Xid and Xid-like immunodeficiencies from a signaling point of view

Bruton's tyrosine kinase (Btk) inhibitor tirabrutinib suppresses osteoclastic bone resorption

Osteoclasts are responsible for bone erosion in osteoporosis and rheumatoid arthritis (RA). Both Btk and Tec kinases have essential functions in osteoclast differentiation. Tirabrutinib is a highly potent and dual oral Btk/Tec inhibitor with an IC₅₀ in the nmol/L range and significantly inhibits the M-CSF and RANKL-driven osteoclast differentiation. It was hypothesized that the in vitro activity of tirabrutinib could be demonstrated in mice bone resorption model. The RANKL model studies show that tirabrutinib significantly suppressed bone loss with the inhibition of serum TRAPCP5b and urinary CTX-1. Bone Mineral Density (BMD) loss in tirabrutinib-treated mice was 55% (P < .05), 87% (P < .001) and 88% (P < .001) for the 3, 10 and 30 mg/kg dose groups respectively.

Btk and Tec are required for osteoclast differentiation and activation based on the genetic evidence obtained from Btk and Tec double deficient mice. Tirabrutinib may be a novel therapeutic target for bone diseases, such as osteoporosis and RA.

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Btk and Tec are activated by RANKL and indispensable for osteoclastogenesis.

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Tirabrutinib is a highly potent and dual oral Btk/Tec inhibitor.

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Osteoclast differentiation is activated by RANKL, M-CSF and ITAM.

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Tirabrutinib inhibited RANKL-induced osteolyrtic bone disease.

Bruton's tyrosine kinase (BTK) function is important to the development and expansion of chronic lymphocytic leukemia (CLL)

Chronic lymphocytic leukemia (CLL) is characterized by constitutive activation of the B-cell receptor (BCR) signaling pathway, but variable responsiveness of the BCR to antigen ligation. Bruton's tyrosine kinase (BTK) shows constitutive activity in CLL and is the target of irreversible inhibition by ibrutinib, an orally bioavailable kinase inhibitor that has shown outstanding activity in CLL. Early clinical results in CLL with other reversible and irreversible BTK inhibitors have been less promising, however, raising the question of whether BTK kinase activity is an important target of ibrutinib and also in CLL. To determine the role of BTK in CLL, we used patient samples and the Eμ-TCL1 (TCL1) transgenic mouse model of CLL, which results in spontaneous leukemia development. Inhibition of BTK in primary human CLL cells by small interfering RNA promotes apoptosis. Inhibition of BTK kinase activity through either targeted genetic inactivation or ibrutinib in the TCL1 mouse significantly delays the development of CLL, demonstrating that BTK is a critical kinase for CLL development and expansion and thus an important target of ibrutinib. Collectively, our data confirm the importance of kinase-functional BTK in CLL.

The B-cell receptor signaling pathway as a therapeutic target in CLL

Targeted therapy with imatinib and other selective tyrosine kinase inhibitors has transformed the treatment of chronic myeloid leukemia. Unlike chronic myeloid leukemia, chronic lymphocytic leukemia (CLL) lacks a common genetic aberration amenable to therapeutic targeting. However, our understanding of normal B-cell versus CLL biology points to differences in properties of B-cell receptor (BCR) signaling that may be amenable to selective therapeutic targeting. The application of mouse models has further expanded this understanding and provides information about targets in the BCR signaling pathway that may have other important functions in cell development or long-term health. In addition, overexpression or knockout of selected targets offers the potential to validate targets genetically using new mouse models of CLL. The initial success of BCR-targeted therapies has promoted much excitement in the field of CLL. At the present time, GS-1101, which reversibly inhibits PI3Kδ, and ibrutinib (PCI-32765), an irreversible inhibitor of Bruton tyrosine kinase, have generated the most promising early results in clinical trials including predominately refractory CLL where durable disease control has been observed. This review provides a summary of BCR signaling, tools for studying this pathway relevant to drug development in CLL, and early progress made with therapeutics targeting BCR-related kinases.

The B-cell receptor signaling pathway as a therapeutic target in CLL

Targeted therapy with imatinib and other selective tyrosine kinase inhibitors has transformed the treatment of chronic myeloid leukemia. Unlike chronic myeloid leukemia, chronic lymphocytic leukemia (CLL) lacks a common genetic aberration amenable to therapeutic targeting. However, our understanding of normal B-cell versus CLL biology points to differences in properties of B-cell receptor (BCR) signaling that may be amenable to selective therapeutic targeting. The application of mouse models has further expanded this understanding and provides information about targets in the BCR signaling pathway that may have other important functions in cell development or long-term health. In addition, overexpression or knockout of selected targets offers the potential to validate targets genetically using new mouse models of CLL. The initial success of BCR-targeted therapies has promoted much excitement in the field of CLL. At the present time, GS-1101, which reversibly inhibits PI3Kδ, and ibrutinib (PCI-32765), an irreversible inhibitor of Bruton tyrosine kinase, have generated the most promising early results in clinical trials including predominately refractory CLL where durable disease control has been observed. This review provides a summary of BCR signaling, tools for studying this pathway relevant to drug development in CLL, and early progress made with therapeutics targeting BCR-related kinases.

Steel factor enhances supraoptimal antigen-induced IL-6 production from mast cells via activation of protein kinase C-beta

Ag-triggered mast cell (MC) activation follows a bell-shaped dose-response curve. Reduced activation in response to supraoptimal Ag concentrations is thought to be due to preferential engagement of inhibitory-acting proteins like SHIP1, Lyn, and protein kinase C (PKC)-delta. We show in this study that short-term prestimulation with Steel factor (SF) prevents supraoptimal Ag inhibition, resulting in synergistic MC degranulation and IL-6 secretion. These events are preceded by synergistic phosphorylation/activation of numerous signaling proteins, e.g., Erk, p38, and LAT. However, these effects of prestimulation with SF appear not to be due to reduced engagement of the attenuator SHIP1. Pharmacological analyses suggest that the activation of conventional PKCs is important for this synergy. Specifically, although we found that the conventional PKC inhibitor, Gö6976, likely has some PKC-independent targets in MCs, it led us to further studies that established SF plus Ag-induced IL-6 secretion was severely impaired in PKC-beta(-/-) MCs, but not PKC-alpha(-/-) MCs. Thus, PKC-beta joins PI3K and Btk as important players in this synergistic MC activation.

Mutations of the Igbeta gene cause agammaglobulinemia in man

Agammaglobulinemia is a rare primary immunodeficiency characterized by an early block of B cell development in the bone marrow, resulting in the absence of peripheral B cells and low/absent immunoglobulin serum levels. So far, mutations in Btk, mu heavy chain, surrogate light chain, Igalpha, and B cell linker have been found in 85-90% of patients with agammaglobulinemia. We report on the first patient with agammaglobulinemia caused by a homozygous nonsense mutation in Igbeta, which is a transmembrane protein that associates with Igalpha as part of the preBCR complex. Transfection experiments using Drosophila melanogaster S2 Schneider cells showed that the mutant Igbeta is no longer able to associate with Igalpha, and that assembly of the BCR complex on the cell surface is abrogated. The essential role of Igbeta for human B cell development was further demonstrated by immunofluorescence analysis of the patient's bone marrow, which showed a complete block of B cell development at the pro-B to preB transition. These results indicate that mutations in Igbeta can cause agammaglobulinemia in man.

Bruton's tyrosine kinase (Btk) enhances transcriptional co-activation activity of BAM11, a Btk-associated molecule of a subunit of SWI/SNF complexes

Bruton's tyrosine kinase (Btk) is required for B cell development and signal transduction through cell-surface molecules such as BCR and IL-5 receptor. We have identified a Btk-associated molecule, BAM11 (hereafter referred to as BAM) that binds to the pleckstrin homology (PH) domain of Btk, and inhibits Btk activity both in vivo and in vitro. In this study, we demonstrate BAM's transcriptional co-activation activity and its functional interaction with Btk. By using transient transcription assays, we demonstrate that the enforced expression of BAM enhances transcriptional activity of the synthetic reporter gene. The C-terminus of BAM is essential for the transcriptional co-activation activity. The ectopic expression of Btk together with BAM enhances BAM's transcriptional co-activation activity. BAM's transcriptional co-activation activity is enhanced through interaction with Btk, and requires both its intact PH domain and functional kinase activity. We also show that enforced expression of TFII-I, another Btk-binding protein with transcriptional activity, together with BAM and Btk, further augments BAM- and Btk-dependent transcriptional co-activation. Furthermore, BAM can be co-immunoprecipitated with the INI1/SNF5 protein, a member of the SWI/SNF complex that remodels chromatin and activates transcription. We propose a model in which Btk regulates gene transcription in B cells by activating BAM and the SWI/SNF transcriptional complex via TFII-I activation.

Cell cycle control mechanisms in B-1 and B-2 lymphoid subsets

An effective humoral response requires that a given B lymphocyte population express a repertoire of receptors capable of recognizing a distinct array of antigens, while at the same time disregarding self-antigens. Mature B cells interacting with antigen via their B cell antigen receptors (BCRs) enter G(1) phase of the cell cycle and, depending on the strength of the signal, can commit to S phase entry. Input from co-receptors, which may function to either enhance or inhibit BCR signals, also influence the decision to proliferate. We review herein recent advances in the biochemistry of G(1)-cyclin holoenzymes that function to integrate BCR-coupled signaling pathways to the phosphorylation (and inactivation) of the retinoblastoma gene product (pRb) in splenic B lymphocytes (B-2 cells). We also highlight differences in the control of G(1)-to-S phase progression between B-2 cells and peritoneal CD5+ B cells (B-1 cells).

PI3K and Btk differentially regulate B cell antigen receptor-mediated signal transduction

Phosphoinositide-3 kinase (PI3K) is thought to activate the tyrosine kinase Btk. However, through analysis of PI3K-/- and Btk-/- mice, B cell antigen receptor (BCR)-induced activation of Btk in mouse B cells was found to be unaffected by PI3K inhibitors or by a lack of PI3K. Consistent with this observation, PI3K-/- Btk-/- double-deficient mice had more severe defects than either single-mutant mouse. NF-kappaB activation along with Bcl-xL and cyclin D2 induction were severely blocked in both PI3K-/- and Btk-/- single-deficient B cells. Transgenic expression of Bcl-xL restored the development and BCR-induced proliferation of B cells in PI3K-/- mice. Our results indicate that PI3K and Btk have unique roles in proximal BCR signaling and that they have a common target further downstream in the activation of NF-kappaB.

PKCbeta modulates antigen receptor signaling via regulation of Btk membrane localization

Mutations in Bruton's tyrosine kinase (Btk) result in X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. While targeted disruption of the protein kinase C-beta (PKCbeta) gene in mice results in an immunodeficiency similar to xid, the overall tyrosine phosphorylation of Btk is significantly enhanced in PKCbeta-deficient B cells. We provide direct evidence that PKCbeta acts as a feedback loop inhibitor of Btk activation. Inhibition of PKCbeta results in a dramatic increase in B-cell receptor (BCR)-mediated Ca2+ signaling. We identified a highly conserved PKCbeta serine phosphorylation site in a short linker within the Tec homology domain of Btk. Mutation of this phosphorylation site led to enhanced tyrosine phosphorylation and membrane association of Btk, and augmented BCR and FcepsilonRI-mediated signaling in B and mast cells, respectively. These findings provide a novel mechanism whereby reversible translocation of Btk/Tec kinases regulates the threshold for immunoreceptor signaling and thereby modulates lymphocyte activation.

B-cell-receptor-dependent positive and negative selection in immature B cells

This review touches on only a small part of the complex biology of B cells, but serves to illustrate the point that the antigen receptor is the most important of many cell-surface receptors affecting cell-fate decisions. Receptor expression is necessary, but not sufficient, for cell survival. It is also essential that a B cell's antigen-receptor specificity be appropriate for its environment. The need to balance reactivity with self tolerance has resulted in an intricate feedback control (affected by both the recombinase and cell survival) that regulates independent selection events at the level of the receptor and the cell.

Genetic analysis of B cell antigen receptor signaling

In B lymphocytes, a signaling complex that contributes to cell fate decisions is the B cell antigen receptor (BCR). Data from knockout experiments in cell lines and mice have revealed distinct functions for the intracellular protein tyrosine kinases (Lyn, Syk, Btk) in BCR signaling and B cell development. Combinations of intracellular signaling pathways downstream of these PTKs determine the quality and quantity of BCR signaling. For example, concerted actions of the PLC-gamma 2 and PI3-K pathways are required for proper calcium responses. Similarly, the regulation of ERK and JNK responses involves both PLC-gamma 2 and GTPases pathways. Since the immune response in vivo is regulated by alteration of these signaling outcomes, achieving a precise understanding of intracellular molecular events leading to B lymphocyte proliferation, deletion, anergy, receptor editing, and survival still remains a challenge for the future.

What do the paralogous regions in the genome tell us about the origin of the adaptive immune system?

During the last decade, our understanding of the immune system of ectothermic vertebrates has advanced significantly. It is now clear that all jawed vertebrates are equipped with the adaptive immune system characterized by the MHC molecules and the rearranging receptors. In contrast, there is no molecular evidence that suggests the existence of adaptive immunity in jawless vertebrates. How did the adaptive immune system emerge? Our recent work suggests that one of the driving forces that enabled the emergence of the adaptive immune system was one or more genome-wide or large-scale chromosomal duplications presumed to have taken place in a common ancestor of jawed vertebrates.

Involvement of Ras in Bruton's tyrosine kinase-mediated JNK activation

Defects in Bruton's tyrosine kinase (Btk) result in B cell immunodeficiencies in humans and mice. Recent studies showed that Btk is required for maximal activation of JNK, a family of stress-activated protein kinases, induced by several extracellular stimuli including interleukin (IL)-3. On the other hand, IL-3-induced JNK activation is dependent on Ras. In the present study we have investigated whether Ras is involved in Btk-mediated JNK activation in BaF3 mouse pro-B cells. Overexpression of wild-type Btk protein in these cells enhanced JNK activation upon IL-3 stimulation, whereas expression of kinase-dead Btk partially suppressed JNK activation. Induced expression of the dominant negative Ras(N17) in the cells overexpressing wild-type Btk suppressed JNK activation. Importantly, overexpression of Btk enhanced the level of the GTP-bound, active form of Ras in response to IL-3 stimulation. Btk overexpression also increased the Shc-Grb2 association induced by IL-3 stimulation. Expression of either N17Ras or V12Ras did not impose any effects on Btk kinase activity. These data collectively indicate that Ras plays a role of an intermediary signaling protein in Btk-mediated JNK activation induced by the IL-3 signaling pathway.

Analysis of antigen receptor signalling using mouse gene targeting

Gene targeting in mice has enabled the study of antigen receptor signalling in primary lymphocytes. Furthermore, it has provided the tools to directly assess the function of individual signalling proteins by mutation of the genes that code for them. Some of the results that gene targeting has produced have confirmed previous views of the function of particular proteins. Others have given surprising results and overturned accepted viewpoints.