c-rel regulation of IL-2 gene expression may be mediated through activation of AP-1

Lactate induces metabolic and epigenetic reprogramming of pro-inflammatory Th17 cells

Increased lactate levels in the tissue microenvironment are a well-known feature of chronic inflammation. However, the role of lactate in regulating T cell function remains controversial. Here, we demonstrate that extracellular lactate predominantly induces deregulation of the Th17-specific gene expression program by modulating the metabolic and epigenetic status of Th17 cells. Following lactate treatment, Th17 cells significantly reduced their IL-17A production and upregulated Foxp3 expression through ROS-driven IL-2 secretion. Moreover, we observed increased levels of genome-wide histone H3K18 lactylation, a recently described marker for active chromatin in macrophages, in lactate-treated Th17 cells. In addition, we show that high lactate concentrations suppress Th17 pathogenicity during intestinal inflammation in mice. These results indicate that lactate is capable of reprogramming pro-inflammatory T cell phenotypes into regulatory T cells.

The many-sided contributions of NF-κB to T-cell biology in health and disease

T cells (or T lymphocytes) exhibit a myriad of functions in immune responses, ranging from pathogen clearance to autoimmunity, cancer and even non-lymphoid tissue homeostasis. Therefore, deciphering the molecular mechanisms orchestrating their specification, function and gene expression pattern is critical not only for our comprehension of fundamental biology, but also for the discovery of novel therapeutic targets. Among the master regulators of T-cell identity, the functions of the NF-κB family of transcription factors have been under scrutiny for several decades. However, a more precise understanding of their pleiotropic functions is only just emerging. In this review we will provide a global overview of the roles of NF-κB in the different flavors of mature T cells. We aim at highlighting the complex and sometimes diverging roles of the five NF-κB subunits in health and disease.

Inhibition of IL-2 or NF-κB Subunit c-Rel-Dependent Signaling Inhibits Expansion of Regulatory T Cells During Acute Friend Retrovirus Infection

In retroviral infections, different immunological mechanisms are involved in the development of a chronic infection. In the Friend virus (FV) model, regulatory T cells (Tregs) were found to induce CD8⁺ T cell dysfunction before viral clearance is achieved and thus contribute to viral chronicity. Although studied for decades, the exact suppressive mechanisms of Tregs in the FV model remain elusive and an unavailable therapeutic target. However, extracellular IL-2 and intracellular NF-κB signaling were shown to be important pathways for Treg expansion and activation. Therefore, we decided to focus on these two pathways to test therapeutic approaches inhibiting Treg activation during FV infection. In this study, we show that the inhibition of either IL-2 or the NF-κB subunit c-Rel, impaired Treg expansion and activation at 2 weeks post-FV infection. Total numbers of Tregs as well as activated Tregs were reduced in FV-infected mice after treatment with anti-IL-2 antibodies or the c-Rel blocking reagent pentoxifylline. Surprisingly, this did not affect the expansion or function of virus-specific CD8⁺ T cells nor viral loads in the spleen. However, our data suggest that neutralization of IL-2 as well as blocking c-Rel efficiently inhibits virus-induced Treg expansion.

The MicroRNA miR-155 Is Essential in Fibrosis

The function of microRNAs (miRNAs) during fibrosis and the downstream regulation of gene expression by these miRNAs have become of great biological interest. miR-155 is consistently upregulated in fibrotic disorders, and its ablation downregulates collagen synthesis. Studies demonstrate the integral role of miR-155 in fibrosis, as it mediates TGF-β1 signaling to drive collagen synthesis. In this review, we summarize recent findings on the association between miR-155 and fibrotic disorders. We discuss the cross-signaling between macrophages and fibroblasts that orchestrates the upregulation of collagen synthesis mediated by miR-155. As miR-155 is involved in the activation of the innate and adaptive immune systems, specific targeting of miR-155 in pathologic cells that make excessive collagen could be a viable option before the depletion of miR-155 becomes an attractive antifibrotic approach.

CD28 Costimulation: From Mechanism to Therapy

Ligation of the CD28 receptor on T cells provides a critical second signal alongside T cell receptor (TCR) ligation for naive T cell activation. Here, we discuss the expression, structure, and biochemistry of CD28 and its ligands. CD28 signals play a key role in many T cell processes, including cytoskeletal remodeling, production of cytokines, survival, and differentiation. CD28 ligation leads to unique epigenetic, transcriptional, and post-translational changes in T cells that cannot be recapitulated by TCR ligation alone. We discuss the function of CD28 and its ligands in both effector and regulatory T cells. CD28 is critical for regulatory T cell survival and the maintenance of immune homeostasis. We outline the roles that CD28 and its family members play in human disease and we review the clinical efficacy of drugs that block CD28 ligands. Despite the centrality of CD28 and its family members and ligands to immune function, many aspects of CD28 biology remain unclear. Translation of a basic understanding of CD28 function into immunomodulatory therapeutics has been uneven, with both successes and failures. Such real-world results might stem from multiple factors, including complex receptor-ligand interactions among CD28 family members, differences between the mouse and human CD28 families, and cell-type specific roles of CD28 family members.

Induced transcription and stability of CELF2 mRNA drives widespread alternative splicing during T-cell signaling

Studies in several cell types have highlighted dramatic and diverse changes in mRNA processing that occur upon cellular stimulation. However, the mechanisms and pathways that lead to regulated changes in mRNA processing remain poorly understood. Here we demonstrate that expression of the splicing factor CELF2 (CUGBP, Elav-like family member 2) is regulated in response to T-cell signaling through combined increases in transcription and mRNA stability. Transcriptional induction occurs within 6 h of stimulation and is dependent on activation of NF-κB. Subsequently, there is an increase in the stability of the CELF2 mRNA that correlates with a change in CELF2 3'UTR length and contributes to the total signal-induced enhancement of CELF2 expression. Importantly, we uncover dozens of splicing events in cultured T cells whose changes upon stimulation are dependent on CELF2 expression, and provide evidence that CELF2 controls a similar proportion of splicing events during human thymic T-cell development. Taken together, these findings expand the physiologic impact of CELF2 beyond that previously documented in developing neuronal and muscle cells to T-cell development and function, identify unappreciated instances of alternative splicing in the human thymus, and uncover novel mechanisms for CELF2 regulation that may broadly impact CELF2 expression across diverse cell types.

Coactosin-like 1 antagonizes cofilin to promote lamellipodial protrusion at the immune synapse

Actin depolymerizing factor-homology (ADF-H) family proteins regulate actin filament dynamics at multiple cellular locations. Herein, we have investigated the function of the ADF-H family member coactosin-like 1 (COTL1) in the regulation of actin dynamics at the T cell immune synapse (IS). We initially identified COTL1 in a genetic screen to identify novel regulators of T cell activation, and subsequently found that it associates with F-actin and localizes at the IS in response to TCR+CD28 stimulation. Live cell microscopy showed that depletion of COTL1 protein impaired T cell spreading in response to TCR ligation and abrogated lamellipodial protrusion at the T cell – B cell contact site, producing only a band of F-actin. Significantly, re-expression of wild type COTL1, but not a mutant deficient in F-actin binding could rescue these defects. In addition, COTL1 depletion reduced T cell migration. In vitro studies showed that COTL1 and cofilin compete with each other for binding to F-actin, and COTL1 protects F-actin from cofilin-mediated depolymerization. While depletion of cofilin enhanced F-actin assembly and lamellipodial protrusion at the IS, concurrent depletion of both COTL1 and cofilin restored lamellipodia formation. Taken together, our results suggest that COTL1 regulates lamellipodia dynamics in part by protecting F-actin from cofilin-mediated disassembly.

Hepatitis C virus core+1/ARF protein decreases hepcidin transcription through an AP1 binding site

Chronic viral hepatitis C is characterized by iron accumulation in the liver, and hepcidin regulates iron absorption. Hepatitis C virus (HCV) core+1/ARFP is a novel protein produced by a second functional ORF within the core gene. Here, using reporter assays and HCV bicistronic replicons, we show that, similarly to core, core+1/ARFP decreases hepcidin expression in hepatoma cells. The activator protein 1 (AP1) binding site of the human hepcidin promoter, shown here to be relevant to basal promoter activity and to the repression by core, is essential for the downregulation by core+1/ARFP while the previously described C/EBP (CCAAT/enhancer binding protein) and STAT (signal transducer and activator of transcription) sites are not. Consistently, expression of the AP1 components c-jun and c-fos obliterated the repressive effect of core and core+1/ARFP. In conclusion, we provide evidence that core+1/ARFP downregulates AP1-mediated transcription, providing new insights into the biological role of core+1/ARFP, as well as the transcriptional modulation of hepcidin, the main regulator of iron metabolism.

Critical Role for an acidic amino acid region in platelet signaling by the HemITAM (hemi-immunoreceptor tyrosine-based activation motif) containing receptor CLEC-2 (C-type lectin receptor-2)

CLEC-2 is a member of new family of C-type lectin receptors characterized by a cytosolic YXXL downstream of three acidic amino acids in a sequence known as a hemITAM (hemi-immunoreceptor tyrosine-based activation motif). Dimerization of two phosphorylated CLEC-2 molecules leads to recruitment of the tyrosine kinase Syk via its tandem SH2 domains and initiation of a downstream signaling cascade. Using Syk-deficient and Zap-70-deficient cell lines we show that hemITAM signaling is restricted to Syk and that the upstream triacidic amino acid sequence is required for signaling. Using surface plasmon resonance and phosphorylation studies, we demonstrate that the triacidic amino acids are required for phosphorylation of the YXXL. These results further emphasize the distinct nature of the proximal events in signaling by hemITAM relative to ITAM receptors.

Rational design and characterization of platelet factor 4 antagonists for the study of heparin-induced thrombocytopenia

Patients with heparin-induced thrombocytopenia (HIT) remain at risk for recurrent thromboembolic complications despite improvements in management. HIT is caused by antibodies that preferentially recognize ultralarge complexes (ULCs) of heparin and platelet factor 4 (PF4) tetramers. We demonstrated previously that a variant PF4(K50E) forms dimers but does not tetramerize or form ULCs. Here, we identified small molecules predicted to bind PF4 near the dimer-dimer interface and that interfere with PF4 tetramerization. Screening a library of small molecules in silico for binding at this site, we identified 4 compounds that inhibited tetramerization at micromolar concentrations, designated PF4 antagonists (PF4As). PF4As also inhibited formation of pathogenic ULCs, and 3 of these PF4As promoted the breakdown of preformed ULCs. To characterize the ability of PF4As to inhibit cellular activation, we developed a robust and reproducible assay that measures cellular activation by HIT antibodies via FcγRIIA using DT40 cells. PF4As inhibit FcγRIIA-dependent activation of DT40 cells by HIT antibodies as well as platelet activation, as measured by serotonin release. PF4As provide new tools to probe the pathophysiology of HIT. They also may provide insight into the development of novel, disease-specific therapeutics for the treatment of thromboembolic complications in HIT.

Phosphorylation at serine 318 is not required for inhibition of T cell activation by ALX

The activation of T cells and the initiation of an immune response is tightly controlled by both positive and negative regulators. Two adaptors which function as negative regulators of T cell activation are ALX and LAX. ALX constitutively associates with LAX in T cells, and T cells from mice deficient in ALX and LAX display similar hyper-responsiveness upon T cell receptor (TCR)/CD28 stimulation, including increased production of interleukin-2. During T cell activation, ALX is inducibly phosphorylated, however the site of ALX phosphorylation had not been previously identified and the role of phosphorylation in the inhibitory function of ALX was not known. Here, using mass spectrometry, we demonstrate that ALX is phosphorylated on a serine at position 318. Substitution of alanine for serine at this position (ALX S318A) leads to an abrogation of the mobility shift in ALX induced upon TCR/CD28 stimulation. However, ALX S318A retained the ability to bind to and stimulate tyrosine phosphorylation of LAX. In addition, overexpression of ALX S318A inhibited RE/AP activation upon TCR/CD28 stimulation to a similar extent as wild-type ALX. Therefore, although ALX is inducibly phosphorylated upon TCR/CD28 stimulation, this phosphorylation is not required for ALX to inhibit T cell activation.

Melatonin protects CD4+ T cells from activation-induced cell death by blocking NFAT-mediated CD95 ligand upregulation

Over the past 20 y, the hormone melatonin was found to be produced in extrapineal sites, including cells of the immune system. Despite the increasing data regarding the biological effects of melatonin on the regulation of the immune system, the effect of this molecule on T cell survival remains largely unknown. Activation-induced cell death plays a critical role in the maintenance of the homeostasis of the immune system by eliminating self-reactive or chronically stimulated T cells. Because activated T cells not only synthesize melatonin but also respond to it, we investigated whether melatonin could modulate activation-induced cell death. We found that melatonin protects human and murine CD4(+) T cells from apoptosis by inhibiting CD95 ligand mRNA and protein upregulation in response to TCR/CD3 stimulation. This inhibition is a result of the interference with calmodulin/calcineurin activation of NFAT that prevents the translocation of NFAT to the nucleus. Accordingly, melatonin has no effect on T cells transfected with a constitutively active form of NFAT capable of migrating to the nucleus and transactivating target genes in the absence of calcineurin activity. Our results revealed a novel biochemical pathway that regulates the expression of CD95 ligand and potentially other downstream targets of NFAT activation.

Influence of apple polyphenols on inflammatory gene expression

Apples (Malus spp., Rosaceae) and products thereof contain high amounts of polyphenols which show diverse biological activities and may contribute to beneficial health effects, like protecting the intestine against inflammation initiated by chronic inflammatory bowel diseases (IBD). IBD are characterized by an excessive release of several proinflammatory cytokines and chemokines by different cell types which results consequently in an increased inflammatory response. In the present study we investigated the preventive effectiveness of polyphenolic juice extracts and single major constituents on inflammatory gene expression in immunorelevant human cell lines (DLD-1, T84, MonoMac6, Jurkat) induced with specific stimuli. Besides the influence on proinflammatory gene expression, the effect on NF-kappaB-, IP-10-, IL-8-promoter-, STAT1-dependent signal transduction, and the relative protein levels of multiple released cytokines and chemokines were studied. DNA microarray analysis of several genes known to be strongly regulated during gastrointestinal inflammation, combined with quantitative real-time PCR (qRT-PCR) revealed that the apple juice extract AE04 (100-200 microg/mL) significantly inhibited the expression of NF-kappaB regulated proinflammatory genes (TNF-alpha, IL-1beta, CXCL9, CXCL10), inflammatory relevant enzymes (COX-2, CYP3A4), and transcription factors (STAT1, IRF1) in LPS/IFN-gamma stimulated MonoMac6 cells without significant effects on the expression of house-keeping genes. A screening of some major compounds of AE04 revealed that the flavan-3-ol dimer procyanidin B(2 )is mainly responsible for the anti-inflammatory activity of AE04. Furthermore, the dihydrochalcone aglycone phloretin and the dimeric flavan-3-ol procyanidin B(1 )significantly inhibited proinflammatory gene expression and repressed NF-kappaB-, IP-10-, IL-8-promoter-, and STAT1-dependent signal transduction in a dose-dependent manner. The influence on proinflammatory gene expression by the applied polyphenols thereby strongly correlated with the increased protein levels investigated by human cytokine array studies. In summary, we evaluated selected compounds responsible for the anti-inflammatory activity of AE04. In particular, procyanidin B(1), procyanidin B(2), and phloretin revealed anti-inflammatory activities in vitro and therefore may serve as transcription-based inhibitors of proinflammatory gene expression.

CLEC-2 activates Syk through dimerization

The C-type lectin receptor CLEC-2 activates platelets through Src and Syk tyrosine kinases, leading to tyrosine phosphorylation of downstream adapter proteins and effector enzymes, including phospholipase-C gamma2. Signaling is initiated through phosphorylation of a single conserved tyrosine located in a YxxL sequence in the CLEC-2 cytosolic tail. The signaling pathway used by CLEC-2 shares many similarities with that used by receptors that have 1 or more copies of an immunoreceptor tyrosine-based activation motif, defined by the sequence Yxx(L/I)x(6-12)Yxx(L/I), in their cytosolic tails or associated receptor chains. Phosphorylation of the conserved immunoreceptor tyrosine-based activation motif tyrosines promotes Syk binding and activation through binding of the Syk tandem SH2 domains. In this report, we present evidence using peptide pull-down studies, surface plasmon resonance, quantitative Western blotting, tryptophan fluorescence measurements, and competition experiments that Syk activation by CLEC-2 is mediated by the cross-linking through the tandem SH2 domains with a stoichiometry of 2:1. In support of this model, cross-linking and electron microscopy demonstrate that CLEC-2 is present as a dimer in resting platelets and converted to larger complexes on activation. This is a unique mode of activation of Syk by a single YxxL-containing receptor.

NKAP is a transcriptional repressor of notch signaling and is required for T cell development

T cell development depends on the coordinated interplay between receptor signaling and transcriptional regulation. Through a genetic complementation screen a transcriptional repressor, NKAP, was identified. NKAP associated with the histone deacetylase HDAC3 and was shown to be part of a DNA-binding complex, as demonstrated by chromatin immunoprecipitation. NKAP also associated with the Notch corepressor complex. The expression of NKAP during T cell development inversely correlated with the expression of Notch target genes, implying that NKAP may modulate Notch-mediated transcription. To examine the function of NKAP in T cell development, we ablated NKAP by Lck(cre). Loss of NKAP blocked development of alphabeta but not gammadelta T cells, and Nkap(fl/o)Lck(cre) DP T cells expressed 8- to 20-fold higher amounts of Hes1, Deltex1, and CD25 mRNA. Thus, NKAP functions as a transcriptional repressor, acting on Notch target genes, and is required for alphabeta T cell development.

Negative regulation of TCR signaling by linker for activation of X cells via phosphotyrosine-dependent and -independent mechanisms

The activation of T cells and the initiation of an immune response is tightly controlled through the crosstalk of both positive and negative regulators. Two adaptors that function as negative regulators of T cell activation are adaptor in lymphocytes of unknown function X (ALX) and linker for activation of X cell (LAX). Previously, we showed that T cells from mice deficient in ALX and LAX display similar hyperresponsiveness, with increased IL-2 production and proliferation upon TCR/CD28 stimulation, and that these adaptors physically associate. In this study, we analyze the nature of the association between ALX and LAX. We demonstrate that this association occurs in the absence of TCR/CD28 signaling via a mechanism independent of both tyrosine phosphorylation of LAX and the SH2 domain of ALX. Cotransfection of ALX with LAX resulted in LAX tyrosine phosphorylation in the absence of TCR/CD28 stimulation. ALX-mediated LAX phosphorylation depends upon the ALX SH2 domain, which functions to recruit Lck to LAX. We also show that LAX, like ALX, can inhibit RE/AP reporter activation. However, in contrast to its inhibition of NFAT, the inhibition of RE/AP by LAX is independent of its tyrosine phosphorylation. Therefore, it can be concluded that inhibition of signaling events involved in T cell activation by LAX occurs through mechanisms both dependent on and independent of its tyrosine phosphorylation.

G6b-B inhibits constitutive and agonist-induced signaling by glycoprotein VI and CLEC-2

Platelets play an essential role in wound healing by forming thrombi that plug holes in the walls of damaged blood vessels. To achieve this, platelets express a diverse array of cell surface receptors and signaling proteins that induce rapid platelet activation. In this study we show that two platelet glycoprotein receptors that signal via an immunoreceptor tyrosine-based activation motif (ITAM) or an ITAM-like domain, namely the collagen receptor complex glycoprotein VI (GPVI)-FcR γ-chain and the C-type lectin-like receptor 2 (CLEC-2), respectively, support constitutive (i.e. agonist-independent) signaling in a cell line model using a nuclear factor of activated T-cells (NFAT) transcriptional reporter assay that can detect low level activation of phospholipase Cγ (PLCγ). Constitutive and agonist signaling by both receptors is dependent on Src and Syk family kinases, and is inhibited by G6b-B, a platelet immunoglobulin receptor that has two immunoreceptor tyrosine-based inhibitory motifs in its cytosolic tail. Mutation of the conserved tyrosines in the two immunoreceptor tyrosine-based inhibitory motifs prevents the inhibitory action of G6b-B. Interestingly, the inhibitory activity of G6b-B is independent of the Src homology 2 (SH2)-domain containing tyrosine phosphatases, SHP1 and SHP2, and the inositol 5′-phosphatase, SHIP. Constitutive signaling via Src and Syk tyrosine kinases is observed in platelets and is associated with tyrosine phosphorylation of GPVI-FcR γ-chain and CLEC-2. We speculate that inhibition of constitutive signaling through Src and Syk tyrosine kinases by G6b-B may help to prevent unwanted platelet activation.

The protein tyrosine phosphatase PTPN4/PTP-MEG1, an enzyme capable of dephosphorylating the TCR ITAMs and regulating NF-kappaB, is dispensable for T cell development and/or T cell effector functions

T cell receptor signaling processes are controlled by the integrated actions of families of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPases). Several distinct cytosolic protein tyrosine phosphatases have been described that are able to negatively regulate TCR signaling pathways, including SHP-1, SHP-2, PTPH1, and PEP. Using PTPase substrate-trapping mutants and wild type enzymes, we determined that PTPN4/PTP-MEG1, a PTPH1-family member, could complex and dephosphorylate the ITAMs of the TCR zeta subunit. In addition, the substrate-trapping derivative augmented basal and TCR-induced activation of NF-kappaB in T cells. To characterize the contribution of this PTPase in T cells, we developed PTPN4-deficient mice. T cell development and TCR signaling events were comparable between wild type and PTPN4-deficient animals. The magnitude and duration of TCR-regulated ITAM phosphorylation, as well as overall protein phosphorylation, was unaltered in the absence of PTPN4. Finally, Th1- and Th2-derived cytokines and in vivo immune responses to Listeria monocytogenes were equivalent between wild type and PTPN4-deficient mice. These findings suggest that additional PTPases are involved in controlling ITAM phosphorylations.

Tumor suppressor cylindromatosis acts as a negative regulator for Streptococcus pneumoniae-induced NFAT signaling

Gram-positive bacterium Streptococcus pneumoniae is an important human pathogen that colonizes the upper respiratory tract and is also the major cause of morbidity and mortality worldwide. S. pneumoniae causes invasive diseases such as pneumonia, meningitis, and otitis media. Despite the importance of pneumococcal diseases, little is known about the molecular mechanisms by which S. pneumoniae-induced inflammation is regulated, especially the negative regulatory mechanisms. Here we show that S. pneumoniae activates nuclear factor of activated T cells (NFAT) signaling pathway and the subsequent up-regulation of inflammatory mediators via a key pneumococcal virulence factor, pneumolysin. We also demonstrate that S. pneumoniae activates NFAT transcription factor independently of Toll-like receptors 2 and 4. Moreover, S. pneumoniae induces NFAT activation via both Ca(2+)-calcineurin and transforming growth factor-beta-activated kinase 1 (TAK1)-mitogen-activated protein kinase kinase (MKK) 3/6-p38alpha/beta-dependent signaling pathways. Interestingly, we found for the first time that tumor suppressor cylindromatosis (CYLD) acts as a negative regulator for S. pneumoniae-induced NFAT signaling pathway via a deubiquitination-dependent mechanism. Finally, we showed that CYLD interacts with and deubiquitinates TAK1 to negatively regulate the activation of the downstream MKK3/6-p38alpha/beta pathway. Our studies thus bring new insights into the molecular pathogenesis of S. pneumoniae infections through the NFAT-dependent mechanism and further identify CYLD as a negative regulator for NFAT signaling, thereby opening up new therapeutic targets for these diseases.

Collagen promotes sustained glycoprotein VI signaling in platelets and cell lines

BACKGROUND

Glycoprotein (GP)VI is the major signaling receptor for collagen on platelets and signals via the associated FcRgamma-chain, which has an immunoreceptor tyrosine-containing activation motif (ITAM).

OBJECTIVE

To determine why GPVI-FcRgamma signals poorly, or not at all, in response to collagen in hematopoietic cell lines, despite robust responses to the GPVI-reactive snake venom toxin convulxin.

METHODS AND RESULTS

Using a nuclear factor of activated T-cells (NFAT) transcriptional reporter assay, a sensitive readout for sustained ITAM signaling, we demonstrate collagen-induced GPVI-FcRgamma signaling in hematopoietic cell lines. This is accompanied by relatively weak but sustained protein tyrosine phosphorylation, in contrast to the stronger but transient response to convulxin. Sustained signaling by collagen is also observed in platelets and is necessary for the maintenance of spreading on collagen. Finally, in cell lines, the inhibitory collagen receptor leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), which is not expressed on platelets but is present on most hematopoietic cells, inhibits GPVI responses to collagen but not convulxin.

CONCLUSION

The inability of previous studies to readily detect GPVI collagen signaling in cell lines is probably because of the weak but sustained nature of the signal and the presence of the inhibitory collagen receptor LAIR-1. In platelets, we propose that GPVI-FcRgamma has evolved to transmit sustained signals in order to maintain spreading over several hours, as well as facilitating rapid activation through release of feedback agonists and integrin activation. The establishment of a cell line NFAT assay will facilitate the molecular dissection of GPVI signaling and the identification of GPVI antagonists in drug discovery.

TCR/CD28-stimulated actin dynamics are required for NFAT1-mediated transcription of c-rel leading to CD28 response element activation

TCR/CD28 engagement triggers the initiation of a variety of signal transduction pathways that lead to changes in gene transcription. Although reorganization of the actin cytoskeleton is required for T cell activation, the molecular pathways controlled by the actin cytoskeleton are ill defined. To this end, we analyzed TCR/CD28-stimulated signaling pathways in cytochalasin D-treated T cells to determine the cytoskeletal requirements for T cell activation. Cytochalasin D treatment impaired T cell activation by causing a reduction in TCR/CD28-mediated calcium flux, and blocked activation of two regulatory elements within the IL-2 promoter, NFAT/AP-1 and CD28RE/AP. Treatment had no effect on signaling leading to the activation of either AP-1 or NF-kappaB. Significantly, we found that NFAT1 is required for optimal c-rel up-regulation in response to TCR/CD28 stimulation. In fact, NFAT1 could be detected bound at the c-rel promoter in response to TCR/CD28 stimulation, and targeting of NFAT1 using RNA interference in human CD4(+) T cells abrogated c-rel transcription. Overall, these findings establish that disrupting actin cytoskeletal dynamics impairs TCR/CD28-mediated calcium flux required for NFAT1-mediated c-rel transcription and, thus, activation of the CD28RE/AP.

The C-type lectin receptors CLEC-2 and Dectin-1, but not DC-SIGN, signal via a novel YXXL-dependent signaling cascade

The two lectin receptors, CLEC-2 and Dectin-1, have been shown to signal through a Syk-dependent pathway, despite the presence of only a single YXXL in their cytosolic tails. In this study, we show that stimulation of CLEC-2 in platelets and in two mutant cell lines is dependent on the YXXL motif and on proteins that participate in signaling by immunoreceptor tyrosine-based activation motif receptors, including Src, Syk, and Tec family kinases, and on phospholipase Cgamma. Strikingly, mutation of either Src homology (SH) 2 domain of Syk blocks signaling by CLEC-2 despite the fact that it has only a single YXXL motif. Furthermore, signaling by CLEC-2 is only partially dependent on the BLNK/SLP-76 family of adapter proteins in contrast to that of immunoreceptor tyrosine-based activation motif receptors. The C-type lectin receptor, Dectin-1, which contains a YXXL motif preceded by the same four amino acids as for CLEC-2 (DEDG), signals like CLEC-2 and also requires the two SH2 domains of Syk and is only partially dependent on the BLNK/SLP-76 family of adapters. In marked contrast, the C-type lectin receptor, DC-SIGN, which has a distinct series of amino acids preceding a single YXXL, signals independent of this motif. A mutational analysis of the DEDG sequence of CLEC-2 revealed that the glycine residue directly upstream of the YXXL tyrosine is important for CLEC-2 signaling. These results demonstrate that CLEC-2 and Dectin-1 signal through a single YXXL motif that requires the tandem SH2 domains of Syk but is only partially dependent on the SLP-76/BLNK family of adapters.

Inhibited cell death, NF-kappaB activity and increased IL-10 in TCR-triggered thymocytes of transgenic mice overexpressing the glucocorticoid-induced protein GILZ

Glucocorticoids promote thymocyte apoptosis and modulate transcription of several genes including GILZ, which is strongly up-regulated in the thymus. We used transgenic mice overexpressing GILZ in the T-cell lineage to investigate TCR-triggered functions of GILZ-overexpressing thymocytes. TCR-triggered apoptosis, but not glucocorticoid-induced apoptosis, was inhibited in transgenic mice compared to their controls. In vivo anti-CD3 administration did not reduce CD4(+)CD8(+) thymocyte number. Analysis of TCR-triggered molecular changes indicated that p65 NF-kappaB nuclear translocation and DNA binding activity was inhibited in transgenic mice, which might be linked with apoptosis inhibition. IL-10 release increased whereas release of IL-2, IFN-gamma, IL-13 and IL-4 remained unchanged. These results support the hypothesis that GILZ regulates, at least in part, T-cell development by influencing thymus function at cellular and molecular levels.

Control of interleukin-2 gene transcription: a paradigm for inducible, tissue-specific gene expression

Interleukin-2 (IL-2) is a key cytokine that controls immune cell function, in particular the adaptive arm of the immune system, through its ability to control the clonal expansion and homeostasis of peripheral T cells. IL-2 is produced almost exclusively by T cells in response to antigenic stimulation and thus provides an excellent example of a cell-specific inducible gene. The mechanisms that control IL-2 gene transcription have been studied in detail for the past 20 years and our current understanding of the nature of the inducible and tissue-specific controls will be discussed.

The carboxyl-terminal segment of the adaptor protein ALX directs its nuclear export during T cell activation

The adaptor protein ALX acts downstream of CD28 to regulate the interleukin-2 (IL-2) promoter during T cell activation. Whereas ALX is predominantly localized to the cytoplasm, ALX partially resides in the nucleus, and the nuclear pool is rapidly depleted in response to T cell receptor (TCR)/CD28 signaling. Here it is shown that this depletion occurs via nuclear export of ALX, which depends on a leucine-rich nuclear export signal (NES) in its carboxyl segment and on the CRM-1 transport protein. Nuclear import of ALX also depends on its carboxyl-terminal segment. Blocking nuclear export of ALX, either pharmacologically, by leptomycin B, or by site-directed mutation of the ALX NES, impairs CD28-mediated phosphorylation of ALX. Additionally, upon overexpression, the ALX NES mutant was found to be impaired in inhibiting TCR/CD28-induced transcriptional up-regulation of the RE/AP composite element from the IL-2 promoter, whereas a truncated form of ALX that is a potent inhibitor of RE/AP activation was found to reside entirely in the cytoplasm. Together, these results show that ALX exerts its effect on IL-2 up-regulation in the cytoplasm and suggest an intricate relationship between the nuclear localization/export, phosphorylation, and activity of ALX in response to TCR and CD28 signaling.

Intramolecular regulatory switch in ZAP-70: analogy with receptor tyrosine kinases

ZAP-70, a Syk family cytoplasmic protein tyrosine kinase (PTK), is required to couple the activated T-cell antigen receptor (TCR) to downstream signaling pathways. It contains two tandem SH2 domains that bind to phosphorylated TCR subunits and a C-terminal catalytic domain. The region connecting the SH2 domains with the kinase domain, termed interdomain B, has previously been shown to have striking regulatory effects on ZAP-70 function, presumed to be due to the recruitment of key substrates. Paradoxically, deletion of interdomain B preserves ZAP-70 function. Recent structural studies of several receptor tyrosine kinases (RTKs) revealed that their juxtamembrane regions negatively regulate their catalytic activities. In EphB2 and several other RTKs, this autoinhibition depends upon interaction between the kinase domain and tyrosine residues within the juxtamembrane region. Autoinhibition is released when these tyrosines become phosphorylated following receptor stimulation. Sequence homology suggested analogous regulation for ZAP-70. Based on mutagenesis analysis of ZAP-70 interdomain B, we find that this region downregulates ZAP-70 catalytic activity in a similar manner as the juxtamembrane region of EphB2. Similar regulation was also noted for the related Syk kinase. These findings suggest that a general autoinhibitory mechanism employed by RTKs is also used by some cytoplasmic tyrosine kinases.

Dynamic recruitment of PAK1 to the immunological synapse is mediated by PIX independently of SLP-76 and Vav1

T cell receptor engagement activates p21-activated kinase 1 (PAK1) through a LAT-SLP-76-Nck-Vav-Rac-dependent pathway. A second independent pathway involving a GIT-PIX-PAK1 trimolecular complex is also activated by T cell receptor ligation. Here we show a Vav-independent pathway exists that leads to PAK1 activation. In addition, PAK1, PIX and GIT1 were recruited to the T cell-antigen-presenting cell contact site independently of SLP-76 and Vav1. PAK1 recruitment to the T cell-antigen-presenting cell interface required interaction with PIX, which also led to optimal PLC-gamma1 activation and T cell receptor-dependent transcriptional responses. These data indicate that a pathway involving the GIT-PIX-PAK1 complex has a crucial function in PAK1 activation by recruiting PAK1 to the immunological synapse.

A novel protein kinase C (PKCepsilon) is required for fMet-Leu-Phe-induced activation of NF-kappaB in human peripheral blood monocytes

We have reported that the chemoattractant, fMet-Leu-Phe (fMLP), induces the activation of NF-kappaB in human peripheral blood monocytes and that this requires the activity of small GTPase, RhoA (Huang, S., Chen, L.-Y., Zuraw, B. L., Ye, R. D., and Pan, Z. K. (2001) J. Biol. Chem. 276, 40977-40981). Here we showed that the novel protein kinase C isozyme, PKCepsilon, associates functionally with RhoA in fMLP-stimulated monocytes and that PKCepsilon acted as a signaling component downstream of the GTPase RhoA during fMLP-induced activation of NF-kappaB. Stimulation of monocytes with fMLP resulted in activation of both PKCepsilon and NF-kappaB. This latter activation was largely blocked by specific inhibitors of PKCepsilon by transient expression of a dominant-negative form of PKCepsilon and by PKCepsilon-specific short interfering RNA. These findings demonstrate, for the first time, that fMLP-induced activation of NF-kappaB utilizes a signaling pathway, which requires activity of PKCepsilon, and that PKCepsilon acts as a signaling component downstream of RhoA in cytokine gene transcription stimulated by a chemoattractant. The specificity of this response suggests an important role for the Rho GTPase-PKCepsilon-NF-kappaB pathway in host defense and represents a novel and potentially important mechanism through which fMLP not only attracts leukocytes but may also contribute directly to inflammation.

SHIP Family Inositol Phosphatases Interact with and Negatively Regulate the Tec Tyrosine Kinase

The Tec family of protein-tyrosine kinases (PTKs), that includes Tec, Itk, Btk, Bmx, and Txk, plays an essential role in phospholipase Cgamma (PLCgamma) activation following antigen receptor stimulation. This function requires activation of phosphatidylinositol 3-kinase (PI 3-kinase), which promotes Tec membrane localization through phosphatidylinositol 3,4,5-trisphosphate (PtdIns 3,4,5-P(3)) generation. The mechanism of negative regulation of Tec family PTKs is poorly understood. In this study, we show that the inositol 5' phosphatases SHIP1 and SHIP2 interact preferentially with Tec, compared with other Tec family members. Four lines of evidence suggest that SHIP phosphatases are negative regulators of Tec. First, SHIP1 and SHIP2 are potent inhibitors of Tec activity. Second, inactivation of the Tec SH3 domain, which is necessary and sufficient for SHIP binding, generates a hyperactive form of Tec. Third, SHIP1 inhibits Tec membrane localization. Finally, constitutively targeting Tec to the membrane relieves SHIP1-mediated inhibition. These data suggest that SHIP phosphatases can interact with and functionally inactivate Tec by de-phosphorylation of local PtdIns 3,4,5-P(3) and inhibition of Tec membrane localization.

The ALX Src homology 2 domain is both necessary and sufficient to inhibit T cell receptor/CD28-mediated up-regulation of RE/AP

Activation of naive T cells occurs when two signals are received. The first signal is received through the T cell antigen receptor (TCR), and a second costimulatory signal is primarily provided by CD28. We have recently identified a novel adaptor molecule, ALX, which is expressed exclusively in hematopoietic cells. ALX contains several sites for potential protein-protein interaction, including an Src homology 2 (SH2) domain, four PXXP polyproline sequences, and two likely sites of tyrosine phosphorylation. Overexpression of ALX inhibits the transcriptional activation of the interleukin 2 promoter during T cell activation, specifically affecting CD28-mediated activation of the RE/AP element of the interleukin 2 promoter. To understand how ALX functions downstream of CD28, we generated a panel of site-directed mutants as well as truncations in which potential protein-binding sites were mutated or absent. We found that the ALX SH2 domain is both necessary and sufficient to mediate inhibition of RE/AP activation. Mutation of the SH2 domain did not affect ALX expression, relative localization in the cytoplasm and nucleus, phosphorylation, or a mobility shift in response to TCR signaling alone. However, an activation-induced mobility shift triggered by CD28 was reduced in the ALX SH2 domain mutant. In addition, the isolated ALX SH2 domain was found to associate with a phosphoprotein from Jurkat T cells on TCR/CD28 stimulation. Therefore, the ALX SH2 domain plays a critical role in ALX function downstream of CD28.

RhoA and Rac1 signals in fMLP-induced NF-kappaB activation in human blood monocytes

GTPase RhoA is required for fMet-Leu-Phe (fMLP)-stimulated NF-kappaB activation in human peripheral blood monocytes. Here we have investigated different members of the Rho family of GTPases Rac1, Cdc42, and RhoA in regulating the transcription factor nuclear factor-kappaB (NF-kappaB) in human peripheral blood monocytes. Stimulation of monocytes with fMLP rapidly activated Rac1, Cdc42, and RhoA and cotransfection of the monocytic THP1 cells with dominant negative forms of Rho GTPases, we found that Rac1 and RhoA, but not Cdc42, involved fMLP-stimulated kappaB reporter gene expression. These results indicate that fMLP stimulates three members of the Rho family of GTPases Rac1, Cdc42, and RhoA activity in monocytes, and that Rac1 and RhoA, but not Cdc42, is required for fMLP-induced NF-kappaB activation. Furthermore, our data also suggest that RhoA is mediated by signals independent of Rac1 in NF-kappaB activation in human peripheral blood monocytes stimulated with bacterial products.

Expression and function of Tec, Itk, and Btk in lymphocytes: evidence for a unique role for Tec

The Tec protein tyrosine kinase is the founding member of a family that includes Btk, Itk, Bmx, and Txk. Btk is essential for B-cell receptor signaling, because mutations in Btk are responsible for X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice, whereas Itk is involved in T-cell receptor signaling. Tec is expressed in both T and B cells, but its role in antigen receptor signaling is not clear. In this study, we show that Tec protein is expressed at substantially lower levels in primary T and B cells relative to Itk and Btk, respectively. However, Tec is up-regulated upon T-cell activation and in Th1 and Th2 cells. In functional experiments that mimic Tec up-regulation, we find that Tec overexpression in lymphocyte cell lines is sufficient to induce phospholipase Cgamma (PLC-gamma) phosphorylation and NFAT (nuclear factor of activated T cells) activation. In contrast, overexpression of Btk, Itk, or Bmx does not induce NFAT activation. Tec-induced NFAT activation requires PLC-gamma, but not the adapters LAT, SLP-76, and BLNK, which are required for Btk and Itk to couple to PLC-gamma. Finally, we show that the unique effector function for Tec correlates with a unique subcellular localization. We hypothesize that Tec functions in activated and effector T lymphocytes to induce the expression of genes regulated by NFAT transcription factors.

CD154 transcriptional regulation in primary human CD4 T cells

CD154 (CD40-ligand) has a wide variety of pleiotropic effects throughout the immune system and is critical to both cellular and humoral immunity. Cell surface and soluble CD154 are primarily expressed by activated CD4 T cells. Expression of CD154 is tightly regulated in a time-dependent manner, and, like most T cell-derived cytokines and other members of the tumor necrosis factor (TNF) superfamily, CD154 is largely regulated at the level of gene transcription. Recently, dysregulated expression of CD154 has been noted in a number of autoimmune disorders, including systemic lupus erythematosus (SLE). In addition, abnormal expression of CD154 has been hypothesized to contribute to a wider array of diseases, from atherosclerosis to Alzheimer's disease. Until recently, very little was known about the transcriptional regulation of CD154. We are exploring CD154 regulation in primary human CD4 T cells in hopes of understanding the cis- and trans-regulatory elements that control its expression in the cells that normally express CD154. Ultimately, we hope to be able to correct abnormal expression of CD154 in various disease states and to help design gene therapy vectors for treating CD154-deficient individuals with hyper-IgM syndrome.

Cloning and characterization of ALX, an adaptor downstream of CD28

T cell activation requires two signals: specific recognition of antigen through the T cell receptor (TCR) and a costimulatory signal provided primarily by CD28 in naïve T cells. We cloned a novel gene with considerable homology to RIBP/TSAd/Lad, an adaptor involved in T cell activation and interleukin-2 (IL-2) promoter activation. Expression of this gene is limited to the spleen and thymus. We have named this gene ALX, adaptor in lymphocytes of unknown function X. Because the related adaptor RIBP is involved in IL-2 regulation, we investigated whether ALX had a similar function. ALX overexpression in Jurkat T cells results in inhibition of IL-2 promoter activation after stimulation with superantigen. The IL-2 promoter contains several binding sites for transcription factors including the composite element RE/AP, which is the primary site of CD28 transcriptional activation. ALX overexpression had the greatest effect on the activation of a RE/AP reporter as opposed to an AP-1 reporter. Interestingly, ALX overexpression strongly inhibited RE/AP activation in response to anti-CD28/phorbol 12-myristate 13-acetate (PMA) stimulation but had minimal effect when anti-TCR/PMA was used. Therefore, it appears that ALX may function downstream of CD28 costimulation during T cell activation. In addition, the mobility of ALX shifts upon TCR/CD28 costimulation to a greater extent than what is observed with either stimulus alone demonstrating that ALX is a target of both TCR and CD28 costimulatory signaling pathways.

Targeting combinatorial transcriptional complex assembly at specific modules within the interleukin-2 promoter by the immunosuppressant SB203580

The proximal promoter sequence of the interleukin-2 (IL-2) gene contains a series of composite sites or modules that controls much of its responsiveness to environmental stimuli. The integrated targeting of these modules is therefore a major mode of regulation. This report describes how multiple functional hierarchies, required for the recruitment of the p300 co-activator to the CD28RE/AP1 (TRE) module of the IL-2 promoter, are selectively disrupted in human T-cells by the immunosuppressive and anti-inflammatory actions of the p38 mitogen-activated protein kinase inhibitor (MAPK), SB203580. The molecular hierarchies targeted by SB203580 include the combinatorial interaction of NF-kappaB and CREB at the CD28RE/AP1 element coupled with the subsequent dynamic co-assembly and activation of p300. Several aspects of this targeting are linked to the ability of SB203580 to inhibit p38 MAPK-controlled pathways. Together, these results provide the molecular basis through which the combinatorial structure and context of the composite elements of the IL-2 promoter dictates mitogen responsiveness and drug susceptibility that are quantitatively and qualitatively distinct from the isolated action of single consensus sequences and/or transcriptional motifs.

The tyrosine phosphatase CD148 is excluded from the immunologic synapse and down-regulates prolonged T cell signaling

CD148 is a receptor-like protein tyrosine phosphatase up-regulated on T cells after T cell receptor (TCR) stimulation. To examine the physiologic role of CD148 in TCR signaling, we used an inducible CD148-expressing Jurkat T cell clone. Expression of CD148 inhibits NFAT (nuclear factor of activated T cells) activation induced by soluble anti-TCR antibody, but not by antigen-presenting cells (APCs) loaded with staphylococcal enterotoxin superantigen (SAg) or immobilized anti-TCR antibody. Immunofluorescence microscopy revealed that the extracellular domain of CD148 mediates its exclusion from the immunologic synapse, sequestering it from potential substrates. Targeting of the CD148 phosphatase domain to the immunologic synapse potently inhibited NFAT activation by all means of triggering through the TCR. These data lead us to propose a model where CD148 function is regulated in part by exclusion from substrates in the immunologic synapse. Upon T cell-APC disengagement, CD148 can then access and dephosphorylate substrates to down-regulate prolongation of signaling.

Regulation of T cell receptor-induced activation of the Ras-ERK pathway by diacylglycerol kinase zeta

T cell development in the thymus and activation of mature T cells in the periphery depend on signals stimulated by engagement of the T cell antigen receptor (TCR). Among the second messenger cascades initiated by TCR ligation include the phosphatidylinositol pathway where the membrane phospholipid, phosphatidylinositol 4,5-bisphosphate, is hydrolyzed to inositol 1,4,5-trisphosphate and diacylglycerol (DAG). Inositol 1,4,5-trisphosphate signals a rise in intracellular free calcium, leading to translocation of nuclear factor of activated T cells into the nucleus. DAG activates RasGRP and protein kinase C theta. Because both RasGRP and protein kinase C theta are essential for thymocyte and T cell function, it is critical to understand how DAG is regulated. In this report, we demonstrate expression of DAG kinase zeta (DGKzeta, the enzyme that catalyzes the conversion of DAG to phosphatidic acid) in multiple lymphoid organs, with highest expression observed within the T cell compartment. Overexpression studies in Jurkat T cells indicate that DGKzeta interferes with TCR-induced Ras and ERK activation, AP-1 induction, and expression of the activation marker CD69. In contrast, TCR-stimulated calcium influx is not altered. Mutational analysis indicates that the kinase and DAG binding domains, but not the ankyrin repeats of DGKzeta, are required for its inhibitory effects. Collectively these studies demonstrate a potential role of DGKzeta to function as a selective negative regulator of DAG signaling on T cell activation and provide the first structure/function analysis of this enzyme in T cells.

Genetic evidence for Shc requirement in TCR-induced c-Rel nuclear translocation and IL-2 expression

Shc, a prototypic adapter molecule, has been implicated in T cell receptor (TCR) signal transduction, but its role has not been identified clearly. Here we report that Shc is essential for TCR-induced IL-2 production but is dispensable for CD69 or CD25 expression. Engagement of TCR in mutant Jurkat T cells lacking Shc fails to produce IL-2 because of impaired mitogen-activated protein kinase activation. Activation of c-Rel, a transcription factor essential for IL-2 expression, was impaired also. In contrast, activation of nuclear factor of activated T cell and expression of CD69/CD25 were comparable between the mutant and wild-type Jurkat cells. These defects were rescued by expression of exogenous Shc. Activation of c-Rel using the estrogen receptor fusion protein restored the activation of the IL-2 promoter in an estrogen-dependent manner. These results show that Shc plays an essential role in the TCR-induced activation of c-Rel and the IL-2 promoter.

The lymphoid protein tyrosine phosphatase Lyp interacts with the adaptor molecule Grb2 and functions as a negative regulator of T-cell activation

OBJECTIVE

Following activation of T cells, phosphorylation of tyrosine residues occurs through a complex signaling process involving protein tyrosine kinases, phosphatases, and a variety of adapter molecules including Grb2. We have attempted to identify new signaling molecules that are important for the activation response.

METHODS

Using a protein interaction screening protocol based on phage display, T-cell signaling components that associate with the adapter molecule, Grb2, the lymphoid-specific tyrosine phosphatase Lyp was identified. Using transcriptional reporter assays, the role of Lyp in T-cell activation was studied by overexpression of wild-type or catalytically inactive mutants of Lyp.

RESULTS

A GST fusion containing the C-terminal SH3 domain of Grb2 bound to the nucleotide exchange factor Sos or Grb2-associated binder 2 (Gab2). In contrast, the N-terminal SH3-containing fusion bound to the protein tyrosine phosphatase Lyp. Grb2 was co-immunoprecipitated with Lyp in 293T cells overexpressing both proteins. Using Northern blot analysis, Lyp was found to be expressed predominantly in hematopoietic tissue, including spleen, lymph node, thymus, peripheral blood leukocytes, bone marrow, and fetal liver. Two human T-cell lines, Jurkat and HuT78, expressed both Lyp mRNA and protein. Overexpression of wild-type Lyp or a catalytically inactive, substrate-trapping mutant (D195A) in Jurkat cells inhibited transcriptional activity initiated by anti-CD3 and anti-CD28 antibodies. In contrast, two other catalytically inactive mutants (R233M or C227S) had no effect.

CONCLUSION

These data demonstrate a novel interaction between the phosphatase Lyp and the adaptor Grb2 and are consistent with a negative regulatory role for Lyp in T-cell signaling.

Hematopoietic progenitor kinase 1 associates physically and functionally with the adaptor proteins B cell linker protein and SLP-76 in lymphocytes

B cell linker protein (BLNK) is a SLP-76-related adaptor protein essential for signal transduction from the BCR. To identify components of BLNK-associated signaling pathways, we performed a phosphorylation-dependent yeast two-hybrid analysis using BLNK probes. Here we report that the serine/threonine kinase hematopoietic progenitor kinase 1 (HPK1), which is activated upon antigen-receptor stimulation and which has been implicated in the regulation of MAP kinase pathways, interacts physically and functionally with BLNK in B cells and with SLP-76 in T cells. This interaction requires Tyr(379) of HPK1 and the Src homology 2 (SH2) domain of BLNK/SLP-76. Via homology modeling, we defined a consensus binding site within ligands for SLP family SH2 domains. We further demonstrate that the SH2 domain of SLP-76 participates in the regulation of AP-1 and NFAT activation in response to T cell receptor (TCR) stimulation and that HPK1 inhibits AP-1 activation in a manner partially dependent on its interaction with SLP-76. Our data are consistent with a model in which full activation of HPK1 requires its own phosphorylation on tyrosine and subsequent interaction with adaptors of the SLP family, providing a mechanistic basis for the integration of this kinase into antigen receptor signaling cascades.

Identification of the minimal tyrosine residues required for linker for activation of T cell function

The linker for activation of T cells (LAT) is essential for signaling through the T cell receptor (TCR). Following TCR stimulation, LAT becomes tyrosine-phosphorylated, creating docking sites for other signaling proteins such as phospholipase C-gamma(1) (PLC-gamma(1)), Grb2, and Gads. In this study, we have attempted to identify the critical tyrosine residues in LAT that mediate TCR activation-induced mobilization of intracellular Ca(2+) and activation of the MAP kinase Erk2. By using the LAT-deficient Jurkat derivative, J.CaM2, stable cell lines were established expressing various tyrosine mutants of LAT. We show that three specific tyrosine residues (Tyr(132), Tyr(171), and Tyr(191)) are necessary and sufficient to achieve a Ca(2+) flux following TCR stimulation. These tyrosine residues function by reconstituting PLC-gamma(1) phosphorylation and recruitment to LAT. However, these same tyrosines can only partially reconstitute Erk activation. Full reconstitution of Erk requires two additional tyrosine residues (Tyr(110) and Tyr(226)), both of which have the Grb2-binding motif YXN. This reconstitution of Erk activation requires that the critical tyrosine residues be on the same molecule of LAT, suggesting that a single LAT molecule nucleates multiple protein-protein interactions required for optimal signal transduction.

Activation of NF-kappa B by nontypeable Hemophilus influenzae is mediated by toll-like receptor 2-TAK1-dependent NIK-IKK alpha /beta-I kappa B alpha and MKK3/6-p38 MAP kinase signaling pathways in epithelial cells

Nontypeable Hemophilus influenzae (NTHi) is an important human pathogen in both children and adults. In children, it causes otitis media, the most common childhood infection and the leading cause of conductive hearing loss in the United States. In adults, it causes lower respiratory tract infections in the setting of chronic obstructive pulmonary disease, the fourth leading cause of death in the United States. The molecular mechanisms underlying the pathogenesis of NTHi-induced infections remain undefined, but they may involve activation of NF-kappa B, a transcriptional activator of multiple host defense genes involved in immune and inflammatory responses. Here, we show that NTHi strongly activates NF-kappa B in human epithelial cells via two distinct signaling pathways, NF-kappa B translocation-dependent and -independent pathways. The NF-kappa B translocation-dependent pathway involves activation of NF-kappa B inducing kinase (NIK)--IKK alpha/beta complex leading to I kappa B alpha phosphorylation and degradation, whereas the NF-kappa B translocation-independent pathway involves activation of MKK3/6--p38 mitogen-activated protein (MAP) kinase pathway. Bifurcation of NTHi-induced NIK-IKK alpha/beta-I kappa B alpha and MKK3/6--p38 MAP kinase pathways may occur at transforming growth factor-beta activated kinase 1 (TAK1). Furthermore, we show that toll-like receptor 2 (TLR2) is required for NTHi-induced NF-kappa B activation. In addition, several key inflammatory mediators including IL-1 beta, IL-8, and tumor necrosis factor-alpha are up-regulated by NTHi. Finally, P6, a 16-kDa lipoprotein highly conserved in the outer membrane of all NTHi and H. influenzae type b strains, appears to also activate NF-kappa B via similar signaling pathways. Taken together, our results demonstrate that NTHi activates NF-kappa B via TLR2-TAK1-dependent NIK--IKK alpha/beta-I kappa B alpha and MKK3/6--p38 MAP kinase signaling pathways. These studies may bring new insights into molecular pathogenesis of NTHi-induced infections and open up new therapeutic targets for these diseases.

Endogenous CD28 expressed on myeloma cells up-regulates interleukin-8 production: implications for multiple myeloma progression

CD28 is the major costimulatory molecule on T cells. CD28 activation, in conjunction with T-cell receptor engagement, up-regulates transcription of several cytokines, including interleukin-2 (IL-2), through transcriptional activation of the RE/AP composite element. Although CD28 is not normally expressed on B cells or plasma cells, more than 90% of extramedullary myelomas (a late stage B-cell neoplasm) express CD28. The functional significance of this is unknown. The results of this study demonstrate that CD28 stimulates transcriptional activation of RE/AP-based reporters in B cells and myeloma cells. However, CD28 stimulation does not up-regulate IL-2 production in myeloma cell lines, demonstrating that the IL-2 promoter may not be a relevant RE/AP-containing target of CD28 in myelomas. Instead, an RE/AP composite element has been identified within the promoter of the IL-8 gene, a chemokine that promotes angiogenesis. Furthermore, stimulation of endogenous CD28 expressed by 3 myeloma cell lines increased IL-8 production. Therefore, the study demonstrates that CD28 is functional in myelomas to up-regulate transcription of endogenous genes, including IL-8. The proposal is made that aberrant expression of CD28 may play a role in the progression of multiple myeloma.

Immunoreceptor tyrosine-based activation motif-dependent signaling by Kaposi's sarcoma-associated herpesvirus K1 protein: effects on lytic viral replication

The Kaposi's sarcoma-associated herpesvirus (KSHV) K1 gene encodes a polypeptide bearing an immunoreceptor tyrosine-based activation motif (ITAM) that is constitutively active for ITAM-based signal transduction. Although ectopic overexpression of K1 in cultured fibroblasts can lead to growth transformation, in vivo this gene is primarily expressed in lymphoid cells undergoing lytic infection. Here we have examined function of K1 in the setting of lytic replication, through the study of K1 mutants lacking functional ITAMs. Expression of such mutants in BJAB cells cotransfected with wild-type K1 results in dramatic inhibition of K1 signal transduction, as judged by impaired activation of Syk kinase and phospholipase C-gamma2 as well as by diminished expression of a luciferase reporter gene dependent upon K1-induced calcium and Ras signaling. Thus, the mutants behave as dominantly acting inhibitors of K1 function. To assess the role of K1 in lytic replication, we introduced these K1 mutants into BCBL-1 cells, a B-cell lymphoma line latently infected with KSHV, and induced lytic replication by ectopic expression of the KSHV ORF50 transactivator. Expression of lytic cycle genes was diminished up to 80% in the presence of a K1 dominant negative mutant. These inhibitory effects could be overridden by tetradecanoyl phorbol acetate treatment, indicating that inhibition was not due to irreversible cell injury and suggesting that other signaling events could bypass the block. We conclude that ITAM-dependent signaling by K1 is not absolutely required for lytic reactivation but functions to modestly augment lytic replication in B cells, the natural reservoir of KSHV.

A viral protein that selectively downregulates ICAM-1 and B7-2 and modulates T cell costimulation

Kaposi's sarcoma-associated (KS-associated) herpesvirus (KSHV) is a B-lymphotropic agent linked to AIDS-related lymphoproliferative disorders and KS. We and others have earlier identified two viral genes, K3 and K5, that encode endoplasmic reticulum proteins that downregulate surface MHC-I chains by enhancing their endocytosis. Here we have examined the ability of these proteins to influence the disposition of other host surface proteins implicated in immune recognition and activation. We report that K5, but not K3, expression in BJAB cells dramatically reduces ICAM-1 and B7-2 surface expression; B7-1 expression is unaffected. This K5-induced reduction can be reversed by coexpression of a dominant negative mutant of dynamin, indicating that the loss of ICAM and B7-2 surface expression is due to their enhanced endocytosis. This downregulation is functionally significant, because K5-transfected B cells show substantial impairment in their ability to induce T cell activation. K5 is thus the first example of a viral modulator of immunological synapse formation and T cell costimulation. We propose that its expression reduces T cell responses to KSHV-infected B cells early in infection, thereby diminishing antiviral cytokine release and the production of stimulatory signals for CTL generation.

The effect of HIV-1 regulatory proteins on cellular genes: derepression of the IL-2 promoter by Tat

In HIV-infected individuals dysregulation of the immune system is characterized by severe disorders of the cytokine network. Increase secretion of IL-2, the major T cell growth and differentiation cytokine, may play a decisive role in sensitization of T cells for activation induced apoptosis and indirect death of activated T cells through augmented virus replication. We investigated the cause of enhanced IL-2 secretion and found that the HIV Tat induces this effect. We demonstrate that increased IL-2 secretion is due to Tat-enhanced IL-2 promoter activation. Tat derepresses and activates the distal AP-1 site (position -185 to -177) in the IL-2 promoter. In nonstimulated T cells a repressor complex containing NF-IL6, JunB, c-Fos and Fra-1 is formed on the AP-1(IL-2/d) site and represses IL-2 promoter activity. After T cell activation, a heterodimeric activator containing p65 and c-Jun binds to the AP-1(IL-2/d) site. HIV Tat enhances activation of NF-kappaB and consequently, activates the AP-1(IL-2/d) site. Our data provide evidence for a novel mechanism by which HIV Tat dysregulates IL-2 production and therefore may contribute to the HIV-1 infection in a way yet to be clarified.

HIV-1 Tat inhibits IL-2 gene transcription through qualitative and quantitative alterations of the cooperative Rel/AP1 complex bound to the CD28RE/AP1 composite element of the IL-2 promoter

Dysregulation of cytokine secretion plays an important role in AIDS pathogenesis. Here, we demonstrate that expression of HIV-1 Tat protein in Jurkat cells induces a severe impairment of IL-2 but not TNF gene transcription. Interestingly, this inhibition correlates with the effect of the viral protein on the transactivation of the CD28RE/AP1 composite element (-164/-154), but not with that observed on the NFAT/AP1 site of the IL-2 gene promoter, neither with the effect on NF-kappa B- nor AP1-independent binding sites. Endogenous expression of Tat induced a decrease in the amount of the specific protein complex bound to the CD28RE/AP1 probe after PMA plus calcium ionophore stimulation. This effect was accompanied by qualitative alterations of the AP1 complex. Thus, in wild-type Jurkat cells, c-jun was absent from the complex, whereas in Tat-expressing cells, c-jun was increasingly recruited overtime. By contrast, similar amounts of c-rel and a small amount of NFAT1 were detected both in wild type and in Jurkat Tat(+) cells. Furthermore, Tat not only induced the participation of c-jun in the cooperative complex but also a decrease in its transactivation activity alone or in combination with c-rel. Thus, the interaction of Tat with the components of this rel/AP1 cooperative complex seems to induce quantitative and qualitative alterations of this complex as activation progresses, resulting in a decrease of IL-2 gene transcription. Altogether our results suggest the existence of tuned mechanisms that allow the viral protein to specifically affect cooperative interactions between transcription factors.

Protein tyrosine phosphatase CD148-mediated inhibition of T-cell receptor signal transduction is associated with reduced LAT and phospholipase Cgamma1 phosphorylation

In this study, we investigate the role of the receptor-like protein tyrosine phosphatase CD148 in T-cell activation. Overexpression of CD148 in the Jurkat T-cell line inhibited activation of the transcription factor nuclear factor of activated T cells following T-cell receptor (TCR) stimulation but not following stimulation through a heterologously expressed G protein-coupled receptor, the human muscarinic receptor subtype 1. Using a tetracycline-inducible expression system, we show that the TCR-mediated activation of both the Ras and calcium pathways was inhibited by expression of CD148 at levels that approximate those found in activated primary T cells. These effects were dependent on the phosphatase activity of CD148. Analysis of TCR-induced protein tyrosine phosphorylation demonstrated that most phosphoproteins were unaffected by CD148 expression. However, phospholipase Cgamma1 (PLCgamma1) and LAT were strikingly hypophosphorylated in CD148-expressing cells following TCR stimulation, whereas the phosphorylation levels of Slp-76 and Itk were modestly reduced. Based on these results, we propose that CD148 negatively regulates TCR signaling by interfering with the phosphorylation and function of PLCgamma1 and LAT.