IkappaB kinases serve as a target of CD28 signaling

Splenic TNF-α signaling potentiates the innate-to-adaptive transition of antiviral NK cells

Natural killer (NK) cells possess both innate and adaptive features. Here, we investigated NK cell activation across tissues during cytomegalovirus infection, which generates antigen-specific clonal expansion and long-lived memory responses. Longitudinal tracking and single-cell RNA sequencing of NK cells following infection revealed enhanced activation in the spleen, as well as early formation of a CD69lo precursor population that preferentially gave rise to adaptive NK cells. Splenic NK cells demonstrated heightened tumor necrosis factor alpha (TNF-α) signaling and increased expression of the receptor TNFR2, which coincided with elevated TNF-α production by splenic myeloid cells. TNFR2-deficient NK cells exhibited impaired interferon gamma (IFN-γ) production and expansion. TNFR2 signaling engaged two distinct nuclear factor κB (NF-κB) signaling arms-innate effector NK cell responses required canonical NF-κB signaling, whereas non-canonical NF-κB signaling enforced differentiation of CD69lo adaptive NK cell precursors. Thus, NK cell priming in the spleen during viral infection promotes an innate-to-adaptive transition, providing insight into avenues for generating adaptive NK cell immunity across diverse settings.

The optimized core peptide derived from CABIN1 efficiently inhibits calcineurin-mediated T-cell activation

The C-terminal fragment of CABIN1 interacts with calcineurin and represses the transcriptional activity of the nuclear factor of activated T cells (NFAT). However, the specific sequences and mechanisms through which it binds to calcineurin are unclear. This study determined that decameric peptide (CABIN1 residues 2146–2155) is minimally required for binding to calcineurin. This peptide contains a unique “PPTP” C-terminal sequence and a “PxIxIT” N-terminal motif. Furthermore, p38MAPK phosphorylated the threonine residue of the “PPTP” sequence under physiological conditions, dramatically enhancing the peptide’s binding affinity to calcineurin. Therefore, the CABIN1 peptide inhibited the calcineurin-NFAT pathway and the activation of T cells more efficiently than the VIVIT peptide without affecting calcineurin’s phosphatase activity. The CABIN1 peptide could thus be a more potent calcineurin inhibitor and provide therapeutic opportunities for various diseases caused by the calcineurin-NFAT pathway.

A peptide with therapeutic potential binds strongly to the cellular enzyme calcineurin and may prove valuable in anti-cancer and autoimmune disease treatments. Many cancers and autoimmune diseases are linked with overactivation of a key calcineurin-related pathway which is heavily involved in T cell activation. This pathway has long been a therapeutic target, but existing drugs show problems with stability and delivery, and can cause serious side effects. One known inhibitor of calcineurin is the protein CABIN1, but precisely how well it binds and how useful it may be is unclear. Now, Hong-Duk Youn at Seoul National University College of Medicine, South Korea, and co-workers have identified how one specific peptide from CABIN1 binds strongly to calcineurin. The CABIN1 peptide was stable and displayed greater efficiency at inhibiting calcineurin than another recently identified peptide candidate.

Expression patterns of NLRC5 and key genes in the STAT1 pathway following infection with Salmonella pullorum

NLRC5, a protein belonging to the NOD-like receptor protein family (NLRs), is highly expressed in immune tissues and cells. NLRC5 plays an important role in the immune response of humans, where its regulatory mechanism has been elucidated. However, the function and regulation of NLRC5 in chickens remains unclear. In this study, temporal expression characteristics of NLRC5 and associated genes in the STAT1 pathway in chickens following infection with Salmonella pullorum were investigated using quantitative real-time polymerase chain reaction and hierarchical cluster analyses. The role of transcription factor STAT1 in NLRC5 promoter activity was studied via point mutation of the STAT1-binding cis-element and dual-luciferase assays. Our results showed a strong correlation between NLRC5 and NF-κB. In addition, STAT1 played a crucial role in NLRC5 promoter activity, and may be activated via the interferon pathway. There was also a close relationship between CD80 and NF-κB, and CD80 may up-regulate NF-κB expression and enhance its protein activity in chickens. These findings reveal the temporal characteristics of chicken NLRC5 and STAT1 genes during S. pullorum infection, and highlight the role of STAT1 in NLRC5 promoter activity. This information aids our understanding of the regulatory mechanisms of NLRC5 and associated genes, and will help elucidate their function in the immune response of chickens.

CD28 costimulatory signals in T lymphocyte activation: Emerging functions beyond a qualitative and quantitative support to TCR signalling

CD28 is one of the most important co-stimulatory receptors necessary for full T lymphocyte activation. By binding its cognate ligands, B7.1/CD80 or B7.2/CD86, expressed on the surface of professional antigen presenting cells (APC), CD28 initiates several signalling cascades, which qualitatively and quantitatively support T cell receptor (TCR) signalling. More recent data evidenced that human CD28 can also act as a TCR-independent signalling unit, by delivering specific signals, which regulate the expression of pro-inflammatory cytokine/chemokines. Despite the enormous progresses made in identifying the mechanisms and molecules involved in CD28 signalling properties, much remains to be elucidated, especially in the light of the functional differences observed between human and mouse CD28. In this review we provide an overview of the current mechanisms and molecules through which CD28 support TCR signalling and highlight recent findings on the specific signalling motifs that regulate the unique pro-inflammatory activity of human CD28.

Molecular mechanisms of T-cell anergy

Anergy is a long-term stable state of T-lymphocyte unresponsiveness to antigenic stimulation associated with the blockade of IL-2 production and proliferation. Anergy is a pathway of peripheral tolerance formation. In this review, mechanisms underlying T-cell tolerization are considered in a classical in vitro model of clonal anergy, and these mechanisms are compared with different pathways of anergy induction in vivo. Special attention is given to regulatory T-lymphocytes because, on one hand, anergy is a specific feature of these cells, and on the other hand anergy is also a mechanism of their action on target cells - effector T-lymphocytes. The role of this phenomenon in the differentiation of regulatory T-cells and also in the development of activation-induced apoptosis in effector T-lymphocytes is discussed.

PKC-theta-mediated signal delivery from the TCR/CD28 surface receptors

Protein kinase C-theta (PKCθ) is a key enzyme in T lymphocytes, where it plays an important role in signal transduction downstream of the activated T cell antigen receptor (TCR) and the CD28 costimulatory receptor. Interest in PKCθ as a potential drug target has increased following recent findings that PKCθ is essential for harmful inflammatory responses mediated by Th2 (allergies) and Th17 (autoimmunity) cells as well as for graft-versus-host disease (GvHD) and allograft rejection, but is dispensable for beneficial responses such as antiviral immunity and graft-versus-leukemia (GvL) response. TCR/CD28 engagement triggers the translocation of the cytosolic PKCθ to the plasma membrane (PM), where it localizes at the center of the immunological synapse (IS), which forms at the contact site between an antigen-specific T cell and antigen-presenting cells (APC). However, the molecular basis for this unique localization, and whether it is required for its proper function have remained unresolved issues until recently. Our recent study resolved these questions by demonstrating that the unique V3 (hinge) domain of PKCθ and, more specifically, a proline-rich motif within this domain, is essential and sufficient for its localization at the IS, where it is anchored to the cytoplasmic tail of CD28 via an indirect mechanism involving Lck protein tyrosine kinase (PTK) as an intermediate. Importantly, the association of PKCθ with CD28 is essential not only for IS localization, but also for PKCθ-mediated activation of downstream signaling pathways, including the transcription factors NF-κB and NF-AT, which are essential for productive T cell activation. Hence, interference with formation of the PKCθ-Lck-CD28 complex provides a promising basis for the design of novel, clinically useful allosteric PKCθ inhibitors. An additional recent study demonstrated that TCR triggering activates the germinal center kinase (GSK)-like kinase (GLK) and induces its association with the SLP-76 adaptor at the IS, where GLK phosphorylates the activation loop of PKCθ, converting it into an active enzyme. This recent progress, coupled with the need to study the biology of PKCθ in human T cells, is likely to facilitate the development of PKCθ-based therapeutic modalities for T cell-mediated diseases.

New insight into the oncogenic mechanism of the retroviral oncoprotein Tax

Human T cell leukemia virus type 1 (HTLV-1), an etiological factor that causes adult T cell leukemia and lymphoma (ATL), infects over 20 million people worldwide. About 1 million of HTLV-1-infected patients develop ATL, a highly aggressive non-Hodgkin's lymphoma without an effective therapy. The pX region of the HTLV-1 viral genome encodes an oncogenic protein, Tax, which plays a central role in transforming CD4+ T lymphocytes by deregulating oncogenic signaling pathways and promoting cell cycle progression. Expression of Tax following viral entry is critical for promoting survival and proliferation of human T cells and is required for initiation of oncogenesis. Tax exhibits diverse functions in host cells, and this oncoprotein primarily targets IκB kinase complex in the cytoplasm, resulting in persistent activation of NF-κB and upregulation of its responsive gene expressions that are crucial for T cell survival and cell cycle progression. We here review recent advances for the pathological roles of Tax in modulating IκB kinase activity. We also discuss our recent observation that Tax connects the IκB kinase complex to autophagy pathways. Understanding Tax-mediated pathogenesis will provide insights into development of new therapeutics in controlling HTLV-1-associated diseases.

A novel association between filamin A and NF-κB inducing kinase couples CD28 to inhibitor of NF-κB kinase α and NF-κB activation

CD28 costimulatory molecule plays a critical role in the activation of NF-κB. Indeed, while stimulation of T cells with either professional APCs or anti-TCR plus anti-CD28 antibodies efficiently activates NF-κB, TCR alone fails to do that. Moreover, CD28 stimulation by B7 in the absence of TCR may activate IκB kinase α (IKKα) and a non-canonical NF-κB2-like pathway, in human primary CD4(+) T cells. Despite its functional relevance in NF-κB activation, the molecules connecting autonomous CD28-mediated signals to IKKα and NF-κB activation remain still unknown. In searching for specific upstream activators linking CD28 to the IKKα/NF-κB cascade, we identify a novel constitutive association between filamin A (FLNa) and the NF-κB inducing kinase (NIK), in both Jurkat and human primary T cells. Following CD28 engagement by B7, in the absence of TCR, FLNa-associated NIK is activated and induces IKKα kinase activity. Both proline (P(208)YAP(211)P(212)) and tyrosine residues (Y(206)QPY(209)APP) within the C-terminal proline-rich motif of CD28 are involved in the recruitment of FLNa/NIK complexes to the membrane as well as in the activation of NIK and IKKα.

An enigmatic tail of CD28 signaling

CD28 costimulation regulates a wide range of cellular processes, from proliferation and survival to promoting the differentiation of specialized T-cell subsets. Since first being identified over 20 years ago, CD28 has remained a subject of intense study because of its profound consequences on T cell function and its potential for therapeutic manipulation. In this review we highlight the signaling cascades initiated by the major signaling motifs in CD28, focusing on PI-3 kinase-dependent and -independent pathways and how these are linked to specific cellular outcomes. Recent studies using gene targeted knockin mice have clarified the relative importance of these motifs on in vivo immune responses; however, much remains to be elucidated. Understanding the mechanism behind costimulation holds great potential for development of new clinically relevant reagents, a fact beginning to be realized with the advent of drugs that prevent CD28 ligation and signaling.

TNFR2-deficient memory CD8 T cells provide superior protection against tumor cell growth

TNF receptor-2 (TNFR2) plays a critical role in promoting the activation and survival of naive T cells during the primary response. Interestingly, anti-CD3 plus IL-2 activated TNFR2(-/-) CD8 T cells are highly resistant to activation-induced cell death (AICD), which correlates with high expression levels of prosurvival molecules such as Bcl-2, survivin, and CD127 (IL-7Ralpha). We determined whether the resistance of activated TNFR2(-/-) CD8 T cells to AICD contributes to more effective protection against tumor cell growth. We found that during a primary tumor challenge, despite initial inferiority in controlling tumor cell growth, TNFR2(-/-) mice were able to more effectively control tumor burden over time compared with wild-type (WT) mice. Furthermore, vaccination of TNFR2(-/-) mice with recombinant Listeria monocytogenes that express OVA confers better protection against the growth of OVA-expressing E.G7 tumor cells relative to similarly vaccinated WT mice. The enhanced protection against tumor cell growth was not due to more effective activation of OVA-specific memory CD8 T cells in vaccinated TNFR2(-/-) mice. In vitro studies indicate that optimally activated OVA-specific TNFR2(-/-) CD8 T cells proliferated to the same extent and possess similar cytotoxicity against E.G7 tumor cells as WT CD8 T cells. However, relative to WT cells, activated OVA-specific TNFR2(-/-) CD8 T cells were highly resistant to AICD. Thus, the enhanced protection against E.G7 in TNFR2(-/-) mice is likely due to the recruitment and activation of OVA-specific memory TNFR2(-/-) CD8 T cells and their prolonged survival at the tumor site.

BCL11B enhances TCR/CD28-triggered NF-kappaB activation through up-regulation of Cot kinase gene expression in T-lymphocytes

BCL11B is a transcriptional regulator with an important role in T-cell development and leukaemogenesis. We demonstrated recently that BCL11B controls expression from the IL (interleukin)-2 promoter through direct binding to the US1 (upstream site 1). In the present study, we provide evidence that BCL11B also participates in the activation of IL-2 gene expression by enhancing NF-kappaB (nuclear factor kappaB) activity in the context of TCR (T-cell receptor)/CD28-triggered T-cell activation. Enhanced NF-kappaB activation is not a consequence of BCL11B binding to the NF-kappaB response elements or association with the NF-kappaB-DNA complexes, but rather the result of higher translocation of NF-kappaB to the nucleus caused by enhanced degradation of IkappaB (inhibitor of NF-kappaB). The enhanced IkappaB degradation in cells with increased levels of BCL11B was specific for T-cells activated through the TCR, but not for cells activated through TNFalpha (tumour necrosis factor alpha) or UV light, and was caused by increased activity of IkappaB kinase, as indicated by its increase in phosphorylation. As BCL11B is a transcription factor, we investigated whether the expression of genes upstream of IkappaB kinase in the TCR/CD28 signalling pathway was affected by increased BCL11B expression, and found that Cot (cancer Osaka thyroid oncogene) kinase mRNA levels were elevated. Cot kinase is known to promote enhanced IkappaB kinase activity, which results in the phosphorylation and degradation of IkappaB and activation of NF-kappaB. The implied involvement of Cot kinase in BCL11B-mediated NF-kappaB activation in response to TCR activation is supported by the fact that a Cot kinase dominant-negative mutant or Cot kinase siRNA (small interfering RNA) knockdown blocked BCL11B-mediated NF-kappaB activation. In support of our observations, in the present study we report that BCL11B enhances the expression of several other NF-kappaB target genes, in addition to IL-2. In addition, we provide evidence that BCL11B associates with intron 2 of the Cot kinase gene to regulate its expression.

Ca2+-Operated Transcriptional Networks: Molecular Mechanisms and In Vivo Models

Calcium is the most universal signal used by living organisms to convey information to many different cellular processes. In this review we present well-known and recently identified proteins that sense and decode the calcium signal and are key elements in the nucleus to regulate the activity of various transcriptional networks. When possible, the review also presents in vivo models in which the genes encoding these calcium sensors-transducers have been modified, to emphasize the critical role of these Ca2+-operated mechanisms in many physiological functions.

3-Hydroxyanthranilic acid inhibits PDK1 activation and suppresses experimental asthma by inducing T cell apoptosis

3-Hydroxyanthranilic acid (HAA), a compound generated during tryptophan metabolism initiated by indoleamine 2,3-dioxygenase, is known to induce T cell death, but its molecular target is not known. Here we report that HAA inhibits NF-kappaB activation upon T cell antigen receptor engagement by specifically targeting PDK1. Inhibition of NF-kappaB by HAA leads to dysfunction and cell death of activated Th2 cells, which in turn suppresses experimental asthma. Inhibition of NF-kappaB and induction of apoptosis is specific to CD4 T cells because HAA does not inhibit NF-kappaB activation or induce cell death upon Toll-like receptor 4 stimulation in dendritic cells. Thus, HAA is a natural inhibitor that restrains T cell expansion and activation.

CD28-mediated regulation of multiple myeloma cell proliferation and survival

Although interactions with bone marrow stromal cells are essential for multiple myeloma (MM) cell survival, the specific molecular and cellular elements involved are largely unknown, due in large part to the complexity of the bone marrow microenvironment itself. The T-cell costimulatory receptor CD28 is also expressed on normal and malignant plasma cells, and CD28 expression in MM correlates significantly with poor prognosis and disease progression. In contrast to T cells, activation and function of CD28 in myeloma cells is largely undefined. We have found that direct activation of myeloma cell CD28 by anti-CD28 mAb alone induces activation of PI3K and NFkappaB, suppresses MM cell proliferation, and protects against serum starvation and dexamethasone (dex)-induced cell death. Coculture with dendritic cells (DCs) expressing the CD28 ligands CD80 and CD86 also elicits CD28-mediated effects on MM survival and proliferation, and DCs appear to preferentially localize within myeloma infiltrates in primary patient samples. Our findings suggest a previously undescribed myeloma/DC cell-cell interaction involving CD28 that may play an important role in myeloma cell survival within the bone marrow stroma. These data also point to CD28 as a potential therapeutic target in the treatment of MM.

Disruption of diacylglycerol metabolism impairs the induction of T cell anergy

Anergic T cells have altered diacylglycerol metabolism, but whether that altered metabolism has a causative function in the induction of T cell anergy is not apparent. To test the importance of diacylglycerol metabolism in T cell anergy, we manipulated diacylglycerol kinases (DGKs), which are enzymes that terminate diacylglycerol-dependent signaling. Overexpression of DGK-alpha resulted in a defect in T cell receptor signaling that is characteristic of anergy. We generated DGK-alpha-deficient mice and found that DGK-alpha-deficient T cells had more diacylglycerol-dependent T cell receptor signaling. In vivo anergy induction was impaired in DGK-alpha-deficient mice. When stimulated in anergy-producing conditions, T cells lacking DGK-alpha or DGK-zeta proliferated and produced interleukin 2. Pharmacological inhibition of DGK-alpha activity in DGK-zeta-deficient T cells that received an anergizing stimulus proliferated similarly to wild-type T cells that received CD28 costimulation and prevented anergy induction. Our findings suggest that regulation of diacylglycerol metabolism is critical in determining whether activation or anergy ensues after T cell receptor stimulation.

Defective activation of protein kinase C and Ras-ERK pathways limits IL-2 production and proliferation by CD4+CD25+ regulatory T cells

Naturally occurring CD4+CD25+ regulatory T cells (Tregs), which play an important role in the maintenance of self-tolerance, proliferate poorly and fail to produce IL-2 following stimulation in vitro with peptide-pulsed or anti-CD3-treated APCs. When TCR proximal and distal signaling events were examined in Tregs, we observed impairments in the amplitude and duration of tyrosine phosphorylation when compared with the response of CD4+CD25- T cells. Defects were also seen in the activity of phospholipase C-gamma and in signals downstream of this enzyme including calcium mobilization, NFAT, NF-kappaB, and Ras-ERK-AP-1 activation. Enhanced stimulation of diacylglycerol-dependent pathways by inhibition of diacylglycerol metabolism could overcome the "anergic state" and support the ability of Tregs to up-regulate CD69, produce IL-2, and proliferate. Our results demonstrate that Tregs maintain their hyporesponsive state by suppressing the induction and propagation of TCR-initiated signals to control the accumulation of second messengers necessary for IL-2 production and proliferation.

Sustained B7/CD28 interactions and resultant phosphatidylinositol 3-kinase activity maintain G1-->S phase transitions at an optimal rate

Twenty-four hours of TCR engagement and CD28 costimulation was found sufficient to elicit an optimal rate of cell division over a 72-h period only when a high concentration of IL-2 was produced in the culture and remained readily available to the CD4(+) T cells. The cell division response could be aborted following 24 h of stimulation by the simultaneous abrogation of IL-2R signaling and the blockade of CD28 or TCR ligands. Biochemical and pharmacologic studies indicated that a phosphatidylinositol 3-kinase-Akt signaling cascade costimulated by the TCR and CD28 maintained the blasting cell division rate at a maximal level beyond 24 h even when IL-2 was withdrawn, neutralized, or exhausted. These data show that CD4(+) T cells remain sensitive to antigens (Ag) and costimulatory signals throughout the clonal expansion response. Furthermore, only those T cells that perceive the presence of a continued threat in the form of Ag/MHC complexes and B7 costimulatory ligands or a high concentration of a growth factor are directed to remain in cell cycle.

Controlling NF-κB activation in T cells by costimulatory receptors

Full and productive activation of T lymphocytes relies on the simultaneous delivery of T cell receptor (TCR)- and coreceptor-derived signals. In naïve T cells engagement of the TCR alone causes anergy, while TCR triggering of preactivated T cells results in activation-induced cell death. Costimulatory signals are prominently mirrored by the activation of NF-kappaB, which needs input from the TCR as well as from coreceptors in order to be fully activated and to fulfil its crucial function in the immune response. Coreceptor-generated signals tightly control the duration and amplitude of the NF-kappaB response. The activation of IkappaB kinase (IKK) complex at the contact zone between a T cell and an antigen-presenting cell offers the unique opportunity to study the spatial organization of IKK activation. Recent studies indicate that coreceptor pathways influence the threshold activities of many signalling mediators and thus act on multiple layers of the NF-kappaB pathway.

CD28 regulates the translation of Bcl-xL via the phosphatidylinositol 3-kinase/mammalian target of rapamycin pathway

In concert with the TCR, CD28 promotes T cell survival by regulating the expression of the antiapoptotic protein Bcl-x(L). The mechanism by which CD28 mediates the induction of Bcl-x(L) remains unknown. We show that although signaling through the TCR is sufficient to stimulate transcription of Bcl-x(L) mRNA, CD28, by activating PI3K and mammalian target of rapamycin, provides a critical signal that regulates the translation of Bcl-x(L) transcripts. We observe that CD28 induced 4E-binding protein-1 phosphorylation, an inhibitor of the translational machinery, and that CD28 costimulation directly augmented the translation of a Bcl-x(L) 5'-untranslated region reporter construct. Lastly, costimulation by CD28 shifted the distribution of Bcl-x(L) mRNA transcripts from the pretranslation complex to the translationally active polyribosomes. These results demonstrate that CD28 relieves the translational inhibition of Bcl-x(L) in a PI3K/mammalian target of rapamycin-dependent manner.

Inducible-costimulator-mediated suppression of human immunodeficiency virus type 1 replication in CD4(+) T lymphocytes

We investigated the effects of signaling through CD28 family molecules on human immunodeficiency virus type 1 (HIV-1) replication in vitro. A monoclonal antibody (mAb) specific for inducible costimulator (ICOS) suppressed both X4 and R5 HIV-1 replication in CD4(+) peripheral blood mononuclear cells (PBMC). This suppression was not attributable to reduced cell growth or viability. CD28 mAb showed variable effects and also suppressed HIV-1 replication when immobilized. Replication of pseudotype viruses with HIV-1-but not with vesicular stomatitis virus G-envelope was efficiently suppressed in CD4(+) PBMC treated with ICOS or CD28 mAbs. However, CD4, CXCR4, and CCR5 expression on the surface was not down-regulated. Moreover, HIV-1 replication in CD4(+) PBMC was suppressed by a soluble form of human B7-H2, a ligand of ICOS, but was enhanced by soluble B7-1, a ligand for CD28. These findings suggest that natural or artificial ligands for ICOS potentially suppress HIV-1 replication mainly at the entry stages.

Proteome Analysis of Lipid Rafts in Jurkat Cells Characterizes a Raft Subset That Is Involved in NF-κB Activation

Lipid rafts are detergent-insoluble membrane domains that play a key role in signal transduction by the T-cell antigen receptor. Proteome analysis revealed the presence of amidosulfobetaine-soluble signal transducing, integral membrane, cytoskeletal, heat shock, and GTP-binding proteins in rafts prepared from Jurkat cells. Several of these proteins were recruited to rafts by CD3/CD28 costimulation. Of particular interest is the inducible association of activated IkappaB kinase complexes with raft vesicles that could be captured with anti-flotillin-1 antibodies. Following amidosulfobetaine solubilization, flotillin-beta and IKKbeta underwent reciprocal co-immunoprecipitation. Treatment of Jurkat cells with methyl-beta-cyclodextrin disrupted the assembly and activation of this raft complex and also interfered in CD3/ CD28-induced activation of a NF-kappaB response element in the IL-2 promoter.

Protein kinase Calpha (PKCalpha) acts upstream of PKCtheta to activate IkappaB kinase and NF-kappaB in T lymphocytes

NF-kappaB is an ubiquitous transcription factor that is a key in the regulation of the immune response and inflammation. T-cell receptor (TCR) cross-linking leads to NF-kappaB activation, an IkappaB kinase (IKK)-dependent process. However, the upstream kinases that regulate IKK activity following TCR activation remain to be fully characterized. Herein, we demonstrate using genetic analysis, pharmacological inhibition, and RNA interference (RNAi) that the conventional protein kinase C (PKC) isoform PKCalpha, but not PKCbeta1, is required for the activation of the IKK complex following T-cell activation triggered by CD3/CD28 cross-linking. We find that in the presence of Ca(2+) influx, the catalytically active PKCalphaA25E induces IKK activity and NF-kappaB-dependent transcription; which is abrogated following the mutations of two aspartates at positions 246 and 248, which are required for Ca(2+) binding to PKCalpha and cell membrane recruitment. Kinetic studies reveal that an early phase (1 to 5 min) of IKK activation following TCR/CD28 cross-linking is PKCalpha dependent and that a later phase (5 to 25 min) of IKK activation is PKCtheta dependent. Activation of IKK- and NF-kappaB-dependent transcription by PKCalphaA25E is abrogated by the PKCtheta inhibitor rottlerin or the expression of the kinase-inactive form of PKCtheta. Taken together, our results suggest that PKCalpha acts upstream of PKCtheta to activate the IKK complex and NF-kappaB in T lymphocytes following TCR activation.

Structure/function analysis of the murine CD95L promoter reveals the identification of a novel transcriptional repressor and functional CD28 response element

CD28 costimulation, an important second signal for antigen-mediated T cell activation, is known to enhance expression of several genes important for the regulation of CD4+ T cell effector function including interleukin-2 and CD154. Previous studies demonstrate CD28-mediated enhancement of the transcription and expression of Fas ligand (CD95L) in T cell lines, suggesting a regulatory link between CD28 and CD95L expression. These results served as the basis for structure/function analysis of the CD95L promoter to elucidate the mechanism for CD28-mediated enhancement of CD95L. In this report, we describe a novel response element, located at -210 to -201 bp upstream of the transcription start site, that confers CD28 responsiveness to the CD95L gene. This response element is homologous to the CD28 response element (CD28RE) previously identified in the IL-2 promoter and bears structural similarities to a newly identified CD28RE in the CD154 promoter. We further demonstrate that CD28-mediated enhancement of promoter activity correlates with enhanced expression of CD95L mRNA, cell surface expression of CD95L protein, and increased apoptosis of CD95+ target cells. These results demonstrate a direct transcriptional regulatory role for CD28 in CD95L-mediated functional activity in CD4+ T cells. Mutational analysis of the CD95L promoter also reveals a novel transcriptional repressor element located approximately 60 bp 5' of the CD28RE. The repressor element bears sequence homology to an activator protein-1 element, constitutively binds c-Fos but not c-Jun, and is activation-independent.

Co-stimulatory members of the TNFR family: keys to effective T-cell immunity?

Interactions between co-stimulatory ligands and their receptors are crucial for the activation of T cells, the prevention of tolerance and the development of T-cell immunity. It is now evident that members of the immunoglobulin-like CD28-B7 co-stimulatory family cannot fully account for an effective long-lasting T-cell response or the generation of memory T cells. Several members of the tumour-necrosis factor receptor (TNFR) superfamily--OX40, 4-1BB, CD27, CD30 and HVEM (herpes-virus entry mediator)--are poised to deliver co-stimulatory signals both early and late after encounter with antigen. The roles of these molecules in initiating and sustaining the T-cell response and in promoting long-lived immunity are discussed.

Discovery of novel and selective IKK-beta serine-threonine protein kinase inhibitors. Part 1

IkappaB kinase beta (IKK-beta) is a serine-threonine protein kinase critically involved in the activation of the transcription factor Nuclear Factor kappa B (NF-kappaB) in response to various inflammatory stimuli. We have identified a small molecule inhibitor of IKK-beta. Optimization of the lead compound resulted in improvements in both in vitro and in vivo potency, and provided IKK-beta inhibitors exhibiting potent activity in an acute cytokine release model (LPS-induced TNFalpha).

It's all Rel-ative: NF-kappaB and CD28 costimulation of T-cell activation

Costimulatory signals complement or modify the signals provided to a lymphocyte through antigen receptors. For productive T-cell activation, the CD28 molecule is apparently the most important, although not the only, costimulatory receptor. CD28 can provide a signal that is at least partially distinct from that delivered by the T cell receptor (TCR)-CD3 complex. Several lines of evidence indicate that the nuclear factor (NF)-kappaB pathway is perhaps the most relevant biochemical or transcriptional target for the costimulatory activity of CD28. Although many questions remain, recent years have witnessed significant progress in understanding the signal transduction pathways leading from the TCR and CD28 to Rel/NF-kappaB-dependent transcription.

NF-kappaB family of transcription factors: central regulators of innate and adaptive immune functions

Transcription factors of the Rel/NF-kappaB family are activated in response to signals that lead to cell growth, differentiation, and apoptosis, and these proteins are critical elements involved in the regulation of immune responses. The conservation of this family of transcription factors in many phyla and their association with antimicrobial responses indicate their central role in the regulation of innate immunity. This is illustrated by the association of homologues of NF-kappaB, and their regulatory proteins, with resistance to infection in insects and plants (M. S. Dushay, B. Asling, and D. Hultmark, Proc. Natl. Acad. Sci. USA 93:10343-10347, 1996; D. Hultmark, Trends Genet. 9:178-183, 1993; J. Ryals et al., Plant Cell 9:425-439, 1997). The aim of this review is to provide a background on the biology of NF-kappaB and to highlight areas of the innate and adaptive immune response in which these transcription factors have a key regulatory function and to review what is currently known about their roles in resistance to infection, the host-pathogen interaction, and development of human disease.

Transcriptional mechanisms underlying lymphocyte tolerance

In lymphocytes, integration of Ca2+ and other signaling pathways results in productive activation, while unopposed Ca2+ signaling leads to tolerance or anergy. We show that the Ca2+-regulated transcription factor NFAT has an integral role in both aspects of lymphocyte function. Ca2+/calcineurin signaling induces a limited set of anergy-associated genes, distinct from genes induced in the productive immune response; these genes are upregulated in vivo in tolerant T cells and are largely NFAT dependent. T cells lacking NFAT1 are resistant to anergy induction; conversely, NFAT1 induces T cell anergy if prevented from interacting with its transcriptional partner AP-1 (Fos/Jun). Thus, in the absence of AP-1, NFAT imposes a genetic program of lymphocyte anergy that counters the program of productive activation mediated by the cooperative NFAT:AP-1 complex.

Protein kinase C(theta) in T cell activation

The novel protein kinase C (PKC) isoform, PKC theta, is selectively expressed in T lymphocytes and is a sine qua non for T cell antigen receptor (TCR)-triggered activation of mature T cells. Productive engagement of T cells by antigen-presenting cells (APCs) results in recruitment of PKC theta to the T cell-APC contact area--the immunological synapse--where it interacts with several signaling molecules to induce activation signals essential for productive T cell activation and IL-2 production. The transcription factors NF-kappa B and AP-1 are the primary physiological targets of PKC theta, and efficient activation of these transcription factors by PKC theta requires integration of TCR and CD28 costimulatory signals. PKC theta cooperates with the protein Ser/Thr phosphatase, calcineurin, in transducing signals leading to activation of JNK, NFAT, and the IL-2 gene. PKC theta also promotes T cell cycle progression and regulates programmed T cell death. The exact mode of regulation and immediate downstream substrates of PKC theta are still largely unknown. Identification of these molecules and determination of their mode of operation with respect to the function of PKC theta will provide essential information on the mechanism of T cell activation. The selective expression of PKC theta in T cells and its essential role in mature T cell activation establish it as an attractive drug target for immunosuppression in transplantation and autoimmune diseases.

T-cell activation through the antigen receptor. Part 1: signaling components, signaling pathways, and signal integration at the T-cell antigen receptor synapse

Part 1 of this review will highlight the basic components and signaling pathways by which the T-cell antigen receptor (TCR) activates mature extrathymic T cells. TCR signaling commences with an early wave of protein tyrosine kinase activation, which is mediated by the Src kinases Lck and Fyn, the 70-kd zeta-associated protein kinase, and members of the Tec kinase family. This early wave of protein tyrosine phosphorylation leads to the activation of downstream signaling pathways, including an increase in intracellular free calcium, protein kinase C, nuclear factor kappaB and Ras-mitogen-activated protein kinase activation. These pathways activate transcription factors, such as activator protein 1, nuclear factor of activated T cells, and Rel proteins, which ultimately lead to the expression of genes that control cellular proliferation, differentiation, anergy, or apoptosis. This review also describes how costimulatory receptors assist in signal transduction and assembly of macromolecular complexes at the TCR contact site with the antigen-presenting cell, also known as the immune synapse. These basic concepts of TCR signal transduction will be used in part 2 to explain how T-cell function can be altered by therapeutic targeting of TCR signaling components, as well as to explain modification of TCR signaling during T(H)1/T(H)2 differentiation, tolerance, and immune senescence.

Opposing roles of serine/threonine kinases MEKK1 and LOK in regulating the CD28 responsive element in T-cells

T-cell activation requires signals from both the T-cell receptor (TcR) and other co-stimulatory molecules such as CD28. TcR- and CD28-mediated signals are integrated during T-cell activation resulting in the expression of cytokine genes such as interleukin-2 (IL-2). An enhancer element (CD28RE) of the IL-2 gene specifically responsive to CD28 signals has been previously identified and characterized. This response element and an adjacent Activated Protein-1 (nuclear factor-interleukin-2B) site together (RE/AP1) were shown to complex with c-rel, AP-1 and other factors. However, details of the signal transduction pathways leading from CD28 to the composite response element remain poorly understood. We present data showing that overexpression of the serine threonine kinase, mitogen-activated protein kinase/extracellular-signal-regulated kinase kinase kinase-1 (MEKK1), but not nuclear factor-kappa B inducing kinase, or MAP kinase/ERK kinase-1 (MEK1), can significantly increase the level of CD28RE/AP1-driven luciferase (Luc) reporter gene expression in Jurkat E6-1 cells. A MEKK1 dominant negative mutant blocked such activation induced by stimulation with Raji B cells and the superantigen staphylococcus enterotoxin E (SEE), as well as via CD3/CD28. Mutations in either site of the RE/AP1 element abolished MEKK1-induced Luc expression. Calcineurin inhibitors, CsA and FK520, or inhibitors of p38 kinase (SB 203580), or MEK1 (PD 098059), did not affect MEKK1-induced reporter activation. These results directly implicate MEKK1 in the CD28 signalling pathway that activates the CD28 response element. Co-expression of the lymphocyte-oriented kinase (LOK) kinase attenuated Raji/SEE-induced IL-2 production in Jurkat cells, as well as MEKK1 and Raji/SEE-induced reporter gene activation. These data suggest that MEKK1 and LOK may have opposing roles in regulating the CD28RE/AP1 element.

Regulation of IL-13 synthesis in human lymphocytes: implications for asthma therapy

1. IL-13 is an important mediator in inflammatory diseases such as asthma. IL-13 is mainly produced by T cells. However, signalling pathways leading to induction of this cytokine are not well-characterized. We analysed the regulation of IL-13 in human peripheral blood mononuclear cells and CD4(+) T cells. 2. Cyclosporine (CsA) and FK-506 inhibited IL-13 synthesis, when cells were stimulated by TPA/ionomycin. However, stimulation by alpha-CD3/alpha-CD28 led to an enhanced IL-13 synthesis. 3. NF-kappa B inhibitor N-tosyl-L-lysine chloromethylketone (TLCK) inhibited IL-13 synthesis more effectively after TPA/ionomycin stimulation. After alpha-CD3/alpha-CD28 stimulation, only 300 microM TLCK inhibited IL-13 synthesis. Dexamethasone inhibited IL-13 equally effective after alpha-CD3/alpha-CD28 and TPA/ionomycin stimulation. 4. p38 MAPK inhibitor SB203580 inhibited IL-13 synthesis only partially. MEK inhibitor U0126 inhibited TPA/ionomycin induced IL-13 synthesis very effectively, whereas alpha-CD3/alpha-CD28 stimulated IL-13 induction was resistant to this drug. 5. These results were confirmed in purified CD4(+) T cells. In difference to PBMCs alpha-CD3/alpha-CD28 stimulated IL-13 synthesis was effectively inhibited by CsA, FK-506 and U0126. 6. Therefore U0126 was tested in an animal model of allergic asthma. We could demonstrate for the first time that inhibition of the MEK - ERK cascade is a therapeutic option for asthma. Intraperitoneal administration of 10 mg kg(-1) U0126 reduced lung eosinophilia in ovalbumin-challenged Brown Norway rats by 44%. 7. These results demonstrate that different signalling pathways are involved in regulating IL-13 synthesis in primary human T cells. Characterizing highly potent inhibitors of IL-13 synthesis can be exploited to identify new drugs to treat immunological diseases such as asthma.

Inhibitor of apoptosis protein from Orgyia pseudotsugata nuclear polyhedrosis virus provides a costimulatory signal required for optimal proliferation of developing thymocytes

The inhibitors of apoptosis proteins (IAPs) constitute a family of endogenous inhibitors that control apoptosis in the cell by inhibiting caspase processing and activity. IAPs are also implicated in cell division, cell cycle regulation, and cancer. To address the role of IAPs in thymus development and homeostasis, we generated transgenic mice expressing IAP generated from the baculovirus Orgyia pseudotsugata nuclear polyhedrosis virus (OpIAP). Developing thymocytes expressing OpIAP show increased nuclear levels of NF-kappaB and reduced cytoplasmic levels of its inhibitor, IkappaBalpha. In mature thymocytes, OpIAP induces optimal activation and proliferation after TCR triggering in the absence of a costimulatory signal. OpIAP expression in immature thymocytes blocks TCR-induced apoptosis. Taken together, our data illustrate the pleiotropism of OpIAP in vivo.

A metalloporphyrin-based superoxide dismutase mimic inhibits adoptive transfer of autoimmune diabetes by a diabetogenic T-cell clone

We present here the first report of a metalloporphyrin-based antioxidant that can prevent or delay the onset of autoimmune diabetes. Type 1 diabetes is an autoimmune process whereby T-cells recognize pancreatic beta-cell antigens and initiate a leukocyte infiltrate that produces proinflammatory cytokines and reactive oxygen species (ROS), ultimately leading to beta-cell destruction. Because islet beta-cells have a reduced capacity to scavenge free radicals, they are very sensitive to ROS action. Metalloporphyrin-based superoxide dismutase (SOD) mimics scavenge ROS and protect cells from oxidative stress and apoptosis. To investigate the effect of SOD mimics and the role of oxidative stress in the development of autoimmune diabetes in vivo, we used a diabetogenic T-cell clone, BDC-2.5, to induce rapid onset of diabetes in young nonobese diabetic-severe combined immunodeficient mice (NOD.scid). Disease was significantly delayed or prevented altogether by treatment of recipient mice with an SOD mimic, AEOL-10113, before transfer of the BDC-2.5 clone. To investigate the mechanisms of protection, in vitro assays for T-cell proliferation and gamma-interferon (IFN-gamma) production were carried out using the T-cell clone BDC-2.5. We found that the SOD mimic significantly inhibited antigen-presenting cell-dependent T-cell proliferation and IFN-gamma production in vitro. In addition, pretreatment of lipopolysaccharide (LPS)-stimulated peritoneal macrophages with SOD mimic inhibited the LPS-dependent increase in TNF-alpha as well as the NADPH oxidase-dependent release of superoxide. Finally, this compound protected NIT-1 insulinoma cells from interleukin-1beta and alloxan cytotoxicity in vitro.

OX40 promotes Bcl-xL and Bcl-2 expression and is essential for long-term survival of CD4 T cells

It is important to understand which molecules are essential for long-lived immunity. We show that OX40 (CD134) is required with CD28 for the survival of CD4 T cells following antigen-driven expansion. In contrast to CD28-/- T cells, which show defects early, OX40-/- T cells are relatively unimpaired in IL-2 production, cell division, and expansion. However, OX40-/- T cells fail to maintain high levels of Bcl-xL and Bcl-2 4-8 days after activation, and undergo apoptosis. Conversely, OX40 stimulation promotes Bcl-xL and Bcl-2 and suppresses apoptosis. Moreover, retroviral transduction of OX40-/- T cells with Bcl-xL or Bcl-2 reverses their survival defect. Thus, a temporal relationship exists between CD28 and OX40, with OX40 being a critical regulator of antigen-driven T cell survival.

Inhibition of the nuclear factor kappa B (NF-kappa B) pathway by tetracyclic kaurene diterpenes in macrophages. Specific effects on NF-kappa B-inducing kinase activity and on the coordinate activation of ERK and p38 MAPK

The anti-inflammatory action of most terpenes has been explained in terms of the inhibition of nuclear factor kappaB (NF-kappaB) activity. Ent-kaurene diterpenes are intermediates of the synthesis of gibberellins and inhibit the expression of NO synthase-2 and the release of tumor necrosis factor-alpha in J774 macrophages challenged with lipopolysaccharide. These diterpenes inhibit NF-kappaB and IkappaB kinase (IKK) activation in vivo but failed to affect in vitro the function of NF-kappaB, the phosphorylation and targeting of IkappaBalpha, and the activity of IKK-2. Transient expression of NF-kappaB-inducing kinase (NIK) activated the IKK complex and NF-kappaB, a process that was inhibited by kaurenes, indicating that the inhibition of NIK was one of the targets of these diterpenes. These results show that kaurenes impair the inflammatory signaling by inhibiting NIK, a member of the MAPK kinase superfamily that interacts with tumor necrosis factor receptor-associated factors, and mediate the activation of NF-kappaB by these receptors. Moreover, kaurenes delayed the phosphorylation of p38, ERK1, and ERK2 MAPKs, but not that of JNK, in response to lipopolysaccharide treatment of J774 cells. The absence of a coordinate activation of MAPK and IKK might contribute to a deficient activation of NF-kappaB that is involved in the anti-inflammatory activity of these molecules.

ZAP-70 and SLP-76 regulate protein kinase C-theta and NF-kappa B activation in response to engagement of CD3 and CD28

The transcription factor NF-kappaB is a critical regulator of T cell function that becomes strongly activated in response to coengagement of TCR and CD28. Although events immediately proximal to NF-kappaB activation are well understood, uncertainty remains over which upstream signaling pathways engaged by TCR and CD28 lead to NF-kappaB activation. By using Jurkat T cell lines that are deficient or replete for either the protein tyrosine kinase ZAP-70 or the cytosolic adapter molecule SLP-76, the role of these proteins in modulating NF-kappaB activation was examined. NF-kappaB was not activated in response to coengagement of TCR and CD28 in either the ZAP-70- or SLP-76-negative cells, whereas stimuli that bypass these receptors (PMA plus A23187, or TNF-alpha) activated NF-kappaB normally. Protein kinase C (PKC) theta activation, which is required for NF-kappaB activation, also was defective in these cells. Reexpression of ZAP-70 restored PKCtheta and NF-kappaB activation in response to TCR and CD28 coengagement. p95(vav) (Vav)-1 tyrosine phosphorylation was largely unperturbed in the ZAP-70-negative cells; however, receptor-stimulated SLP-76/Vav-1 coassociation was greatly reduced. Wild-type SLP-76 fully restored PKCtheta and NF-kappaB activation in the SLP-76-negative cells, whereas 3YF-SLP-76, which lacks the sites of tyrosine phosphorylation required for Vav-1 binding, only partially rescued signaling. These data illustrate the importance of the ZAP-70/SLP-76 signaling pathway in CD3/CD28-stimulated activation of PKC theta and NF-kappaB, and suggest that Vav-1 association with SLP-76 may be important in this pathway.

Multiple signals required for cyclic AMP-responsive element binding protein (CREB) binding protein interaction induced by CD3/CD28 costimulation

The optimal activation of cAMP-responsive element binding protein (CREB), similar to the full activation of T lymphocytes, requires the stimulation of both CD3 and CD28. Using a reporter system to detect interaction of CREB and CREB-binding protein (CBP), in this study we found that CREB binds to CBP only by engagement of both CD3 and CD28. CD3/CD28-promoted CREB-CBP interaction was dependent on p38 mitogen-activated protein kinase (MAPK) and calcium/calmodulin-dependent protein kinase (CaMK) IV in addition to the previously identified extracellular signal-regulated kinase pathway. Extracellular signal-regulated kinase, CaMKIV, and p38 MAPK were also the kinases involved in CREB Ser(133) phosphorylation induced by CD3/CD28. A reconstitution experiment illustrated that optimum CREB-CBP interaction and CREB trans-activation were attained when these three kinase pathways were simultaneously activated in T cells. Our results demonstrate that coordinated activation of different kinases leads to full activation of CREB. Notably, CD28 ligation activated p38 MAPK and CaMKIV, the kinases stimulated by CD3 engagement, suggesting that CD28 acts by increasing the activation extent of p38 MAPK and CaMKIV. These results support the model of a minimum activation threshold for CREB-CBP interaction that can be reached only when both CD3 and CD28 are stimulated.

Role of TNF receptor-associated factor 2 and p38 mitogen-activated protein kinase activation during 4-1BB-dependent immune response

4-1BB is a costimulatory member of the TNFR family, expressed on activated CD4(+) and CD8(+) T cells. Previous results showed that 4-1BB-mediated T cell costimulation is CD28-independent and involves recruitment of TNFR-associated factor 2 (TRAF2) and activation of the stress-activated protein kinase cascade. Here we describe a role for the p38 mitogen-activated protein kinase (MAPK) pathway in 4-1BB signaling. Aggregation of 4-1BB alone induces p38 activation in a T cell hybridoma, whereas, in normal T cells, p38 MAPK is activated synergistically by immobilized anti-CD3 plus immobilized 4-1BB ligand. 4-1BB-induced p38 MAPK activation is inhibited by the p38-specific inhibitor SB203580 in both a T cell hybridoma and in murine T cells. T cells from TRAF2 dominant-negative mice are impaired in 4-1BB-mediated p38 MAPK activation. A link between TRAF2 and the p38 cascade is provided by the MAPK kinase kinase, apoptosis-signal-regulating kinase 1. A T cell hybrid transfected with a kinase-dead apoptosis-signal-regulating kinase 1 fails to activate p38 MAPK in response to 4-1BB signaling. To assess the role of p38 activation in an immune response, T cells were stimulated in an MLR in the presence of SB203580. In a primary MLR, SB203580 blocked IL-2, IFN-gamma, and IL-4 secretion whether the costimulatory signal was delivered via 4-1BB or CD28. In contrast, following differentiation into Th1 or Th2 cells, p38 inhibition blocked IL-2 and IFN-gamma without affecting IL-4 secretion. Nevertheless, IL-4 secretion by Th2 cells remained costimulation-dependent. Thus, critical T cell signaling events diverge following Th1 vs Th2 differentiation.

Domain-specific interaction with the I kappa B kinase (IKK)regulatory subunit IKK gamma is an essential step in tax-mediated activation of IKK

The human T-cell leukemia virus type 1 Tax oncoprotein deregulates the NF-kappa B signaling pathway by persistently stimulating a key signal transducer, the I kappa B kinase (IKK). Tax physically associates with the IKK regulatory subunit, IKK gamma, although the underlying biochemical mechanism and functional significance remain unclear. We show that the Tax-IKK gamma interaction requires two homologous leucine zipper domains located within IKK gamma. These leucine zipper domains are unique for the presence of a conserved upstream region that is essential for Tax binding. Site-directed mutagenesis analysis revealed that a leucine-repeat region of Tax is important for IKK gamma binding. Interestingly, all the Tax mutants defective in IKK gamma binding failed to engage the IKK complex or stimulate IKK activity, and these functional defects can be rescued by fusing the Tax mutants to IKK gamma. These results provide mechanistic insights into how Tax specifically targets and functionally activates the cellular kinase IKK.

Activation of I-kappaB kinase by the HTLV type 1 Tax protein: mechanistic insights into the adaptor function of IKKgamma

The Tax protein encoded by human T cell leukemia virus type 1 (HTLV-1) induces constitutive nuclear expression of the transcription factor NF-kappaB, causing aberrant expression of a large array of cellular genes. Tax activates NF-kappaB by stimulating the activity of the I-kappaB kinase (IKK), which in turn leads to phosphorylation and degradation of the NF-kappaB inhibitor I-kappaBalpha. In normal T cells, IKK activation occurs transiently on cellular stimulation through the T cell receptor (TCR) and the CD28 costimulatory molecule. However, this inducible kinase is constitutively activated in Tax-expressing and HTLV-1-infected T cells, which contributes to the deregulated nuclear expression of NF-kappaB. As a genetic approach to dissect the pathways mediating IKK activation by Tax and T cell activation signals, somatic cell mutagenesis was performed to isolate signaling-defective mutant Jurkat T cell lines. One of the mutant cell lines was shown to have a defect in NF-kappaB activation by both T cell mitogens and Tax. Interestingly, this mutant cell line lacks expression of the IKK regulatory protein, IKKgamma. Expression of exogenous IKKgamma in the mutant cells restored NF-kappaB activation, thus confirming the essential role of this regulatory factor in IKK activation by the cellular and viral stimuli. Mechanistic studies have shown that Tax physically interacts with IKKgamma via specific domains, including two homologous leucine zipper motifs present in IKKgamma. The Tax/IKKgamma interaction serves to recruit Tax to the IKK catalytic subunits, IKKalpha and IKKbeta, and this recruitment appears to be an essential mechanism by which Tax stimulates the activity of IKK.

Abnormal immune function of hemopoietic cells from alymphoplasia (aly) mice, a natural strain with mutant NF-kappa B-inducing kinase

Alymphoplasia (aly) mice, a natural strain with a mutant NF-kappa B-inducing kinase (NIK) gene, manifest a unique phenotype; they lack lymph nodes and Peyer's patches, have a disturbed spleen architecture, and exhibit defects in both Ab and cellular immune responses. Although a stromal defect caused by impaired lymphotoxin-beta receptor signaling accounts for their abnormal lymphoid organogenesis, the exact mechanisms underlying the development of immunodeficiency in aly mice are poorly understood. We therefore investigated the contribution of hemopoietic cells with the aly NIK mutation to the development of immunodeficiency. Transfer of aly/aly bone marrow cells into aly/+ mice resulted in poorly developed B cell follicles and lack of support for the development of germinal centers and isotype switching, indicating that the hemopoietic cells of aly mice contain an autonomous defect. However, follicular dendritic cell clusters were maintained in the spleens of these bone marrow chimeras, suggesting that the lack of follicular dendritic cell clusters in aly mice is probably due to the stromal defect. The aly mice lacked marginal zone B cells in their spleens, and aly/aly B cells showed an impaired proliferative response after in vitro stimulation. IL-2 production by activated T cells was also impaired. By contrast, the dendritic cells of aly mice exhibited grossly normal development and function. Supporting the concept of an autonomous cell defect, Rel protein expression was altered in aly/aly spleens. Thus, the aly NIK mutation affects hemopoietic cell function in an intrinsic fashion and, together with the stromal defect, may contribute to the development of immunodeficiency in aly mice.

Negative regulation of the nuclear factor kappa B-inducing kinase by a cis-acting domain

Nuclear factor kappaB (NF-kappaB)-inducing kinase (NIK) participates in the activation of NF-kappaB, a family of eukaryotic transcription factors that mediate cell growth and transformation. NIK activates the IkappaB kinase both in vivo and in vitro, although how the activity of NIK is regulated has remained unclear. Here we show that the N-terminal region of NIK contains a negative-regulatory domain (NRD), which is composed of a basic motif and a proline-rich repeat motif. Deletion of these motifs leads to a marked enhancement of NIK function. We further demonstrate that the N-terminal NRD interacts with the C-terminal region of NIK, thereby inhibiting the binding of NIK to its substrate IkappaB kinase. Consistently, when expressed alone, the NRD potently inhibits NIK-mediated NF-kappaB signaling. These results provide a new insight into the mechanism of NIK regulation.

Tyrosine-phosphorylated Vav1 as a point of integration for T-cell receptor- and CD28-mediated activation of JNK, p38, and interleukin-2 transcription

In this study we identified tyrosine-phosphorylated Vav1 as an early point of integration between the signaling routes triggered by the T-cell receptor and CD28 in human T-cell leukemia cells. Costimulation resulted in a prolonged and sustained phosphorylation and membrane localization of Vav1 in comparison to T-cell receptor activation alone. T-cell stimulation induced the recruitment of Vav1 to an inducible multiprotein T-cell activation signaling complex at the plasma membrane. Vav1 activated the mitogen-activated protein kinases JNK and p38. The Vav1-mediated activation of JNK employed a pathway involving Rac, HPK1, MLK3, and MKK7. The costimulation-induced activation of p38 was inhibited by dominant negative forms of Vav1, Rac, and MKK6. Here we show that Vav1 also induces transcription factors that bind to the CD28RE/AP element contained in the interleukin-2 promoter. A detailed mutational analysis of Vav1 revealed a series of constitutively active and nonfunctional forms of Vav1. Almost all inactive versions were mutated in their Dbl homology domain and behaved as dominant negative mutants that impaired costimulation-induced activation of JNK, p38, and CD28RE/AP-dependent transcription. In contrast to NF-AT-dependent transcription, Vav1-mediated transcriptional induction of the CD28RE/AP element in the interleukin-2 promoter could only partially be inhibited by cyclosporin A, suggesting a dual role of Vav1 for controlling Ca(2+)-dependent and -independent events.

Mixed-lineage kinase 3 delivers CD3/CD28-derived signals into the IkappaB kinase complex

The phosphorylation of IkappaB by the multiprotein IkappaB kinase complex (IKC) precedes the activation of transcription factor NF-kappaB, a key regulator of the inflammatory response. Here we identified the mixed-lineage group kinase 3 (MLK3) as an activator of NF-kappaB. Expression of the wild-type form of this mitogen-activated protein kinase kinase kinase (MAPKKK) induced nuclear immigration, DNA binding, and transcriptional activity of NF-kappaB. MLK3 directly phosphorylated and thus activated IkappaB kinase alpha (IKKalpha) and IKKbeta, revealing its function as an IkappaB kinase kinase (IKKK). MLK3 cooperated with the other two IKKKs, MEKK1 and NF-kappaB-inducing kinase, in the induction of IKK activity. MLK3 bound to components of the IKC in vivo. This protein-protein interaction was dependent on the central leucine zipper region of MLK3. A kinase-deficient version of MLK3 strongly impaired NF-kappaB-dependent transcription induced by T-cell costimulation but not in response to tumor necrosis factor alpha or interleukin-1. Accordingly, endogenous MLK3 was phosphorylated and activated by T-cell costimulation but not by treatment of cells with tumor necrosis factor alpha or interleukin-1. A dominant negative version of MLK3 inhibited NF-kappaB- and CD28RE/AP-dependent transcription elicited by the Rho family GTPases Rac and Cdc42, thereby providing a novel link between these GTPases and the IKC.

Protein kinase C-theta participates in NF-kappaB activation induced by CD3-CD28 costimulation through selective activation of IkappaB kinase beta

The NF-kappaB/Rel family of eukaryotic transcription factors plays an essential role in the regulation of inflammatory, antiapoptotic, and immune responses. NF-kappaB is activated by many stimuli including costimulation of T cells with ligands specific for the T-cell receptor (TCR)-CD3 complex and CD28 receptors. However, the signaling intermediates that transduce these costimulatory signals from the TCR-CD3 and CD28 surface receptors leading to nuclear NF-kappaB expression are not well defined. We now show that protein kinase C-theta (PKC-theta), a novel PKC isoform, plays a central role in a signaling pathway induced by CD3-CD28 costimulation leading to activation of NF-kappaB in Jurkat T cells. We find that expression of a constitutively active mutant of PKC-theta potently induces NF-kappaB activation and stimulates the RE/AP composite enhancer from the interleukin-2 gene. Conversely, expression of a kinase-deficient mutant or antisense PKC-theta selectively inhibits CD3-CD28 costimulation, but not tumor necrosis factor alpha-induced activation of NF-kappaB in Jurkat T cells. The induction of NF-kappaB by PKC-theta is mediated through the activation of IkappaB kinase beta (IKKbeta) in the absence of detectable IKKalpha stimulation. PKC-theta acts directly or indirectly to stimulate phosphorylation of IKKbeta, leading to activation of this enzyme. Together, these results implicate PKC-theta in one pathway of CD3-CD28 costimulation leading to NF-kappaB activation that is apparently distinct from that involving Cot and NF-kappaB-inducing kinase (NIK). PKC-theta activation of NF-kappaB is mediated through the selective induction of IKKbeta, while the Cot- and NIK-dependent pathway involves induction of both IKKalpha and IKKbeta.

NF-kappa B activation induced by T cell receptor/CD28 costimulation is mediated by protein kinase C-theta

Protein kinase C-theta (PKCtheta) is a Ca(2+)-independent member of the PKC family that is selectively expressed in skeletal muscle and T lymphocytes and plays an important role in T cell activation. However, the molecular basis for the important functions of PKCtheta in T cells and the manner in which it becomes coupled to the T cell receptor-signaling machinery are unknown. We addressed the functional relationship between PKCtheta and CD28 costimulation, which plays an essential role in T cell receptor-mediated IL-2 production. Here, we provide evidence that PKCtheta is functionally coupled to CD28 costimulation by virtue of its selective ability to activate the CD28RE/activator protein-1 (AP-1) element in the IL-2 gene promoter. First, CD28 costimulation enhanced the membrane translocation and catalytic activation of PKCtheta. Second, among several PKC isoforms, PKCtheta was the only one capable of activating NF-kappaB or CD28RE/AP-1 reporters in T cells (but not in 293T cells). Third, wild-type PKCtheta synergized with CD28/CD3 signals to activate CD28RE/AP-1. In addition, PKCtheta selectively synergized with Tat to activate a CD28RE/AP-1 reporter. Fourth, CD3/CD28-induced CD28RE/AP-1 activation and NF-kappaB nuclear translocation were blocked by a selective PKCtheta inhibitor. Last, PKCtheta-mediated activation of the same reporter was inhibited by the proteasome inhibitor MG132 (which blocks IkappaB degradation) and was found to involve IkappaB-kinase beta. These findings identify a unique PKCtheta-mediated pathway for the costimulatory action of CD28, which involves activation of the IkappaB-kinase beta/IkappaB/NF-kappaB-signaling cascade.

NF- B activation induced by T cell receptor/CD28 costimulation is mediated by protein kinase C-

Protein kinase C-theta (PKCtheta) is a Ca(2+)-independent member of the PKC family that is selectively expressed in skeletal muscle and T lymphocytes and plays an important role in T cell activation. However, the molecular basis for the important functions of PKCtheta in T cells and the manner in which it becomes coupled to the T cell receptor-signaling machinery are unknown. We addressed the functional relationship between PKCtheta and CD28 costimulation, which plays an essential role in T cell receptor-mediated IL-2 production. Here, we provide evidence that PKCtheta is functionally coupled to CD28 costimulation by virtue of its selective ability to activate the CD28RE/activator protein-1 (AP-1) element in the IL-2 gene promoter. First, CD28 costimulation enhanced the membrane translocation and catalytic activation of PKCtheta. Second, among several PKC isoforms, PKCtheta was the only one capable of activating NF-kappaB or CD28RE/AP-1 reporters in T cells (but not in 293T cells). Third, wild-type PKCtheta synergized with CD28/CD3 signals to activate CD28RE/AP-1. In addition, PKCtheta selectively synergized with Tat to activate a CD28RE/AP-1 reporter. Fourth, CD3/CD28-induced CD28RE/AP-1 activation and NF-kappaB nuclear translocation were blocked by a selective PKCtheta inhibitor. Last, PKCtheta-mediated activation of the same reporter was inhibited by the proteasome inhibitor MG132 (which blocks IkappaB degradation) and was found to involve IkappaB-kinase beta. These findings identify a unique PKCtheta-mediated pathway for the costimulatory action of CD28, which involves activation of the IkappaB-kinase beta/IkappaB/NF-kappaB-signaling cascade.

Somatic mutagenesis studies of NF-kappa B signaling in human T cells: evidence for an essential role of IKK gamma in NF-kappa B activation by T-cell costimulatory signals and HTLV-I Tax protein

NF-kappa B plays a pivotal role in normal T-cell activation and may also mediate human T-cell leukemia virus (HTLV)-induced T-cell transformation. Activation of NF-kappa B by both T-cell costimulatory signals and the HTLV Tax protein involves stimulation of I kappa B kinase (IKK). As a genetic approach to dissect the intermediate steps involved in NF-kappa B activation in human T cells, we performed somatic cell mutagenesis to isolate signaling-defective mutant Jurkat T-cell lines. One of the mutant cell lines was shown to have a specific blockade in the IKK signaling pathway but remained competent in the c-Jun N-terminal kinase and MAP kinase pathways. Interestingly, this mutant cell line lacks expression of IKK gamma, a non-catalytic component of the IKK complex. Expression of exogenous IKK gamma in the mutant cells restored NF-kappa B activation by both the T-cell costimulation agents and Tax. These findings provide genetic evidence for the requirement of IKK gamma in NF-kappa B signaling triggered by both T-cell costimulatory signals and HTLV-I Tax protein.

IKKepsilon is part of a novel PMA-inducible IkappaB kinase complex

Here we report the identification of a novel PMA-inducible IkappaB kinase complex, distinct from the well-characterized high-molecular weight IkappaB kinase complex containing IKKalpha, IKKbeta, and IKKgamma. We have characterized one kinase from this complex, which we designate IKKepsilon. Although recombinant IKKepsilon directly phosphorylates only serine 36 of IKBalpha, the PMA-activated endogenous IKKepsilon complex phosphorylates both critical serine residues. Remarkably, this activity is due to the presence of a distinct kinase in this complex. A dominant-negative mutant of IKKepsilon blocks induction of NF-kappaB by both PMA and activation of the T cell receptor but has no effect on the activation of NF-KB by TNFalpha or IL-1. These observations indicate that the activation of NF-kappaB requires multiple distinct IkappaB kinase complexes, which respond to both overlapping and discrete signaling pathways.