administered, Bruton's tyrosine kinase (BTK) inhibitor that abrogates the critical signaling downstream of the B-cell receptor (BCR). This signaling is required for B-cell survival, proliferation and interaction with the microenvironment. Ibrutinib proved active in preclinical models of lymphoproliferative diseases and achieved impressive response rates in heavily pretreated relapsed and refractory (R/R) patients with chronic lymphocytic leukemia (CLL). Ibrutinib prolonged survival compared to standard therapy and mitigated the effect of most poor prognostic factors in CLL, thus becoming the main therapeutic option in high-risk populations. Moreover, compared with standard chemoimmunotherapy (CIT) for adults, ibrutinib causes fewer cytopenias and infections, while having its own unique toxicity profile. Its efficacy in relapsed patients as well as its tolerability have led to its increased use in previously untreated patients, especially in those with poor prognostic markers and/or the elderly. This review elaborates on ibrutinib's unique toxicity profile and the mechanisms of acquired resistance leading to progression on ibrutinib, since both are critical for understanding the obstacles to its first-line use. We will further evaluate the data from ongoing clinical trials in this setting and explore future options for combination therapy.

Inhibition of BTK protects lungs from trauma-hemorrhagic shock-induced injury in rats

The present study aimed to investigate the role of Bruton's tyrosine kinase (BTK) in the pathogenesis of lung injury induced by trauma‑hemorrhagic shock (THS), and to examine the pulmonary protective effects of BTK inhibition. Male Sprague‑Dawley rats were divided into four groups (n=12/group): i) A Sham group, which received surgery without induced trauma; ii) a THS‑induced injury group; iii) a THS‑induced injury group that also received treatment with the BTK inhibitor LFM‑A13 prior to trauma induction; and iv) a Sham group that was pretreated with LFM‑A13 prior to surgery but did not receive induced trauma. The expression of phosphorylated‑BTK protein in the lungs was measured by immunohistochemistry and western blot analysis. The bronchoalveolar lavage fluid (BALF) protein concentration, total leukocyte and eosinophil numbers, and the expression levels of peripheral blood proinflammatory factors were measured. Morphological alterations in the lungs were detected by hematoxylin and eosin staining. Pulmonary nitric oxide (NO) concentration and inducible NO synthase (iNOS) expression were also assessed. Activities of the nuclear factor (NF)‑κB and mitogen‑activated protein kinase (MAPK) signaling pathways were determined by western blotting or electrophoretic mobility shift assay. BTK was notably activated in lungs of THS rats. BALF protein concentration, total leukocytes and eosinophils, peripheral blood expression levels of tumor necrosis factor‑α, interleukin (IL)‑1β, IL‑6 and monocyte chemotactic protein 1 were significantly upregulated after THS induction, and each exhibited decreased expression upon LFM‑A13 treatment. THS‑induced interstitial hyperplasia, edema and neutrophilic infiltration in lungs were improved by the inhibition of BTK. In addition, THS‑induced NO release, iNOS overexpression, and NF‑κB and MAPK signaling were suppressed by BTK inhibition. Results from the present study demonstrate that BTK may serve a pivotal role in the pathogenesis of THS‑related lung injury, and the inhibition of BTK may significantly alleviate THS‑induced lung damage. These results provide a potential therapeutic application for the treatment of THS‑induced lung injury.

Mediation of transitional B cell maturation in the absence of functional Bruton's tyrosine kinase

X-linked immune-deficient (Xid) mice, carrying a mutation in Bruton’s tyrosine kinase (Btk), have multiple B cell lineage differentiation defects. We now show that, while Xid mice showed only mild reduction in the frequency of the late transitional (T2) stage of peripheral B cells, the defect became severe when the Xid genotype was combined with either a CD40-null, a TCRbeta-null or an MHC class II (MHCII)-null genotype. Purified Xid T1 and T2 B cells survived poorly in vitro compared to wild-type (WT) cells. BAFF rescued WT but not Xid T1 and T2 B cells from death in culture, while CD40 ligation equivalently rescued both. Xid transitional B cells ex vivo showed low levels of the p100 protein substrate for non-canonical NF-kappaB signalling. In vitro, CD40 ligation induced equivalent activation of the canonical but not of the non-canonical NF-kappaB pathway in Xid and WT T1 and T2 B cells. CD40 ligation efficiently rescued p100-null T1 B cells from neglect-induced death in vitro. These data indicate that CD40-mediated signals, likely from CD4 T cells, can mediate peripheral transitional B cell maturation independent of Btk and the non-canonical NF-kappaB pathway, and thus contribute to the understanding of the complexities of peripheral B cell maturation.

From identification of the BTK kinase to effective management of leukemia

BTK is a cytoplasmic protein-tyrosine kinase, whose corresponding gene was isolated in the early 1990s. BTK was initially identified by positional cloning of the gene causing X-linked agammaglobulinemia and independently in a search for new kinases. Given the phenotype of affected patients, namely lack of B-lymphocytes and plasma cells with the ensuing inability to mount humoral immune responses, BTK inhibitors were anticipated to have beneficial effects on antibody-mediated pathologies, such as autoimmunity. In contrast to, for example, the SRC-family of cytoplasmic kinases, there was no obvious way in which structural alterations would yield constitutively active forms of BTK, and such mutations were also not found in leukemias or lymphomas. In 2007, the first efficient inhibitor, ibrutinib, was reported and soon became approved both in the United States and in Europe for the treatment of three B-cell malignancies, mantle cell lymphoma, chronic lymphocytic leukemia and Waldenström's macroglobulinemia. Over the past few years, additional inhibitors have been developed, with acalabrutinib being more selective, and recently demonstrating fewer clinical adverse effects. The antitumor mechanism is also not related to mutations in BTK. Instead tumor residency in lymphoid organs is inhibited, making these drugs highly versatile. BTK is one of the only 10 human kinases that carry a cysteine in the adenosine triphosphate-binding cleft. As this allows for covalent, irreversible inhibitor binding, it provides these compounds with a highly advantageous character. This quality may be crucial and bodes well for the future of BTK-modifying medicines, which have been estimated to reach annual multi-billion dollar sales in the future.

Pharmacokinetic and pharmacodynamic evaluation of ibrutinib for the treatment of chronic lymphocytic leukemia: rationale for lower doses

INTRODUCTION

Ibrutinib, a first-in-class covalent inhibitor of Bruton's tyrosine kinase (BTK), is approved in many countries for the treatment of relapsed/refractory chronic lymphocytic leukemia (CLL) and for previously untreated disease with a 17p deletion and, most recently, as a frontline therapy for CLL. In controlled trials in CLL, ibrutinib produced high response rates and improved survival in both the frontline and relapsed settings. While ibrutinib controls CLL with impressive efficacy, it only infrequently induces complete remissions, particularly of relapsed CLL, and does not eradicate minimal residual disease. Finally, ibrutinib is extremely expensive, has off-target toxicities, and requires indefinite therapy. Areas covered: In this article, we provide an overview of the CLL therapeutic landscape and discuss the pharmacokinetic and pharmacodynamic aspects of ibrutinib. Major clinical trials of ibrutinib in CLL are summarized, and its safety profile explored. Expert opinion: Ibrutinib represents a transformative advance in CLL management and has validated BTK as a therapeutic target in this disease, but has some limitations, leading to the emergence of other BTK inhibitors and mechanism-based combination strategies. Given complete BTK occupancy at lower doses of ibrutinib and declining levels of BTK on ibrutinib therapy, lower doses of ibrutinib in CLL are being explored.

Targeting tumor tolerance: A new hope for pancreatic cancer therapy?

With a 5-year survival rate of just 8%, pancreatic cancer (PC) is projected to be the second leading cause of cancer deaths by 2030. Most PC patients are not eligible for surgery with curative intent upon diagnosis, emphasizing a need for more effective therapies. However, PC is notoriously resistant to chemoradiation regimens. As an alternative, immune modulating strategies have recently achieved success in melanoma, prompting their application to other solid tumors. For such therapeutic approaches to succeed, a state of immunologic tolerance must be reversed in the tumor microenvironment and that has been especially challenging in PC. Nonetheless, knowledge of the PC immune microenvironment has advanced considerably over the past decade, yielding new insights and perspectives to guide multimodal therapies. In this review, we catalog the historical groundwork and discuss the evolution of the cancer immunology field to its present state with a specific focus on PC. Strategies currently employing immune modulation in PC are reviewed, specifically highlighting 66 clinical trials across the United States and Europe.

Protein kinase C in the immune system: from signalling to chromatin regulation

Protein kinase C (PKC) form a key family of enzymes involved in signalling pathways that specifically phosphorylates substrates at serine/threonine residues. Phosphorylation by PKC is important in regulating a variety of cellular events such as cell proliferation and the regulation of gene expression. In the immune system, PKCs are involved in regulating signal transduction pathways important for both innate and adaptive immunity, ultimately resulting in the expression of key immune genes. PKCs act as mediators during immune cell signalling through the immunological synapse. PKCs are traditionally known to be cytoplasmic signal transducers and are well embedded in the signalling pathways of cells to mediate the cells' response to a stimulus from the plasma membrane to the nucleus. PKCs are also found to transduce signals within the nucleus, a process that is distinct from the cytoplasmic signalling pathway. There is now growing evidence suggesting that PKC can directly regulate gene expression programmes through a non-traditional role as nuclear kinases. In this review, we will focus on the role of PKCs as key cytoplasmic signal transducers in immune cell signalling, as well as its role in nuclear signal transduction. We will also highlight recent evidence for its newly discovered regulatory role in the nucleus as a chromatin-associated kinase.

Splice-correction strategies for treatment of X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a primary immunodeficiency disease caused by mutations in the gene coding for Bruton’s tyrosine kinase (BTK). Deficiency of BTK leads to a developmental block in B cell differentiation; hence, the patients essentially lack antibody-producing plasma cells and are susceptible to various infections. A substantial portion of the mutations in BTK results in splicing defects, consequently preventing the formation of protein-coding mRNA. Antisense oligonucleotides (ASOs) are therapeutic compounds that have the ability to modulate pre-mRNA splicing and alter gene expression. The potential of ASOs has been exploited for a few severe diseases, both in pre-clinical and clinical studies. Recently, advances have also been made in using ASOs as a personalized therapy for XLA. Splice-correction of BTK has been shown to be feasible for different mutations in vitro, and a recent proof-of-concept study demonstrated the feasibility of correcting splicing and restoring BTK both ex vivo and in vivo in a humanized bacterial artificial chromosome (BAC)-transgenic mouse model. This review summarizes the advances in splice correction, as a personalized medicine for XLA, and outlines the promises and challenges of using this technology as a curative long-term treatment option.

Therapeutic potential of new B cell-targeted agents in the treatment of elderly and unfit patients with chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL), the most common adult leukemia in the Western world, is primarily a disease of the elderly, with most patients ≥65 years of age and having at least one major comorbidity. Aggressive chemoimmunotherapy regimens recommended to achieve remission and improve survival in young, fit patients are often poorly tolerated in elderly and/or less physiologically fit (“unfit”) patients, necessitating alternative treatment options. Although patient age, fitness, and comorbidities are key considerations in the selection of a treatment regimen, historically, clinical trials have been limited to young, fit patients by virtue of the ethical concerns associated with potential end organ toxic effects that could worsen comorbidities. However, the availability of new therapies promises a shift to a research paradigm that encompasses the identification of optimal treatments for elderly and unfit patients. Anti-CD20 monoclonal antibody therapy, which overall has improved response rates and survival in patients with CLL, has only recently been evaluated elderly and unfit patients. B cell-targeted agents such as the Bruton’s tyrosine kinase inhibitor ibrutinib and the phosphatidylinositol 3-kinase inhibitor idelalisib are the first of a new generation of oral agents for CLL. Available clinical data suggest that these therapies have the potential to address the unmet need in elderly and unfit patients with CLL and result in clinical remission, and not merely symptom palliation and improved quality of life, which, by themselves, are also a reasonable goal.

B-cell receptor signaling in the pathogenesis of lymphoid malignancies

B-cell receptor (BCR) signaling pathway plays a central role in B-lymphocyte development and initiation of humoral immunity. Recently, BCR signaling pathway has been shown as a major driver in the pathogenesis of B-cell malignancies. As a result, a vast array of BCR-associated kinases has emerged as rational therapeutic targets changing treatment paradigms in B cell malignancies. Based on high efficacy in early-stage clinical trials, there is rapid clinical development of inhibitors targeting BCR signaling pathway. Here, we describe the essential components of BCR signaling, their function in normal and pathogenic signaling and molecular effects of their inhibition in vitro and in vivo.

Synergistic induction of apoptosis by combination of BTK and dual mTORC1/2 inhibitors in diffuse large B cell lymphoma

Diffuse large B cell lymphoma is generally treated by chemotherapy and there is an unmet medical need for novel targeted therapies or combination therapies. Using in vitro screening, we have identified the combination of ibrutinib, an inhibitor of the tyrosine kinase BTK, and AZD2014, an mTOR catalytic inhibitor, as being highly synergistic in killing ABC-subtype DLBCL cell lines. Simultaneous inhibition of BTK and mTOR causes apoptosis both in vitro and in vivo and results in tumor regression in a xenograft model. We identify two parallel mechanisms that underlie apoptosis in this setting: cooperative inhibition of cap-dependent translation, and the inhibition of an NF-κB/IL10/STAT3 autocrine loop. Combined disruption of these pathways is required for apoptosis. These data represent a rational basis for the dual inhibition of BTK and mTOR as a potential treatment for ABC-subtype DLBCL.

Role of Bruton's tyrosine kinase inhibitors in HIV-1-infected cells

Many cellular cofactors have been documented to be critical for various stages of viral replication. Using high-throughput proteomic assays, we have previously identified Bruton's tyrosine kinase (BTK) as a host protein that was uniquely upregulated in the plasma membrane of human immunodeficiency virus (HIV-1)-infected T cells. Here, we have further characterized the BTK expression in HIV-1 infection and show that this cellular factor is specifically expressed in infected myeloid cells. Significant upregulation of the phosphorylated form of BTK was observed in infected cells. Using size exclusion chromatography, we found BTK to be virtually absent in the uninfected U937 cells; however, new BTK protein complexes were identified and distributed in both high molecular weight (∼600 kDa) and a small molecular weight complex (∼60-120 kDa) in the infected U1 cells. BTK levels were highest in cells either chronically expressing virus or induced/infected myeloid cells and that BTK translocated to the membrane following induction of the infected cells. BTK knockdown in HIV-1-infected cells using small interfering RNA (siRNA) resulted in selective death of infected, but not uninfected, cells. Using BTK-specific antibody and small-molecule inhibitors including LFM-A13 and a FDA-approved compound, ibrutinib (PCI-32765), we have found that HIV-1-infected cells are sensitive to apoptotic cell death and result in a decrease in virus production. Overall, our data suggests that HIV-1-infected cells are sensitive to treatments targeting BTK expressed in infected cells.

Rewiring of sIgM-Mediated Intracellular Signaling through the CD180 Toll-like Receptor

Chronic lymphocytic leukemia (CLL) development and progression are thought to be driven by unknown antigens/autoantigens through the B cell receptor (BCR) and environmental signals for survival and expansion including toll-like receptor (TLR) ligands. CD180/RP105, a membrane-associated orphan receptor of the TLR family, induces normal B cell activation and proliferation and is expressed by approximately 60% of CLL samples. Half of these respond to ligation with anti-CD180 antibody by increased activation/phosphorylation of protein kinases associated with BCR signaling. Hence CLL cells expressing both CD180 and the BCR could receive signals via both receptors. Here we investigated cross-talk between BCR and CD180-mediated signaling on CLL cell survival and apoptosis. Our data indicate that ligation of CD180 on responsive CLL cells leads to activation of either prosurvival Bruton tyrosine kinase (BTK)/phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/AKT-mediated, or proapoptotic p38 mitogen-activated protein kinase (p38MAPK)-mediated signaling pathways, while selective immunoglobulin M (sIgM) ligation predominantly engages the BTK/PI3K/AKT pathway. Furthermore, pretreatment of CLL cells with anti-CD180 redirects IgM-mediated signaling from the prosurvival BTK/PI3K/AKT toward the proapoptotic p38MAPK pathway. Thus preengaging CD180 could prevent further prosurvival signaling mediated via the BCR and, instead, induce CLL cell apoptosis, opening the door to therapeutic profiling and new strategies for the treatment of a substantial cohort of CLL patients.

Bruton's tyrosine kinase and protein kinase C µ are required for TLR7/9-induced IKKα and IRF-1 activation and interferon-β production in conventional dendritic cells

Stimulation of TLR7/9 by their respective ligands leads to the activation of IκB kinase α (IKKα) and Interferon Regulatory Factor 1 (IRF-1) and results in interferon (IFN)-β production in conventional dendritic cells (cDC). However, which other signaling molecules are involved in IKKα and IRF-1 activation during TLR7/9 signaling pathway are not known. We and others have shown that Bruton's Tyrosine Kinase (BTK) played a part in TLR9-mediated cytokine production in B cells and macrophages. However, it is unclear if BTK participates in TLR7/9-induced IFN-β production in cDC. In this study, we show that BTK is required for IFN-β synthesis in cDC upon TLR7/9 stimulation and that stimulated BTK-deficient cDC are defective in the induction of IKKα/β phosphorylation and IRF-1 activation. In addition, we demonstrate that Protein Kinase C µ (PKCµ) is also required for TLR7/9-induced IRF-1 activation and IFN-β upregulation in cDC and acts downstream of BTK. Taken together, we have uncovered two new molecules, BTK and PKCµ, that are involved in TLR7/9-triggered IFN-β production in cDC.

A balance between B cell receptor and inhibitory receptor signaling controls plasma cell differentiation by maintaining optimal Ets1 levels

Signaling through the BCR can drive B cell activation and contribute to B cell differentiation into Ab-secreting plasma cells. The positive BCR signal is counterbalanced by a number of membrane-localized inhibitory receptors that limit B cell activation and plasma cell differentiation. Deficiencies in these negative signaling pathways may cause autoantibody generation and autoimmune disease in both animal models and human patients. We have previously shown that the transcription factor Ets1 can restrain B cell differentiation into plasma cells. In this study, we tested the roles of the BCR and inhibitory receptors in controlling the expression of Ets1 in mouse B cells. We found that Ets1 is downregulated in B cells by BCR or TLR signaling through a pathway dependent on PI3K, Btk, IKK2, and JNK. Deficiencies in inhibitory pathways, such as a loss of the tyrosine kinase Lyn, the phosphatase Src homology region 2 domain-containing phosphatase 1 (SHP1) or membrane receptors CD22 and/or Siglec-G, result in enhanced BCR signaling and decreased Ets1 expression. Restoring Ets1 expression in Lyn- or SHP1-deficient B cells inhibits their enhanced plasma cell differentiation. Our findings indicate that downregulation of Ets1 occurs in response to B cell activation via either BCR or TLR signaling, thereby allowing B cell differentiation and that the maintenance of Ets1 expression is an important function of the inhibitory Lyn → CD22/SiglecG → SHP1 pathway in B cells.

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.

Discovery of a potent, covalent BTK inhibitor for B-cell lymphoma

BTK is a member of the TEC family of non-receptor tyrosine kinases whose deregulation has been implicated in a variety of B-cell-related diseases. We have used structure-based drug design in conjunction with kinome profiling and cellular assays to develop a potent, selective, and irreversible BTK kinase inhibitor, QL47, which covalently modifies Cys481. QL47 inhibits BTK kinase activity with an IC50 of 7 nM, inhibits autophosphorylation of BTK on Tyr223 in cells with an EC50 of 475 nM, and inhibits phosphorylation of a downstream effector PLCγ2 (Tyr759) with an EC50 of 318 nM. In Ramos cells QL47 induces a G1 cell cycle arrest that is associated with pronounced degradation of BTK protein. QL47 inhibits the proliferation of B-cell lymphoma cancer cell lines at submicromolar concentrations.

Single cell imaging of Bruton's tyrosine kinase using an irreversible inhibitor

A number of Bruton's tyrosine kinase (BTK) inhibitors are currently in development, yet it has been difficult to visualize BTK expression and pharmacological inhibition in vivo in real time. We synthesized a fluorescent, irreversible BTK binder based on the drug Ibrutinib and characterized its behavior in cells and in vivo. We show a 200 nM affinity of the imaging agent, high selectivity, and irreversible binding to its target following initial washout, resulting in surprisingly high target-to-background ratios. In vivo, the imaging agent rapidly distributed to BTK expressing tumor cells, but also to BTK-positive tumor-associated host cells.

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.