Mutation of unique region of Bruton's tyrosine kinase in immunodeficient XID mice

Dual phosphorylation of Btk by Akt/protein kinase b provides docking for 14-3-3ζ, regulates shuttling, and attenuates both tonic and induced signaling in B cells

Bruton's tyrosine kinase (Btk) is crucial for B-lymphocyte activation and development. Mutations in the Btk gene cause X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. Using tandem mass spectrometry, 14-3-3ζ was identified as a new binding partner and negative regulator of Btk in both B-cell lines and primary B lymphocytes. The activated serine/threonine kinase Akt/protein kinase B (PKB) phosphorylated Btk on two sites prior to 14-3-3ζ binding. The interaction sites were mapped to phosphoserine pS51 in the pleckstrin homology domain and phosphothreonine pT495 in the kinase domain. The double-alanine, S51A/T495A, replacement mutant failed to bind 14-3-3ζ, while phosphomimetic aspartate substitutions, S51D/T495D, caused enhanced interaction. The phosphatidylinositol 3-kinase (PI3-kinase) inhibitor LY294002 abrogated S51/T495 phosphorylation and binding. A newly characterized 14-3-3 inhibitor, BV02, reduced binding, as did the Btk inhibitor PCI-32765 (ibrutinib). Interestingly, in the presence of BV02, phosphorylation of Btk, phospholipase Cγ2, and NF-κB increased strongly, suggesting that 14-3-3 also regulates B-cell receptor (BCR)-mediated tonic signaling. Furthermore, downregulation of 14-3-3ζ elevated nuclear translocation of Btk. The loss-of-function mutant S51A/T495A showed reduced tyrosine phosphorylation and ubiquitination. Conversely, the gain-of-function mutant S51D/T495D exhibited intense tyrosine phosphorylation, associated with Btk ubiquitination and degradation, likely contributing to the termination of BCR signaling. Collectively, this suggests that Btk could become an important new candidate for the general study of 14-3-3-mediated regulation.

Role of Bruton's tyrosine kinase in myeloma cell migration and induction of bone disease

Myeloma cells typically grow in bone, recruit osteoclast precursors and induce their differentiation and activity in areas adjacent to tumor foci. Bruton's tyrosine kinase (BTK), of the TEC family, is expressed in hematopoietic cells and is particularly involved in B-lymphocyte function and osteoclastogenesis. We demonstrated BTK expression in clinical myeloma plasma cells, interleukin (IL)-6- or stroma-dependent cell lines and osteoclasts. SDF-1 induced BTK activation in myeloma cells and BTK inhibition by small hairpin RNA or the small molecule inhibitor, LFM-A13, reduced their migration toward stromal cell-derived factor-1 (SDF-1). Pretreatment with LFM-A13 also reduced in vivo homing of myeloma cells to bone using bioluminescence imaging in the SCID-rab model. Enforced expression of BTK in myeloma cell line enhanced cell migration toward SDF-1 but had no effect on short-term growth. BTK expression was correlated with cell-surface CXCR4 expression in myeloma cells (n = 33, r = 0.81, P < 0.0001), and BTK gene and protein expression was more profound in cell-surface CXCR4-expressing myeloma cells. BTK was not upregulated by IL-6 while its inhibition had no effect on IL-6 signaling in myeloma cells. Human osteoclast precursors also expressed BTK and cell-surface CXCR4 and migrated toward SDF-1. LFM-A13 suppressed migration and differentiation of osteoclast precursors as well as bone-resorbing activity of mature osteoclasts. In primary myeloma-bearing SCID-rab mice, LFM-A13 inhibited osteoclast activity, prevented myeloma-induced bone resorption and moderately suppressed myeloma growth. These data demonstrate BTK and cell-surface CXCR4 association in myeloma cells and that BTK plays a role in myeloma cell homing to bone and myeloma-induced bone disease. Am. J. Hematol. 88:463-471, 2013. © 2013 Wiley Periodicals, Inc.

Targeting pathological B cell receptor signalling in lymphoid malignancies

Signalling through the B cell receptor (BCR) is central to the development and maintenance of B cells. In light of the numerous proliferative and survival pathways activated downstream of the BCR, it comes as no surprise that malignant B cells would co-opt this receptor to promote their own growth and survival. However, direct evidence for BCR signalling in human lymphoma has only come to light recently. Roles for antigen-dependent and antigen-independent, or tonic, BCR signalling have now been described for several different lymphoma subtypes. Furthermore, correlative data implicate antigen-dependent BCR signalling in many other forms of lymphoma. A host of therapeutic agents targeting effectors of the BCR signalling pathway are now in clinical trials and have shown initial success against multiple forms of lymphoma.

Tollip-induced down-regulation of MARCH1

In addition to their classical antigen presenting functions, MHC class II molecules potentiate the TLR-triggered production of pro-inflammatory cytokines. Here, we have addressed the effect of Tollip and MARCH1 on the regulation of MHC II trafficking and TLR signaling. Our results show that MARCH1-deficient mice splenocytes are impaired in their capacity to produce pro-inflammatory cytokines in response to poly(I:C) and that TLR3 and MHC II molecules interact in the endocytic pathway. Knocking down Tollip expression in human CIITA(+) HeLa cells increased expression of HLA-DR but reduced the proportion of MHC II molecules associated with the CLIP peptide. Truncation of the HLA-DR cytoplasmic tails abrogated the effect of Tollip on MHC class II expression. While overexpression of Tollip did not affect HLA-DR levels, it antagonized the function of co-transfected MARCH1. We found that Tollip strongly reduced MARCH1 protein levels and that the two molecules appear to compete for binding to MHC II molecules. Altogether, our results demonstrate that Tollip regulates MHC class II trafficking and that MARCH1 may represent a new Tollip target.

Targeting the SYK-BTK axis for the treatment of immunological and hematological disorders: recent progress and therapeutic perspectives

Spleen Tyrosine Kinase (SYK) and Bruton's Tyrosine Kinase (BTK) are non-receptor cytoplasmic tyrosine kinases that are primarily expressed in cells of hematopoietic lineage. Both are key mediators in coupling activated immunoreceptors to downstream signaling events that affect diverse biological functions, from cellular proliferation, differentiation and adhesion to innate and adaptive immune responses. As such, pharmacological inhibitors of SYK or BTK are being actively pursued as potential immunomodulatory agents for the treatment of autoimmune and inflammatory disorders. Deregulation of SYK or BTK activity has also been implicated in certain hematological malignancies. To date, from a clinical perspective, pharmacological inhibition of SYK activity has demonstrated encouraging efficacy in patients with rheumatoid arthritis (RA), while patients with relapsed or refractory chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL) have benefited from covalent inhibitors of BTK in early clinical studies. Here, we review and discuss recent insights into the emerging role of the SYK-BTK axis in innate immune cell function as well as in the maintenance of survival and homing signals for tumor cell progression. The current progress on the clinical development of SYK and BTK inhibitors is also highlighted.

Protein kinase Cβ is critical for the metabolic switch to glycolysis following B-cell antigen receptor engagement

Signals derived from the BCR (B-cell antigen receptor) control survival, development and antigenic responses. One mechanism by which BCR signals may mediate these responses is by regulating cell metabolism. Indeed, the bioenergetic demands of naïve B-cells increase following BCR engagement and are characterized by a metabolic switch to aerobic glycolysis; however, the signalling pathways involved in this metabolic reprogramming are poorly defined. The PKC (protein kinase C) family plays an integral role in B-cell survival and antigenic responses. Using pharmacological inhibition and mice deficient in PKCβ, we demonstrate an essential role of PKCβ in BCR-induced glycolysis in B-cells. In contrast, mice deficient in PKCδ exhibit glycolytic rates comparable with those of wild-type B-cells following BCR cross-linking. The induction of several glycolytic genes following BCR engagement is impaired in PKCβ-deficient B-cells. Moreover, blocking glycolysis results in decreased survival of B-cells despite BCR engagement. The results establish a definitive role for PKCβ in the metabolic switch to glycolysis following BCR engagement of naïve B-cells.

Signaling of the ITK (interleukin 2-inducible T cell kinase)-SYK (spleen tyrosine kinase) fusion kinase is dependent on adapter SLP-76 and on the adapter function of the kinases SYK and ZAP70

The inducible T cell kinase-spleen tyrosine kinase (ITK-SYK) oncogene consists of the Tec homology-pleckstrin homology domain of ITK and the kinase domain of SYK, and it is believed to be the cause of peripheral T cell lymphoma. We and others have recently demonstrated that this fusion protein is constitutively tyrosine-phosphorylated and is transforming both in vitro and in vivo. To gain a deeper insight into the molecular mechanism(s) underlying its activation and signaling, we mutated a total of eight tyrosines located in the SYK portion of the chimera into either phenylalanine or to the negatively charged glutamic acid. Although mutations in the interdomain-B region affected ITK-SYK kinase activity, they only modestly altered downstream signaling events. In contrast, mutations that were introduced in the kinase domain triggered severe impairment of downstream signaling. Moreover, we show here that SLP-76 is critical for ITK-SYK activation and is particularly required for the ITK-SYK-dependent phosphorylation of SYK activation loop tyrosines. In Jurkat cell lines, we demonstrate that expression of ITK-SYK fusion requires an intact SLP-76 function and significantly induces IL-2 secretion and CD69 expression. Furthermore, the SLP-76-mediated induction of IL-2 and CD69 could be further enhanced by SYK or ZAP-70, but it was independent of their kinase activity. Notably, ITK-SYK expression in SYF cells phosphorylates SLP-76 in the absence of SRC family kinases. Altogether, our data suggest that ITK-SYK exists in the active conformation state and is therefore capable of signaling without SRC family kinases or stimulation of the T cell receptor.

The phosphoinositide 3-kinase signaling pathway in normal and malignant B cells: activation mechanisms, regulation and impact on cellular functions

The phosphoinositide 3-kinase (PI3K) pathway is a central signal transduction axis controlling normal B cell homeostasis and activation in humoral immunity. The p110δ PI3K catalytic subunit has emerged as a critical mediator of multiple B cell functions. The activity of this pathway is regulated at multiple levels, with inositol phosphatases PTEN and SHIP both playing critical roles. When deregulated, the PI3K pathway can contribute to B cell malignancies and autoantibody production. This review summarizes current knowledge on key mechanisms that activate and regulate the PI3K pathway and influence normal B cell functional responses including the development of B cell subsets, antigen presentation, immunoglobulin isotype switch, germinal center responses, and maintenance of B cell anergy. We also discuss PI3K pathway alterations reported in select B cell malignancies and highlight studies indicating the functional significance of this pathway in malignant B cell survival and growth within tissue microenvironments. Finally, we comment on early clinical trial results, which support PI3K inhibition as a promising treatment of chronic lymphocytic leukemia.

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 role of the tec kinase Bruton's tyrosine kinase (Btk) in leukocyte recruitment

Recruitment of leukocytes into inflamed tissue is a key component of the immune system. The activation of integrins on leukocytes is required for their recruitment into the inflamed tissue. Btk is a cytoplasmic nonreceptor tyrosine kinase belonging to the Tec-kinase family. It plays a key role in B-cell development and function, and recently published studies revealed important roles of Btk in myeloid cells. Btk might be activated through a variety of receptors leading to activation of integrins as the pivotal element in leukocyte recruitment. This review focuses on the role of Btk in B-lymphocyte homing and in neutrophil recruitment.

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.

Tyrosine kinase inhibitors as potential drugs for B-cell lymphoid malignancies and autoimmune disorders

INTRODUCTION

In the last few years, several tyrosine kinase inhibitors (TKIs) have been synthesized and become available for preclinical studies and clinical trials. This article summarizes recent achievements in the mechanism of action, pharmacological properties, and clinical activity and toxicity, as well as the emerging role of TKIs in lymphoid malignancies, allergic diseases, and autoimmune disorders.

AREAS COVERED

A literature review was conducted of the MEDLINE database PubMed for articles in English. Publications from 2000 through January 2012 were scrutinized. The search terms used were Bruton's tyrosine kinase (Btk) inhibitors, PCI-32765, GDC-0834, LFM-A13, AVL-101, AVL-292, spleen tyrosine kinase (Syk) inhibitors, R343, R406, R112, R788, fostamatinib, BAY-61-3606, C-61, piceatannol, Lyn, imatinib, nilotinib, bafetinib, dasatinib, GDC-0834, PP2, SU6656 in conjunction with lymphoid malignancy, NHL, CLL, autoimmune disease, allergic disease, asthma, and rheumatoid arthritis. Conference proceedings from the previous 5 years of the American Society of Hematology, European Hematology Association, American Society of Clinical Oncology, and ACR/ARHP Annual Scientific Meetings were searched manually. Additional relevant publications were obtained by reviewing the references from the chosen articles.

EXPERT OPINION

The use of TKIs, especially inhibitors of Btk, Syk, and Lyn, is a promising new strategy for targeted treatment of B-cell lymphoid malignancies, autoimmune disorders and allergic diseases. However, definitive data from ongoing and future clinical trials will aid in better defining the status of TKIs in the treatment of these disorders.

Differential evolutionary wiring of the tyrosine kinase Btk

BACKGROUND

A central question within biology is how intracellular signaling pathways are maintained throughout evolution. Btk29A is considered to be the fly-homolog of the mammalian Bruton's tyrosine kinase (Btk), which is a non-receptor tyrosine-kinase of the Tec-family. In mammalian cells, there is a single transcript splice-form and the corresponding Btk-protein plays an important role for B-lymphocyte development with alterations within the human BTK gene causing the immunodeficiency disease X-linked agammaglobulinemia in man and a related disorder in mice. In contrast, the Drosophila Btk29A locus encodes two splice-variants, where the type 2-form is the more related to the mammalian Btk gene product displaying more than 80% homology. In Drosophila, Btk29A displays a dynamic pattern of expression through the embryonic to adult stages. Complete loss-of-function of both splice-forms is lethal, whereas selective absence of the type 2-form reduces the adult lifespan of the fly and causes developmental abnormalities in male genitalia.

METHODOLOGY/PRINCIPAL FINDINGS

Out of 7004-7979 transcripts expressed in the four sample groups, 5587 (70-79%) were found in all four tissues and strains. Here, we investigated the role of Btk29A type 2 on a transcriptomic level in larval CNS and adult heads. We used samples either selectively defective in Btk29A type 2 (Btk29A(ficP)) or revertant flies with restored Btk29A type 2-function (Btk29A(fic Exc1-16)). The whole transcriptomic profile for the different sample groups revealed Gene Ontology patterns reflecting lifespan abnormalities in adult head neuronal tissue, but not in larvae.

CONCLUSIONS

In the Btk29A type 2-deficient strains there was no significant overlap between transcriptomic alterations in adult heads and larvae neuronal tissue, respectively. Moreover, there was no significant overlap of the transcriptomic changes between flies and mammals, suggesting that the evolutionary conservation is confined to components of the proximal signaling, whereas the corresponding, downstream transcriptional regulation has been differentially wired.

Regulation of nucleocytoplasmic shuttling of Bruton's tyrosine kinase (Btk) through a novel SH3-dependent interaction with ankyrin repeat domain 54 (ANKRD54)

Bruton's tyrosine kinase (Btk), belonging to the Tec family of tyrosine kinases (TFKs), is essential for B-lymphocyte development. Abrogation of Btk signaling causes human X-linked agammaglobulinemia (XLA) and murine X-linked immunodeficiency (Xid). We employed affinity purification of Flag-tagged Btk, combined with tandem mass spectrometry, to capture and identify novel interacting proteins. We here characterize the interaction with ankryin repeat domain 54 protein (ANKRD54), also known as Lyn-interacting ankyrin repeat protein (Liar). While Btk is a nucleocytoplasmic protein, the Liar pool was found to shuttle at a higher rate than Btk. Importantly, our results suggest that Liar mediates nuclear export of both Btk and another TFK, Txk/Rlk. Liar-mediated Btk shuttling was enriched for activation loop, nonphosphorylated Btk and entirely dependent on Btk's SH3 domain. Liar also showed reduced binding to an aspartic acid phosphomimetic SH3 mutant. Three other investigated nucleus-located proteins, Abl, estrogen receptor β (ERβ), and transcription factor T-bet, were all unaffected by Liar. We mapped the interaction site to the C terminus of the Btk SH3 domain. A biotinylated, synthetic Btk peptide, ARDKNGQEGYIPSNYVTEAEDS, was sufficient for this interaction. Liar is the first protein identified that specifically influences the nucleocytoplasmic shuttling of Btk and Txk and belongs to a rare group of known proteins carrying out this activity in a Crm1-dependent manner.

Agammaglobulinemia and absent B lineage cells in a patient lacking the p85α subunit of PI3K

Whole exome sequencing was used to determine the causative gene in patients with B cell defects of unknown etiology. A homozygous premature stop codon in exon 6 of PIK3R1 was identified in a young woman with colitis and absent B cells. The mutation results in the absence of p85α but normal expression of the p50α and p55α regulatory subunits of PI3K. Bone marrow aspirates from the patient showed <0.1% CD19(+) B cells with normal percentages of TdT(+)VpreB(+)CD19(-) B cell precursors. This developmental block is earlier than that seen in patients with defects in the B cell receptor signaling pathway or in a strain of engineered mice with a similar defect in p85α. The number and function of the patient's T cells were normal. However, Western blot showed markedly decreased p110δ, as well as absent p85α, in patient T cells, neutrophils, and dendritic cells. The patient had normal growth and development and normal fasting glucose and insulin. Mice with p85α deficiency have insulin hypersensitivity, defective platelet function, and abnormal mast cell development. In contrast, the absence of p85α in the patient results in an early and severe defect in B cell development but minimal findings in other organ systems.

PCI-32765: a novel Bruton's tyrosine kinase inhibitor for the treatment of lymphoid malignancies

INTRODUCTION

There has been a significant paradigm shift in the manner in which lymphoid malignancies are treated and managed. Treatment has been moving away from conventional chemotherapy and towards targeted therapy. The success of new classes of agents such as monoclonal antibodies, proteasome inhibitors and immunomodulatory derivatives has sparked further searches for novel pathways to inhibit. The Bruton's tyrosine kinase (Btk) pathway is a downstream mediator of the B-cell receptor (BCR) pathway, which is crucial in B-cell production and maintenance, and a potential therapeutic target.

AREAS COVERED

This review will summarize the current knowledge of the Btk pathway and its role in lymphoid malignancies. It will also discuss the present data about PCI-32765 in both the preclinical and clinical setting.

EXPERT OPINION

PCI-32765 is an oral irreversible Btk inhibitor with high potency and both preclinical and clinical activity in chronic lymphocytic leukemia (CLL) and non-Hodgkin's lymphoma (NHL). Phase I studies have demonstrated that it is well tolerated and has an excellent safety profile. Further studies are ongoing as a single agent and in combination with other targeted and conventional therapies. PCI-32765 is a very promising targeted therapy, and the data from these trials will ultimately decide its future role and success.

Immunodeficiency due to defects in store-operated calcium entry

Mutations in genes encoding the calcium-release activated calcium (CRAC) channel abolish calcium influx in cells of the immune system and cause severe congenital immunodeficiency. Patients with autosomal recessive mutations in the CRAC channel gene ORAI1, its activator stromal interaction molecule 1 (STIM1), and mice with targeted deletion of Orai1, Stim1, and Stim2 genes reveal important roles for CRAC channels in adaptive and innate immune responses to infection and in autoimmunity. Because CRAC channels have important functions outside the immune system, deficiency of either ORAI1 or STIM1 is associated with a unique clinical phenotype. This review will give an overview of CRAC channel function in the immune system, examine the consequences of CRAC channel deficiency for immunity in human patients and mice, and discuss genetic defects in immunoreceptor-associated signaling molecules that compromise calcium influx and cause immunodeficiency.

TEC and MAPK Kinase Signalling Pathways in T helper (TH) cell Development, TH2 Differentiation and Allergic Asthma

Significant advances in our understanding of the signalling events during T cell development and differentiation have been made in the past few decades. It is clear that ligation of the T cell receptor (TCR) triggers a series of proximal signalling cascades regulated by an array of protein kinases. These orchestrated and highly regulated series of events, with differential requirements of particular kinases, highlight the disparities between αβ+CD4+ T cells. Throughout this review we summarise both new and old studies, highlighting the role of Tec and MAPK in T cell development and differentiation with particular focus on T helper 2 (TH2) cells. Finally, as the allergy epidemic continues, we feature the role played by TH2 cells in the development of allergy and provide a brief update on promising kinase inhibitors that have been tested in vitro, in pre-clinical disease models in vivo and into clinical studies.

Toll-like receptor function in primary B cell defects

Primary immunodeficiency diseases include more than 150 different genetic defects, classified on the basis of the mutations or physiological defects involved. The first immune defects to be well recognized were those of adaptive immunity affecting B cell function and resulting in hypogammaglobulinemia and defects of specific antibody production; more recently, novel defects of innate immunity have been described, some involving Toll-like receptors (TLRs) and their signaling pathways. Furthermore, it is increasingly evident that the innate and adaptive pathways intersect and reinforce each other. B cells express a number of TLRs, which when activated lead to cell activation, up-regulation of co-stimulatory molecules, secretion of cytokines, up-regulation of recombination enzymes, isotype switch and immune globulin production. TLR activation of antigen presenting cells leads to heightened cytokine production, providing additional stimuli for B cell development and maturation. Recent studies have demonstrated that patients with common variable immunodeficiency (CVID) and X-linked agammaglobulinemia (XLA) have altered TLR responsiveness. We review TLR defects in these disorders of B cell development, and discuss how B cell gene defects may modulate TLR signaling.

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

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

New frontiers of primary antibody deficiencies

Primary antibody deficiencies (PAD) form the largest group of inherited disorders of the immune system. They are characterized by a marked reduction or absence of serum immunoglobulins (Ig) due to disturbed B cell differentiation and by a poor response to vaccination. PAD can be divided into agammaglobulinemia, Ig class switch recombination deficiencies, and idiopathic hypogammaglobulinemia. Over the past 20 years, defects have been identified in 18 different genes, but in many PAD patients the underlying gene defects have not been found. Diagnosis of known PAD and discovery of new PAD is important for good patient care. In this review, we present the effects of genetic defects in the context of normal B cell differentiation, and we discuss how new technical developments can support understanding and discovering new genetic defects in PAD.

Toll-like receptor 4-, 7-, and 8-activated myeloid cells from patients with X-linked agammaglobulinemia produce enhanced inflammatory cytokines

BACKGROUND

Bruton tyrosine kinase (BTK) is a component of signaling pathways downstream from Toll-like receptors (TLRs) 2, 4, 7, 8, and 9. Previous work in BTK-deficient mice, cell lines, and cultured cells from patients with X-linked agammaglobulinemia (XLA) suggested defective TLR-driven cytokine production.

OBJECTIVE

We sought to compare TLR-4-, TLR-7-, and TLR-8-induced cytokine production of primary cells from patients with XLA with that seen in control cells.

METHODS

PBMCs from patients with XLA, freshly isolated plasmacytoid dendritic cells, monocytes, and monocytoid dendritic cells were activated with TLR-4, TLR-7, and TLR-8 agonists. Signaling intermediates and intracellular and secreted cytokine levels were compared with those seen in control cells.

RESULTS

Although TLR-4, TLR-7, and TLR-8 activation of nuclear factor κB and mitogen-activated protein kinase pathways in cells from patients with XLA and control cells were comparable, TLR-activated freshly isolated monocytes and monocytoid dendritic cells from patients with XLA produced significantly more TNF-α, IL-6, and IL-10 than control cells. TLR-7/8-activated plasmacytoid dendritic cells produced normal amounts of IFN-α. In murine models BTK regulates the degradation of Toll-IL-1 receptor domain-containing adaptor protein, terminating TLR-4-induced cytokine production. Although this might explain the heightened TLR-4-driven cytokine production we observed, Toll-IL-1 receptor domain-containing adaptor protein degradation is intact in cells from patients with XLA, excluding this explanation.

CONCLUSION

In contrast to previous studies with BTK-deficient mice, cell lines, and cultured cells from patients with XLA suggesting impaired TLR-driven cytokine production, these data suggest that BTK inhibits TLR-induced cytokine production in primary human cells.

Are mouse models of human mycobacterial diseases relevant? Genetics says: 'yes!'

Relevance and accuracy of experimental mouse models of tuberculosis (TB) are the subject of constant debate. This article briefly reviews genetic aspects of this problem and provides a few examples of mycobacterial diseases with similar or identical genetic control in mice and humans. The two species display more similarities than differences regarding both genetics of susceptibility/severity of mycobacterial diseases and the networks of protective and pathological immune reactions. In the opinion of the author, refined mouse models of mycobacterial diseases are extremely useful for modelling the corresponding human conditions, if genetic diversity is taken into account.

Formation of B-1 B cells from neonatal B-1 transitional cells exhibits NF-κB redundancy

The stages of development leading up to the formation of mature B-1 cells have not been identified. As a result, there is no basis for understanding why various genetic defects, and those in the classical or alternative NF-κB pathways in particular, differentially affect the B-1 and B-2 B cell lineages. In this article, we demonstrate that B-1 B cells are generated from transitional cell intermediates that emerge in a distinct neonatal wave of development that is sustained for ~2 wk after birth and then declines as B-2 transitional cells predominate. We further show that, in contrast to the dependence of B-2 transitional cells on the alternative pathway, the survival of neonatal B-1 transitional cells and their maturation into B-1 B cells occurs as long as either alternative or classical NF-κB signaling is intact. On the basis of these results, we have generated a model of B-1 development that allows the defects in B-1 and B-2 cell production observed in various NF-κB-deficient strains of mice to be placed into a coherent cellular context.

The Bruton tyrosine kinase inhibitor PCI-32765 ameliorates autoimmune arthritis by inhibition of multiple effector cells

INTRODUCTION

The aim was to determine the effect of the Bruton tyrosine kinase (Btk)-selective inhibitor PCI-32765, currently in Phase I/II studies in lymphoma trials, in arthritis and immune-complex (IC) based animal models and describe the underlying cellular mechanisms.

METHODS

PCI-32765 was administered in a series of murine IC disease models including collagen-induced arthritis (CIA), collagen antibody-induced arthritis (CAIA), reversed passive anaphylactic reaction (RPA), and passive cutaneous anaphylaxis (PCA). Clinical and pathologic features characteristic of each model were examined following treatment. PCI-32765 was then examined in assays using immune cells relevant to the pathogenesis of arthritis, and where Btk is thought to play a functional role. These included proliferation and calcium mobilization in B cells, cytokine and chemokine production in monocytes/macrophages, degranulation of mast cells and its subsequent cytokine/chemokine production.

RESULTS

PCI-32765 dose-dependently and potently reversed arthritic inflammation in a therapeutic CIA model with an ED(50) of 2.6 mg/kg/day. PCI-32765 also prevented clinical arthritis in CAIA models. In both models, infiltration of monocytes and macrophages into the synovium was completely inhibited and importantly, the bone and cartilage integrity of the joints were preserved. PCI-32765 reduced inflammation in the Arthus and PCA assays. In vitro, PCI-32765 inhibited BCR-activated primary B cell proliferation (IC(50) = 8 nM). Following FcγR stimulation, PCI-32765 inhibited TNFα, IL-1β and IL-6 production in primary monocytes (IC(50) = 2.6, 0.5, 3.9 nM, respectively). Following FcεRI stimulation of cultured human mast cells, PCI-32765 inhibited release of histamine, PGD(2), TNF-α, IL-8 and MCP-1.

CONCLUSIONS

PCI-32765 is efficacious in CIA, and in IC models that do not depend upon autoantibody production from B cells. Thus PCI-32765 targets not only B lymphocytes but also monocytes, macrophages and mast cells, which are important Btk-expressing effector cells in arthritis.

Bruton's tyrosine kinase is required for TLR-dependent heme oxygenase-1 gene activation via Nrf2 in macrophages

Heme oxygenase (HO)-1 is the inducible isoform of the rate-limiting enzyme of heme degradation and provides cytoprotection against oxidative stress by its products carbon monoxide and biliverdin. More recently, HO-1 has also been shown to exert immunomodulatory functions via cell type-specific anti-inflammatory effects in myeloid/macrophage cells. In the current study, it is demonstrated that Bruton's tyrosine kinase (Btk), the gene of which is mutated in the human immunodeficiency X-linked agammaglobulinemia, is involved in the upregulation of HO-1 gene expression via TLR signaling in macrophages. The specific Btk inhibitor LFM-A13 blocked HO-1 induction by the classical TLR4 ligand LPS in cell cultures of RAW264.7 monocytic cells and primary mouse alveolar macrophages. Moreover, upregulation of HO-1 gene expression was abrogated in LPS-stimulated alveolar macrophages from Btk(-/-) mice. Transfection studies with luciferase reporter gene constructs demonstrated that LPS-dependent induction of HO-1 promoter activity was attenuated by pharmacological Btk inhibition and by an overexpressed dominant-negative mutant of Btk. This induction was mediated by the transcription factor Nrf2, which is a master regulator of the antioxidant cellular defense. Accordingly, nuclear translocation of Nrf2 in LPS-treated macrophages was reduced by Btk inhibition. The generation of reactive oxygen species, but not that of NO, was involved in this regulatory pathway. Btk-dependent induction of HO-1 gene expression was also observed upon macrophage stimulation with ligands of TLR2, TLR6, TLR7, and TLR9, suggesting that Btk is required for HO-1 gene activation by major TLR pathways.

Btk is a positive regulator in the TREM-1/DAP12 signaling pathway

The triggering receptor expressed on myeloid cells 1 (TREM-1) has been implicated in the production of proinflammatory cytokines and chemokines during bacterial infection and sepsis. For downstream signal transduction, TREM-1 is coupled to the ITAM-containing adaptor DAP12. Here, we demonstrate that Bruton tyrosine kinase (Btk), a member of the Tec kinases, becomes phosphorylated upon TREM-1 triggering. In U937-derived cell lines, in which expression of Btk was diminished by shRNA-mediated knockdown, phosphorylation of Erk1/2 and PLCγ1 and Ca²⁺ mobilization were reduced after TREM-1 stimulation. Importantly, TREM-1-induced production of the pro-inflammatory cytokines, TNF-α and IL-8, and up-regulation of activation/differentiation cell surface markers were impaired in Btk knockdown cells. Similar results were obtained upon TREM-1 stimulation of BMDCs of Btk(-/-) mice. The analysis of cells containing Btk mutants revealed that intact membrane localization and a functional kinase domain were required for TREM-1-mediated signaling. Finally, after TREM-1 engagement, TNF-α production by PBMCs was reduced in the majority of patients suffering from X-linked agammaglobulinemia (XLA), a rare hereditary disease caused by mutations in the BTK gene. In conclusion, our data identify Btk as a positive regulator in the ITAM-mediated TREM-1/DAP12 pathway and suggest its implication in inflammatory processes.

Biology and novel treatment options for XLA, the most common monogenetic immunodeficiency in man

INTRODUCTION

X-linked agammaglobulinemia (XLA) is the most common primary immunodeficiency in man, and is caused by a single genetic defect. Inactivating mutations in the Bruton's tyrosine kinase (BTK) gene are invariably the cause of XLA,. XLA is characterized by a differentiation arrest at the pre-B cell stage, the absence of immunoglobulins and recurrent bacterial infections, making it an insidious disease that gradually disables the patient, and can result in death due to chronic lung disease. Current treatment involves prophylactic antibiotics and immunoglobulin infusions, which are non-curative. This disease is a good candidate for curative hematopoietic stem cell (HSC)-based gene therapy, which could correct the B cell and myeloid deficiencies.

AREAS COVERED

This paper reviews the basic biology of BTK in B cell development, the clinical features of XLA, and the possibilities of gene therapy for XLA, covering the literature from 1995 to 2010.

EXPERT OPINION

Work from various laboratories demonstrates the feasibility of using gene-corrected HSCs to complement the immune defects of Btk-deficiency in mice. We propose that it is timely to start clinical programs to develop stem cell based therapy for XLA, using gene-corrected autologous HSC.

TEC family kinases in health and disease--loss-of-function of BTK and ITK and the gain-of-function fusions ITK-SYK and BTK-SYK

The TEC family is ancient and constitutes the second largest family of cytoplasmic tyrosine kinases. In 1993, loss-of-function mutations in the BTK gene were reported as the cause of X-linked agammaglobulinemia. Of all the existing 90 tyrosine kinases in humans, Bruton's tyrosine kinase (BTK) is the kinase for which most mutations have been identified. These experiments of nature collectively provide a form of mutation scanning with direct implications for the several hundred endogenous signaling proteins carrying domains also found in BTK. In 2009, an inactivating mutation in the ITK gene was shown to cause susceptibility to lethal Epstein-Barr virus infection. Both kinases represent interesting targets for inhibition: in the case of BTK, as an immunosuppressant, whereas there is evidence that the inhibition of inducible T-cell kinase (ITK) could influence the infectivity of HIV and also have anti-inflammatory activity. Since 2006, several patients carrying a fusion protein, originating from a translocation joining genes encoding the kinases ITK and spleen tyrosine kinase (SYK), have been shown to develop T-cell lymphoma. We review these disease processes and also describe the role of the N-terminal pleckstrin homology-Tec homology (PH-TH) domain doublet of BTK and ITK in the downstream intracellular signaling of such fusion proteins.

Highly efficient purification of protein complexes from mammalian cells using a novel streptavidin-binding peptide and hexahistidine tandem tag system: application to Bruton's tyrosine kinase

Tandem affinity purification (TAP) is a generic approach for the purification of protein complexes. The key advantage of TAP is the engineering of dual affinity tags that, when attached to the protein of interest, allow purification of the target protein along with its binding partners through two consecutive purification steps. The tandem tag used in the original method consists of two IgG-binding units of protein A from Staphylococcus aureus (ProtA) and the calmodulin-binding peptide (CBP), and it allows for recovery of 20-30% of the bait protein in yeast. When applied to higher eukaryotes, however, this classical TAP tag suffers from low yields. To improve protein recovery in systems other than yeast, we describe herein the development of a three-tag system comprised of CBP, streptavidin-binding peptide (SBP) and hexa-histidine. We illustrate the application of this approach for the purification of human Bruton's tyrosine kinase (Btk), which results in highly efficient binding and elution of bait protein in both purification steps (>50% recovery). Combined with mass spectrometry for protein identification, this TAP strategy facilitated the first nonbiased analysis of Btk interacting proteins. The high efficiency of the SBP-His₆ purification allows for efficient recovery of protein complexes formed with a target protein of interest from a small amount of starting material, enhancing the ability to detect low abundance and transient interactions in eukaryotic cell systems.

Emergence of the PI3-kinase pathway as a central modulator of normal and aberrant B cell differentiation

Phosphoinositide 3-kinase (PI3K) defines a family of lipid kinases that direct a wide range of cellular processes and cell fate decisions. Since its discovery, and that of its enzymatic antagonist PTEN, much of the focus on PI3K has been on its oncogenic potential. In recent years, studies on PI3K signaling in B lymphocytes have established the importance of this pathway in effecting B cell differentiation and associated molecular events such as V(D)J recombination and class switch recombination. Intriguing new findings also indicate that there is specificity in the PI3K pathway in B cells, including preferential expression or usage of particular PI3K isoforms and counter-regulation by the PTEN and SHIP phosphatases. The role of PI3K adaptor proteins (CD19, BCAP, and TC21) has also undergone revision to reflect both shared and unique properties. The emergence of Foxo1 as a critical PI3K regulatory target for B cell differentiation has united membrane proximal regulatory events orchestrated by PI3K/PTEN/SHIP with key transcriptional targets. Insights into the regulation and impact of PI3K signaling have been brought to bear in new treatments for B cell malignancies, and will also be an important topic of consideration for B cell-dependent autoimmune diseases.

Tec family kinases: regulation of FcεRI-mediated mast-cell activation

Mast cells express the high-affinity receptor for IgE (FcεRI) and are key players in type I hypersensitivity reactions. They are critically involved in the development of allergic rhinitis, allergic asthma and systemic anaphylaxis, however, they also regulate normal physiological processes that link innate and adaptive immune responses. Thus, their activation has to be tightly controlled. One group of signaling molecules that are activated upon FcεRI stimulation is formed by Tec family kinases, and three members of this kinase family (Btk, Itk and Tec) are expressed in mast cells. Many studies have revealed important functions of Tec kinases in signaling pathways downstream of the antigen receptors in lymphocytes. This review summarizes the current knowledge about the function of Tec family kinases in FcεRI-mediated signaling pathways in mast cell.

Neutrophil development and function critically depend on Bruton tyrosine kinase in a mouse model of X-linked agammaglobulinemia

Bruton tyrosine kinase (Btk) is essential for B cell development and function and also appears to be important for myeloid cells. The bone marrow of Btk-deficient mice shows enhanced granulopoiesis compared with that of wild-type mice. In purified granulocyte-monocyte-progenitors (GMP) from Btk-deficient mice, the development of granulocytes is favored at the expense of monocytes. However, Btk-deficient neutrophils are impaired in maturation and function. Using bone marrow chimeras, we show that this defect is cell-intrinsic to neutrophils. In GMP and neutrophils, Btk plays a role in GM-CSF- and Toll-like receptor-induced differentiation. Molecular analyses revealed that expression of the lineage-determining transcription factors C/EBPα, C/EBPβ, and PU.1, depends on Btk. In addition, expression of several granule proteins, including myeloperoxidase, neutrophilic granule protein, gelatinase and neutrophil elastase, is Btk-dependent. In the Arthus reaction, an acute inflammatory response, neutrophil migration into tissues, edema formation, and hemorrhage are significantly reduced in Btk-deficient animals. Together, our findings implicate Btk as an important regulator of neutrophilic granulocyte maturation and function in vivo.

B-1 cells temper endotoxemic inflammatory responses

Sepsis syndrome is caused by inappropriate immune activation due to bacteria and bacterial components released during infection. This syndrome is the leading cause of death in intensive care units. Specialized B-lymphocytes located in the peritoneal and pleural cavities are known as B-1 cells. These cells produce IgM and IL-10, both of which are potent regulators of cell-mediated immunity. It has been suggested that B-1 cells modulate the systemic inflammatory response in sepsis. In this study, we conducted in vitro and in vivo experiments in order to investigate a putative role of B-1 cells in a murine model of LPS-induced sepsis. Macrophages and B-1 cells were studied in monocultures and in co-cultures. The B-1 cells produced the anti-inflammatory cytokine IL-10 in response to LPS. In the B-1 cell-macrophage co-cultures, production of proinflammatory mediators (TNF-α, IL-6 and nitrite) was lower than in the macrophage monocultures, whereas that of IL-10 was higher in the co-cultures. Co-culture of B-1 IL-10(-/-) cells and macrophages did not reduce the production of the proinflammatory mediators (TNF-α, IL-6 and nitrite). After LPS injection, the mortality rate was higher among Balb/Xid mice, which are B-1 cell deficient, than among wild-type mice (65.0% vs. 0.0%). The Balb/Xid mice also presented a proinflammatory profile of TNF-α, IL-6 and nitrite, as well as lower levels of IL-10. In the early phase of LPS stimulation, B-1 cells modulate the macrophage inflammatory response, and the main molecular pathway of that modulation is based on IL-10-mediated intracellular signaling.

The Bruton tyrosine kinase inhibitor PCI-32765 blocks B-cell activation and is efficacious in models of autoimmune disease and B-cell malignancy

Activation of the B-cell antigen receptor (BCR) signaling pathway contributes to the initiation and maintenance of B-cell malignancies and autoimmune diseases. The Bruton tyrosine kinase (Btk) is specifically required for BCR signaling as demonstrated by human and mouse mutations that disrupt Btk function and prevent B-cell maturation at steps that require a functional BCR pathway. Herein we describe a selective and irreversible Btk inhibitor, PCI-32765, that is currently under clinical development in patients with B-cell non-Hodgkin lymphoma. We have used this inhibitor to investigate the biologic effects of Btk inhibition on mature B-cell function and the progression of B cell-associated diseases in vivo. PCI-32765 blocked BCR signaling in human peripheral B cells at concentrations that did not affect T cell receptor signaling. In mice with collagen-induced arthritis, orally administered PCI-32765 reduced the level of circulating autoantibodies and completely suppressed disease. PCI-32765 also inhibited autoantibody production and the development of kidney disease in the MRL-Fas(lpr) lupus model. Occupancy of the Btk active site by PCI-32765 was monitored in vitro and in vivo using a fluorescent affinity probe for Btk. Active site occupancy of Btk was tightly correlated with the blockade of BCR signaling and in vivo efficacy. Finally, PCI-32765 induced objective clinical responses in dogs with spontaneous B-cell non-Hodgkin lymphoma. These findings support Btk inhibition as a therapeutic approach for the treatment of human diseases associated with activation of the BCR pathway.

T-cell signaling regulated by the Tec family kinase, Itk

The Tec family tyrosine kinases regulate lymphocyte development, activation, and differentiation. In T cells, the predominant Tec kinase is Itk, which functions downstream of the T-cell receptor to regulate phospholipase C-gamma. This review highlights recent advances in our understanding of Itk kinase structure and enzymatic regulation, focusing on Itk protein domain interactions and mechanisms of substrate recognition. We also discuss the role of Itk in the development of conventional versus innate T-cell lineages, including both alphabeta and gammadelta T-cell subsets. Finally, we describe the complex role of Itk signaling in effector T-cell differentiation and the regulation of cytokine gene expression. Together, these data implicate Itk as an important modulator of T-cell signaling and function.

The Src, Syk, and Tec family kinases: distinct types of molecular switches

The Src, Syk, and Tec family kinases are three of the most well characterized tyrosine kinase families found in the human genome. Members of these kinase families function downstream of antigen and F(c) receptors in hematopoietic cells and transduce signals leading to calcium mobilization, altered gene expression, cytokine production, and cell proliferation. Over the last several years, structural and biochemical studies have begun to uncover the molecular mechanisms regulating activation of these kinases. It appears that each kinase family functions as a distinct type of molecular switch. This review discusses the activation of the Src, Syk, and Tec kinases from the perspective of structure, phosphorylation, allosteric regulation, and kinetics. The multiple factors that regulate the Src, Syk, and Tec families illustrate the important role played by each of these kinases in immune cell signaling.

B cells delay neutrophil migration toward the site of stimulus: tardiness critical for effective bacillus Calmette-Guérin vaccination against tuberculosis infection in mice

Mutations in the btk gene encoding Bruton's tyrosine kinase cause X-linked immune deficiency, with impaired B lymphocyte function as the major phenotype. Earlier, we demonstrated that CBA/N-xid mice, unlike the wild-type CBA mice, were not protected by bacillus Calmette-Guérin (BCG) vaccination against tuberculosis infection. Because IFN-gamma-producing T cells and activated macrophages are key elements of antituberculosis protection, it remained unclear how the mutation predominantly affecting B cell functions interferes with responses along the T cell-macrophage axis. In this study, we show that B cell deficiency leads to an abnormally rapid neutrophil migration toward the site of external stimulus. Using adoptive cell transfers and B cell genetic knockout, we demonstrate a previously unappreciated capacity of B cells to downregulate neutrophil motility. In our system, an advanced capture of BCG by neutrophils instead of macrophages leads to a significant decrease in numbers of IFN-gamma-producing T cells and impairs BCG performance in X-linked immune-deficient mice. The defect is readily compensated for by the in vivo neutrophil depletion.

Clinical characteristics and genotype-phenotype correlation in 62 patients with X-linked agammaglobulinemia

INTRODUCTION

X-linked agammagobulinemia (XLA) is a primary immunodeficiency disorder caused by Bruton's tyrosine kinase (Btk) gene mutation. Recent studies suggested genotype-phenotype correlation in XLA, but a definitive association remains controversial.

PATIENTS AND METHODS

We examined the relationship between specific Btk gene mutations and severity of clinical presentation in 62 patients with XLA. Disease severity was assessed by the age of disease onset and the presence of severe infections, while mutations were classified into severe and mild based on structural and functional consequence by bioinformatics analysis.

RESULTS

Fifty-six Btk mutations were identified in 62 patients from 57 kindreds. Variation in phenotypes was observed, and there was a tendency of association between genotype and age of disease onset as well as occurrence of severe infections.

CONCLUSION

A critical analysis of the circumstances upon presentation also revealed that under-recognition of recurrent infections and relevant family history are important hurdles to timely diagnosis of XLA.

Tec protein tyrosine kinase inhibits CD25 expression in human T-lymphocyte

The Tec protein tyrosine kinase (PTK) belongs to a group of structurally related nonreceptor PTKs that also includes Btk, Itk, Rlk, and Bmx. Previous studies have suggested that these kinases play important roles in hematopoiesis and in the lymphocyte signaling pathway. Despite evidence suggesting the involvement of Tec in the T-lymphocyte activation pathway via T-cell receptor (TCR) and CD28, Tec's role in T-lymphocytes remains unclear because of the lack of apparent defects in T-lymphocyte function in Tec-deficient mice. In this study, we investigated the role of Tec in human T-lymphocyte using the Jurkat T-lymphoid cell line stably transfected with a cDNA encoding Tec. We found that the expression of wild-type Tec inhibited the expression of CD25 induced by TCR cross-linking. Second, we observed that LFM-A13, a selective inhibitor of Tec family PTK, rescued the suppression of TCR-induced CD25 expression observed in wild-type Tec-expressing Jurkat cells. In addition, expression of kinase-deleted Tec did not alter the expression level of CD25 after TCR ligation. We conclude that Tec PTK mediates signals that negatively regulate CD25 expression induced by TCR cross-linking. This, in turn, implies that this PTK plays a role in the attenuation of IL-2 activity in human T-lymphocytes.

ITK inhibitors in inflammation and immune-mediated disorders

Interleukin-2-inducible T cell kinase (ITK) is a non-receptor tyrosine kinase expressed in T cells, NKT cells and mast cells which plays a crucial role in regulating the T cell receptor (TCR), CD28, CD2, chemokine receptor CXCR4, and FcepsilonR-mediated signaling pathways. In T cells, ITK is an important mediator for actin reorganization, activation of PLCgamma, mobilization of calcium, and activation of the NFAT transcription factor. ITK plays an important role in the secretion of IL-2, but more critically, also has a pivotal role in the secretion of Th2 cytokines, IL-4, IL-5 and IL-13. As such, ITK has been shown to regulate the development of effective Th2 response during allergic asthma as well as infections of parasitic worms. This ability of ITK to regulate Th2 responses, along with its pattern of expression, has led to the proposal that it would represent an excellent target for Th2-mediated inflammation. We discuss here the possibilities and pitfalls of targeting ITK for inflammatory disorders.

Characterization of a novel interaction between transcription factor TFII-I and the inducible tyrosine kinase in T cells

TCR signaling leads to the activation of kinases such as inducible tyrosine kinase (Itk), a key regulatory protein in T-lymphocyte activation and function. The homolog of Itk in B cells is Bruton's tyrosine kinase, previously shown to bind and phosphorylate the transcription factor TFII-I. TFII-I plays major roles in transcription and signaling. Our purpose herein was twofold: first, to identify some of the molecular determinants involved in TFII-I activation downstream of receptor crosslinking in T cells and second, to uncover the existence of Itk-TFII-I signaling in T lymphocytes. We report for the first time that TFII-I is tyrosine phosphorylated upon TCR, TCR/CD43, and TCR/CD28 co-receptor engagement in human and/or murine T cells. We show that Itk physically interacts with TFII-I and potentiates TFII-I-driven c-fos transcription. We demonstrate that TFII-I is phosphorylated upon co-expression of WT, but not kinase-dead, or kinase-dead/R29C mutant Itk, suggesting these residues are important for TFII-I phosphorylation, presumably via an Itk-dependent mechanism. Structural analysis of TFII-I-Itk interactions revealed that the first 90 residues of TFII-I are dispensable for Itk binding. Mutations within Itk's kinase, pleckstrin-homology, and proline-rich regions did not abolish TFII-I-Itk binding. Our results provide an initial step in understanding the biological role of Itk-TFII-I signaling in T-cell function.