The physical association of protein kinase C theta with a lipid raft-associated inhibitor of kappa B factor kinase (IKK) complex plays a role in the activation of the NF-kappa B cascade by TCR and CD28

PDK1 nucleates T cell receptor-induced signaling complex for NF-kappaB activation

Activation of the transcription factor NF-kappaB after engagement of the T cell receptor (TCR) is important for T cell proliferation and activation during the adaptive immune response. Recent reports have elucidated a signaling pathway that involves the protein kinase C (PKC), the scaffold protein CARD11 (also called CARMA-1), the caspase recruitment domain (CARD)-containing protein Bcl10, and the paracaspase (protease related to caspases) MALT1 as critical intermediates linking the TCR to the IkappaB kinase (IKK) complex. However, the events proximal to the TCR that initiate the activation of this signaling pathway remain poorly defined. We demonstrate that 3-phosphoinositide-dependent kinase 1 (PDK1) has an essential role in this pathway by regulating the activation of PKC and through signal-dependent recruiting of both PKC and CARD11 to lipid rafts. PDK1-associated PKC recruits the IKK complex, whereas PDK1-associated CARD11 recruits the Bcl10-MALT1 complex, thereby allowing activation of the IKK complex through Bcl10-MALT1-dependent ubiquitination of the IKK complex subunit known as NEMO (NF-kappaB essential modifier). Hence, PDK1 plays a critical role by nucleating the TCR-induced NF-kappaB activation pathway in T cells.

Inhibition of RICK/Nuclear Factor-κB and p38 Signaling Attenuates the Inflammatory Response in a Murine Model of Crohn Disease

Nuclear factor-kappaB (NF-kappaB) is the main target of anti-inflammatory therapies in human chronic inflammatory bowel diseases (IBD), Crohn disease, and ulcerative colitis. This study investigates the molecular anti-inflammatory mechanisms of SB203580, an inhibitor of the mitogen-activated protein kinase p38. The murine trinitrobenzene sulfonic acid (TNBS)-induced colitis was used as an established model of human Crohn disease. Here we show that SB203580 improved the clinical condition, reduced intestinal inflammation, and suppressed mRNA levels of pro-inflammatory cytokines elevated upon induction of colitis. Besides p38 kinase activity, the "classical" IkappaB-dependent NF-kappaB pathway was strongly up-regulated during colitis induction, whereas the "alternative" was not. SB203580 treatment resulted in a drastic down-regulation of p38 and NF-kappaB activity. The molecular analysis of NF-kappaB activation revealed that Rip-like interacting caspase-like apoptosis-regulatory protein kinase (RICK), a key component of a pathway leading to NF-kappaB induction, is also strongly inhibited by SB203580. In contrast, SB203580 had no effect on the colitis-induced activation of other potential NF-kappaB-activating kinases such as protein kinase C (PKC), mixed lineage kinase 3, and the oncogene product Cot/TPL2. Thus, the inhibitory effect of SB203580 on NF-kappaB activation is to a large extent mediated by RICK inhibition. RICK is the effector kinase of the intracellular receptor of bacterial peptidoglycan NOD. Because bacterial products are suggested to be the key pathogenic agents triggering IBD, inhibition of the NOD/RICK pathway may serve as a novel target of future therapies in human IBD.

Inhibition of IKK down-regulates antigen + IgE-induced TNF production by mast cells: a role for the IKK-IκB-NF-κB pathway in IgE-dependent mast cell activation

Mast cells (MC) are major effector cells for allergic diseases. Cross-linking of immunoglobulin E (IgE) and its high-affinity receptor, FcepsilonRI, by antigen initiates a cascade of signaling events leading to nuclear factor (NF)-kappaB activation and tumor necrosis factor (TNF) production. Here, we demonstrated that inhibition of inhibitor of kappaB (IkappaB) kinase (IKK) by a peptide IKK inhibitor or by four individual chemical IKK inhibitors including 15-deoxy-prostaglandin J(2), BMS-345541, SC-514, or sulindac significantly blocked IgE + trinitrophenyl (TNP)-induced TNF production by mouse bone marrow-derived MC (BMMC). Moreover, IgE + TNP induced a rapid phosphorylation of IKKalpha but not IKKbeta in BMMC. IgE + TNP-induced phosphorylation of IKKalpha was accompanied with phosphorylation and degradation of IkappaBalpha, subsequent NF-kappaB activation, and TNF production. Inhibition of IKK by sulindac decreased IKKalpha phosphorylation, IkappaBalpha phosphorylation and degradation, NF-kappaB activation, and TNF production by BMMC. It is interesting that IgE + TNP stimulation also induced a prominent synthesis of IKKalpha and IkappaBalpha. Inhibition of NF-kappaB activity by pyrrolidine dithiocarbomate (PDTC) blocked IgE + TNP-induced IkappaBalpha synthesis. NF-kappaB activity and TNF production were also inhibited when PDTC was used even after IgE + TNP stimulation, suggesting a potential role for the newly synthesized IkappaBalpha in MC activation. In addition, IgE + TNP-induced IKKalpha and IkappaBalpha phosphorylation was inhibited by a protein kinase C (PKC) inhibitor Ro 31-8220. Taken together, our results support a role for the IKK-IkappaB-NF-kappaB pathway, which likely involves PKC in IgE-dependent TNF production by MC. Thus, IKK may serve as a new target for the regulation of MC function in allergy.

Differentiation and expansion of T cells with regulatory function from human peripheral lymphocytes by stimulation in the presence of TGF-{beta}

T cells with immunoregulatory function have been described in human and mouse systems. In both systems these cells can be differentiated either in the thymus or from peripheral T cells. To date, more progress has been made in the study of murine regulatory T cells, because it has been very difficult to isolate human regulatory T cells of sufficient purity and in sufficient numbers to permit detailed examinations of their biochemistry. We report in this study that human T cells with regulatory function can be differentiated in vitro from naive (CD4(+)CD45RA(+)) cord blood or peripheral T cells by stimulation with anti-CD3 and anti-CD28 in the presence of TGF-beta. Cells derived in this manner express a surface phenotype (CD25(+), CD122(+), HLA-DR(+), glucocorticoid-induced TNF receptor-related gene(+), CD103(+), CTLA-4(+)) described for human and mouse regulatory T cells and express protein and message for the transcription factor forkhead/winged helix transcription factor (FOXP3). They produce primarily TGF-beta and IL-10, with lesser amounts of IFN-gamma and IL-13, when stimulated through their TCRs and are capable of inhibiting cytokine production and proliferation by stimulated naive T cells. Unlike Th1 and Th2 cells, these TGF-beta-derived regulatory T cells do not appear to be dependent on the protein kinase Ctheta; pathway of NF-kappaB activation for Ag-induced responses.

The molecular adapter Carma1 controls entry of IkappaB kinase into the central immune synapse

Carma1 (also known as caspase recruitment domain [CARD]11, Bimp3) is a CARD-containing membrane-associated guanylate kinase family protein that plays an essential role in antigen receptor-induced nuclear factor kappaB activation. We investigated the role of Carma1 in the assembly of signaling molecules at the immune synapse using a peptide-specific system. We report that Carma1 is essential for peptide-induced interleukin 2 and interferon gamma production, but dispensable for proliferation in T cells. Recruitment and distribution of T cell receptor, lymphocyte function associated 1, lipid rafts, and protein kinase C (PKC)theta; to central and peripheral immune synapse regions occur normally in Carma1-/- T cells. Carma1 controls entry of IkappaB kinase (IKK) into lipid raft aggregates and the central region of the immune synapse, as well as activation of IKK downstream of PKC. Our data provide the first genetic evidence on a new class of molecular scaffold that controls entry of defined signaling components, IKK, into the central supramolecular activation cluster at T cell-antigen-presenting cell interfaces without having any apparent effect on the overall organization and formation of immune synapses.

T cell receptor-induced lipid raft recruitment of the I kappa B kinase complex is necessary and sufficient for NF-kappa B activation occurring in the cytosol

TCR-induced NF-kappa B activation is necessary for the innate immune response and involves induced lipid raft recruitment of the I kappa B kinase (IKK) complex. In this study, we systematically investigated lipid raft recruitment of members of the NF-kappa B activation pathway in human T cells. All upstream components leading to IKK activation were found constitutively or inducibly in lipid rafts, while the NF-kappa B/I kappa B complex and phosphorylated forms of IKK alpha/beta, I kappa B alpha and p65 are exclusively found in the cytosolic fraction. Disruption of raft organization precluded NF-kappaB activation induced by T cell costimulation, but IL-1-triggered NF-kappa B activation remained unaffected. Targeting of the IKK complex to lipid rafts caused constitutive IKK activation and NF-kappa B DNA binding, which was further triggered upon T cell costimulation. Various experimental approaches revealed that costimulation-induced IKK alpha/beta activation loop phosphorylation is independent from IKK beta-mediated transautophosphorylation, but rather involves phosphorylation by the IKK-interacting protein NIK and its upstream activator COT.

Regulation of T-cell receptor signalling by membrane microdomains

There is now considerable evidence suggesting that the plasma membrane of mammalian cells is compartmentalized by functional lipid raft microdomains. These structures are assemblies of specialized lipids and proteins and have been implicated in diverse biological functions. Analysis of their protein content using proteomics and other methods revealed enrichment of signalling proteins, suggesting a role for these domains in intracellular signalling. In T lymphocytes, structure/function experiments and complementary pharmacological studies have shown that raft microdomains control the localization and function of proteins which are components of signalling pathways regulated by the T-cell antigen receptor (TCR). Based on these studies, a model for TCR phosphorylation in lipid rafts is presented. However, despite substantial progress in the field, critical questions remain. For example, it is unclear if membrane rafts represent a homogeneous population and if their structure is modified upon TCR stimulation. In the future, proteomics and the parallel development of complementary analytical methods will undoubtedly contribute in further delineating the role of lipid rafts in signal transduction mechanisms.

Signaling to NF-kappaB

The transcription factor NF-kappaB has been the focus of intense investigation for nearly two decades. Over this period, considerable progress has been made in determining the function and regulation of NF-kappaB, although there are nuances in this important signaling pathway that still remain to be understood. The challenge now is to reconcile the regulatory complexity in this pathway with the complexity of responses in which NF-kappaB family members play important roles. In this review, we provide an overview of established NF-kappaB signaling pathways with focus on the current state of research into the mechanisms that regulate IKK activation and NF-kappaB transcriptional activity.

Nuclear Factor 3B Activation and Antiapoptosis in Mucosa-Associated Lymphoid Tissue Lymphoma

Three distinct chromosomal translocations, t(11;18), t(1;14), and t(14;18), involving the API2-MALT1 fusion protein, BCL10, and MALT1 have been convincingly implicated in the pathogenesis of mucosa-associated lymphoid tissue (MALT) lymphomas. Recent genetic and biochemical studies have indicated that BCL10 and MALT1 form a physical and functional complex and are both essential for nuclear factor kappaB (NF-kappaB) activation by antigen receptor stimulation in lymphocytes. API2-MALT1 can bypass the BCL10/MALT1 pathway linking to NF-kappaB activation, thereby inducing antigen receptor-independent events of lymphocytes. BCL10/MALT1- and API2-MALT1-induced NF-kappaB activation can be assumed to be able to contribute to antiapoptosis, probably through NF-kappaB-mediated up-regulation of several apoptotic inhibitor genes. We also have provided direct evidence that API2-MALT1 can exert an antiapoptotic effect, in part through its direct interaction with apoptotic regulators. We therefore hypothesize that the antiapoptotic effect of API2-MALT1 may be mediated by the interaction with apoptotic regulators as well as by the up-regulation of apoptotic inhibitor genes. Finally, we hope that further studies will stimulate research leading to the development of therapeutic drugs that specifically inhibit the antigen receptor-stimulated NF-kappaB activation pathway. Such drugs should be useful for interfering with inappropriate proliferation of lymphocytes associated with inflammatory and neoplastic disorders, including MALT lymphomas.

PKCtheta signals activation versus tolerance in vivo

Understanding the pathways that signal T cell tolerance versus activation is key to regulating immunity. Previous studies have linked CD28 and protein kinase C-θ (PKCθ) as a potential signaling pathway that influences T cell activation. Therefore, we have compared the responses of T cells deficient for CD28 and PKCθ in vivo and in vitro. Here, we demonstrate that the absence of PKCθ leads to the induction of T cell anergy, with a phenotype that is comparable to the absence of CD28. Further experiments examined whether PKCθ triggered other CD28-dependent responses. Our data show that CD4 T cell–B cell cooperation is dependent on CD28 but not PKCθ, whereas CD28 costimulatory signals that augment proliferation can be uncoupled from signals that regulate anergy. Therefore, PKCθ relays a defined subset of CD28 signals during T cell activation and is critical for the induction of activation versus tolerance in vivo.

NF-kappaB signaling in lymphocytes: a new cast of characters

Cell-surface antigen receptors on B and T lymphocytes are complex, multisubunit assemblies that must recruit several accessory proteins and activate multiple signaling pathways in order to illicit a proper immune response. One pathway culminates in the activation of specific protein kinase C (PKC) isoforms, which is necessary for the ultimate activation of the NF-kappaB transcription factor. Since NF-kappaB plays a crucial role in the adaptive immune response (e.g. in lymphocyte proliferation and cytokine production), it is important to understand the molecular mechanisms by which NF-kappaB is regulated. Nevertheless, the connection between PKC activation and NF-kappaB has remained a mystery that has now been at least partly solved. Recent findings implicate a new scaffolding protein, Bimp3/CARMA1/CARD11, as a key factor in bridging PKC activation with the downstream activation of Bcl10 and MALT1, which ultimately stimulates NF-kappaB. Since some of these signaling components are lymphocyte specific, therapeutic agents that block this pathway could blunt the inappropriate proliferation of lymphocytes associated with certain inflammatory and neoplastic disorders. Alternatively, agents that specifically augment this pathway, thereby enhancing immune function in immunodeficiency, may be developed.

The golli-myelin basic protein negatively regulates signal transduction in T lymphocytes

Protein kinase C (PKC) plays a critical role in signal transduction controlling T lymphocyte activation. Both positive and negative regulation of signal transduction is needed for proper control of T lymphocyte activation. We have found that a golli product of the myelin basic protein (MBP) gene can serve as a negative regulator of signaling pathways in the T lymphocyte, particularly the PKC pathway. Increased expression of golli BG21 in Jurkat T cells strongly inhibits anti-CD3-induced IL-2-luciferase activity, an indicator of T lymphocyte activation. Golli BG21 can be phosphorylated by PKC in vitro and its phosphorylation increases in PMA-activated Jurkat cells. BG21 inhibits the PMA-induced increase in AP-1 or NF-kappaB activation, consistent with golli acting in a PKC-mediated cellular event. Golli BG21 inhibition of the PKC pathway is not due to a direct action on PKC activation but in the cascade following PKC activation, since BG21 neither reduces PKC enzyme activity nor blocks the membrane association of PKCtheta brought on by T lymphocyte activation. The inhibitory function of BG21 is independent of its phosphorylation by PKC because a mutant BG21, in which the PKC sites have been mutated, is as effective as the wild type BG21 in inhibiting the PMA-induced AP-1 activation. Structure-function assays indicate that BG21 inhibitory activity resides in the golli domain rather than in MBP domain of the molecule. These results reveal a novel role for MBP gene products in T lymphocytes within the immune system.

SPAK kinase is a substrate and target of PKCtheta in T-cell receptor-induced AP-1 activation pathway

Protein kinase C-theta (PKCtheta) plays an important role in T-cell activation via stimulation of AP-1 and NF-kappaB. Here we report the isolation of SPAK, a Ste20-related upstream mitogen-activated protein kinase (MAPK), as a PKCtheta-interacting kinase. SPAK interacted with PKCtheta (but not with PKCalpha) via its 99 COOH-terminal residues. TCR/CD28 costimulation enhanced this association and stimulated the catalytic activity of SPAK. Recombinant SPAK was phosphorylated on Ser-311 in its kinase domain by PKCtheta, but not by PKCalpha. The magnitude and duration of TCR/CD28-induced endogenous SPAK activation were markedly impaired in PKCtheta-deficient T cells. Transfected SPAK synergized with constitutively active PKCtheta to activate AP-1, but not NF-kappaB. This synergistic activity, as well as the receptor-induced SPAK activation, required the PKCtheta-interacting region of SPAK, and Ser-311 mutation greatly reduced these activities of SPAK. Conversely, a SPAK-specific RNAi or a dominant-negative SPAK mutant inhibited PKCtheta- and TCR/CD28-induced AP-1, but not NF-kappaB, activation. These results define SPAK as a substrate and target of PKCtheta in a TCR/CD28-induced signaling pathway leading selectively to AP-1 (but not NF-kappaB) activation.

Calcineurin imposes T cell unresponsiveness through targeted proteolysis of signaling proteins

Sustained calcium signaling induces a state of anergy or antigen unresponsiveness in T cells, mediated through calcineurin and the transcription factor NFAT. We show here that Ca(2+)-induced anergy is a multistep program that is implemented at least partly through proteolytic degradation of specific signaling proteins. Calcineurin increased mRNA and protein of the E3 ubiquitin ligases Itch, Cbl-b and GRAIL and induced expression of Tsg101, the ubiquitin-binding component of the ESCRT-1 endosomal sorting complex. Subsequent stimulation or homotypic cell adhesion promoted membrane translocation of Itch and the related protein Nedd4, resulting in degradation of two key signaling proteins, PKC-theta and PLC-gamma1. T cells from Itch- and Cbl-b-deficient mice were resistant to anergy induction. Anergic T cells showed impaired calcium mobilization after TCR triggering and were unable to maintain a mature immunological synapse, instead showing late disorganization of the outer ring containing lymphocyte function-associated antigen 1. Our results define a complex molecular program that links gene transcription induced by calcium and calcineurin to a paradoxical impairment of signal transduction in anergic T cells.

Antigen dose, type of antigen-presenting cell and time of differentiation contribute to the T helper 1/T helper 2 polarization of naive T cells

Antigenic encounter by T cells induces immunological synapse formation and T-cell activation. Using different concentrations of toxic shock syndrome toxin-1 (TSST-1) as stimulus, we examined the capacities of dendritic cells (DC) and macrophages (Mphi) to prime syngeneic naive T cells. DCs were, under all experimental settings, more efficient than Mphi at clustering T cells. Translocation of the T-cell receptor (TCR) to the contact area was found to be induced by DCs, as well as by Mphi, in an antigen-dependent manner, although Mphi were less efficient at inducing TCR translocation. Capping of protein kinase C theta (PKCtheta) was also antigen dependent but induced exclusively by DCs. Likewise, DCs were found to be more potent inducers of interleukin-2 (IL-2) production and proliferation of naive T cells than Mphi. After 3 days of culture, DCs presenting 100 ng/ml TSST-1 induced interferon-gamma (IFN-gamma)-secreting cells, whereas Mphi did not. After 7 days of culture, DCs presenting 0.1 ng/ml TSST-1, and Mphi presenting high (as well as low) doses of TSST-1, induced IL-4-producing cells. We therefore provide evidence to show that antigen dose, type of antigen-presenting cell and time of differentiation can contribute to T-cell differentiation.

CD3/CD28 costimulation-induced NF-kappaB activation is mediated by recruitment of protein kinase C-theta, Bcl10, and IkappaB kinase beta to the immunological synapse through CARMA1

CARMA1 (also known as CARD11) is a scaffold molecule and contains a caspase-recruitment domain (CARD) and a membrane-associated guanylate kinase-like (MAGUK) domain. It plays an essential role in mediating CD3/CD28 costimulation-induced NF-kappaB activation. However, the molecular mechanism by which CARMA1 mediates costimulatory signals remains to be determined. Here, we show that CARMA1 is constitutively associated with the cytoplasmic membrane. This membrane association is essential for the function of CARMA1, since a mutant of CARMA1, CARMA1(L808P), that is defective in the membrane association cannot rescue CD3/CD28 costimulation-induced NF-kappaB activation in JPM50.6 CARMA1-deficient T cells. Although CD3/CD28 costimulation effectively induces the formation of the immunological synapse in CARMA1-deficient T cells, the recruitment of protein kinase C-theta (PKC-theta), Bcl10, and IkappaB kinase beta (IKKbeta) into lipid rafts of the immunological synapse is defective. Moreover, expression of wild-type CARMA1, but not CARMA1(L808P), restores the recruitment of PKC-theta, Bcl10, and IKKbeta into lipid rafts in CARMA1-deficient T cells. Consistently, expression of a mutant CARMA1, CARMA1(DeltaCD), that cannot associate with Bcl10 failed to restore CD3/CD28 costimulation-induced NF-kappaB activation in JPM50.6 cells, whereas expression of Bcl10-CARMA(DeltaCD) fusion protein effectively restored this NF-kappaB activation. Together, these results indicate that CARMA1 mediates CD3/CD28 costimulation-induced NF-kappaB activation by recruiting downstream signaling components into the immunological synapse.

Emerging and diverse roles of protein kinase C in immune cell signalling

Members of the protein kinase C (PKC) family are expressed in many different cell types, where they are known to regulate a wide variety of cellular processes that impact on cell growth and differentiation, cytoskeletal remodelling and gene expression in the response to diverse stimuli. The broad tissue distribution and redundancy of in vitro function have often hampered the identification of definitive roles for each PKC family member. However, recent in vivo studies of PKC isoenzyme-selective knockout and transgenic mice have highlighted distinct functions of individual PKCs in the immune system. These genetic analyses, along with biochemical studies utilizing PKC isoenzyme-specific cDNA (wild-type, constitutively active and dominant-negative), antisense oligonucleotides (ASO), RNA interference (RNAi), and pharmacological inhibitors, indicate that PKC-regulated signalling pathways play a significant role in many aspects of immune responses, from development, differentiation, activation and survival of lymphocytes to macrophage activation. The importance of PKCs in cellular immune responses suggests that improved understanding of the molecular events that govern their actions could point to new avenues for development of treatments for immune disorders.

Role of the kinase activation loop on protein kinase C theta activity and intracellular localisation

Multiple protein kinase C (PKC) theta species, identified in an erythroleukaemia cell line, have been characterised in terms of their molecular properties and intracellular distribution. PKCthetas localised in the detergent-soluble cell fraction have an Mr of 76 kDa (theta-76) and contain Thr538 or pThr538 in the kinase activation loop. In contrast, PKCthetas localised in the Golgi complex have an Mr of 85 kDa (theta-85) and, although unphosphorylated at Thr538, are catalytically active. Strikingly, only theta-76 species which are unphosphorylated at Thr538 can undergo autocatalytic conversion to theta-85. Moreover, a Thr538-->Ala PKCtheta mutant is constitutively localised in the Golgi complex, confirming that changes in the phosphorylation state of this residue play a pivotal role in the overall control of catalytic properties and localisation of this kinase.

Protein kinase C theta modulates nuclear receptor-corepressor interaction during T cell activation

Transcriptional repression by nuclear receptor corepressors plays a critical role in T cell development. However, the role of these corepressors in T cell activation is poorly understood. We report that T cell activation silenced transcription driven by nuclear receptors retinoic acid receptor, retinoid X receptor, and thyroid hormone receptor and induced silencing mediator of retinoic acid and thyroid hormone receptors (SMRT)-receptor interaction. Whereas the expression of a dominant active mutant of protein kinase C theta(PKC theta) induced strong SMRT-receptor interaction in the absence of T cell activation, a dominant negative mutant of PKC theta decreased the interaction. Loss of PKC theta expression by induction of "RNA interference" resulted in the attenuation of basal and activation-induced SMRT-receptor interaction. We suggest that T cell activation silences nuclear receptor-dependent transactivation in part through PKC theta-dependent enhancement of SMRT-receptor interaction.

Cardioprotection involves activation of NF-kappa B via PKC-dependent tyrosine and serine phosphorylation of I kappa B-alpha

Previous studies indicated that activation of PKC and Src tyrosine kinases by ischemic preconditioning (PC) may participate in the activation of NF-kappa B. However, the molecular mechanisms underlying activation of NF-kappa B during ischemic PC remain unknown. In the hearts of conscious rabbits, it was found that ischemic PC (6 cycles of 4-min coronary occlusion and 4-min reperfusion) significantly induced both tyrosine (+226.9 +/- 42%) and serine (+137.0 +/- 36%) phosphorylation of the NF-kappa B inhibitory protein I kappa B-alpha, concomitant with increased activation of the I kappa B-alpha kinases IKK alpha (+255.0 +/- 46%) and IKK beta (+173.1 +/- 35%). Furthermore, both tyrosine and serine phosphorylation of I kappa B-alpha were blocked by pretreatment with either the nonreceptor tyrosine kinase inhibitor lavendustin-A (LD-A) or the PKC inhibitor chelerythrine (Che) (both given at doses previously shown to block ischemic PC). Interestingly, Che completely abolished PC-induced activation of IKK alpha/beta, whereas LD-A had no effect. In addition, I kappa B-alpha protein level did not change during ischemic PC. Together, these data indicate that ischemic PC-induced activation of NF-kappa B occurs through both tyrosine and serine phosphorylation of I kappa B-alpha and is regulated by nonreceptor tyrosine kinases and PKC.

Location is everything: lipid rafts and immune cell signaling

The cells of both the adaptive and innate immune systems express a dizzying array of receptors that transduce and integrate an enormous amount of information about the environment that allows the cells to mount effective immune responses. Over the past several years, significant advances have been made in elucidating the molecular details of signal cascades initiated by the engagement of immune cell receptors by their ligands. Recent evidence indicates that immune receptors and components of their signaling cascades are spatially organized and that this spatial organization plays a central role in the initiation and regulation of signaling. A key organizing element for signaling receptors appears to be cholesterol- and sphingolipid-rich plasma membrane microdomains termed lipid rafts. Research into the molecular basis of the spatial segregation and organization of signaling receptors provided by rafts is adding fundamentally to our understanding of the initiation and prolongation of signals in the immune system.

The flotillins are integral membrane proteins in lipid rafts that contain TCR-associated signaling components: implications for T-cell activation

Lipid rafts play an important role in signal integration and cellular activation by the T-cell antigen receptor (TCR). We demonstrate that flotillin-1 and flotillin-2 are important structural raft components, which redistribute to the site of TCR engagement. An antibody to flotillin-1 was able to immobilize other TCR-associated raft components. Although rafts purified from unstimulated cells demonstrated abundant Lck but inabundant LAT, rafts from stimulated cells include an abundance of both components. This suggests dynamic changes in lipid raft composition during CD3/CD28 costimulation. Stimulation of primary human CD4(+) T cells leads to increased GM1 and flotillin-1 expression in the surface membrane, where these components colocalize. This may reconstitute new signaling complexes required for T-cell activation. Altered lipid raft composition and function may play a role in the decline of antigen responsiveness in senescent T cells. In this regard, we observed an increase in the raft-associated gangliolipid, GM1, in resting human CD4(+) and CD8(+) lymphocytes with aging.

Ephrin B2 induces T cell costimulation

Eph kinases form the largest family of receptor tyrosine kinases, and their ligands are ephrins (EFNs), which are cell surface proteins. Some Eph kinases and EFNs are expressed on T cells, B cells, and dendritic cells, but their functions in the immune system are largely unknown. In this study, we investigated the effect of EFNB2 on murine T cells. EFNB2 mRNA was expressed in the cortex of the thymus and white pulp of the spleen. At the protein level, it was expressed on T cells and monocytes/macrophages, but not on B cells. EFNB2Rs were expressed mainly on T cells. Solid-phase EFNB2 along with suboptimal anti-CD3 strongly stimulated T cell proliferation, with concomitant augmentation of IFN-gamma but not IL-2 or IL-4 secretion. The activity of cytotoxic T cells was also significantly enhanced in the presence of solid-phase EFNB2. These results indicate that EFNB2R cross-linking results in costimulation of T cells. EFNB2Rs were normally scattered on the T cell surface; after TCR cross-linking, they rapidly congregated to capped TCR complexes and then to patched rafts. This provides a morphological base for EFNB2Rs to participate in T cell costimulation. We also demonstrated that EFNB2R signaling led to augmented p38 and p44/42 mitogen-activated protein kinase activation. Our study shows that EFNB2 plays important roles in immune regulation.

Protein kinase C theta affects Ca2+ mobilization and NFAT cell activation in primary mouse T cells

Protein kinase C (PKC)theta is an established component of the immunological synapse and has been implicated in the control of AP-1 and NF-kappaB. To study the physiological function of PKCtheta, we used gene targeting to generate a PKCtheta null allele in mice. Consistently, interleukin 2 production and T cell proliferative responses were strongly reduced in PKCtheta-deficient T cells. Surprisingly, however, we demonstrate that after CD3/CD28 engagement, deficiency of PKCtheta primarily abrogates NFAT transactivation. In contrast, NF-kappaB activation was only partially reduced. This NFAT transactivation defect appears to be secondary to reduced inositol 1,4,5-trisphosphate generation and intracellular Ca2+ mobilization. Our finding suggests that PKCtheta plays a critical and nonredundant role in T cell receptor-induced NFAT activation.

Transducing signals from antigen receptors to nuclear factor kappaB

Signaling leading to the survival or apoptosis of immune system cells must be balanced to ensure the normal mounting and extinguishing of immune responses. One of the essential regulators of immune cell survival is the transcription factor nuclear factor kappaB (NF-kappaB). NF-kappaB is critical for the activation of T and B lymphocytes and is a central coordinator of innate and adaptive immunity. Pathogen recognition, whether mediated via the Toll-like receptors or via the antigen-specific T- and B-cell receptors, initiates the activation of distinct signal transduction pathways that activate NF-kappaB. Activation of NF-kappaB by these pathways is necessary for lymphocyte activation, expansion, and effector function in response to infection. In addition, recent work has shown that the aberrant activation of NF-kappaB by these pathways can contribute to the development of autoimmunity, chronic inflammation, or lymphoid malignancy. There is thus an urgent need to understand the exact molecular details of these signal transduction cascades so that we may develop novel therapeutics. This article will review the specific signal transduction pathways that mediate NF-kappaB activation in response to antigen receptor ligation in T and B lymphocytes. These newly defined pathways, which are essential for adaptive immune responses, are built around the key adapter protein, Bcl-10. Bcl-10 is known to participate in chromosomal translocations in human mucosa-associated lymphoid tissue lymphomas.

Enhancement of cytokine production and AP-1 transcriptional activity in T cells by thalidomide-related immunomodulatory drugs

CC-4047 (Actimid) and CC-5013 (Revimid) belong to a class of thalidomide analogs collectively known as the immunomodulatory drugs (IMiDs), which are currently being assessed in the treatment of patients with multiple myeloma and other cancers. IMiDs potently enhance T cell and natural killer cell responses and inhibit tumor necrosis factor-alpha, interleukin (IL)-1 beta, and IL-12 production from LPS-stimulated peripheral blood mononuclear cells. However, the molecular mechanism of action for these compounds is unknown. Herein, we report on the ability of the IMiDs to up-regulate production of IL-2 from activated human CD4+ and CD8+ peripheral blood T cells, production of IL-2 and IFN-gamma from T helper (Th)1-type cells, and production of IL-5 and IL-10 from Th2-type cells. Elevation of IL-2 production from Jurkat T cells was observed as early as 6 h poststimulation and correlated with an increase in IL-2 promoter activity that was dependent upon the proximal but not the distal AP-1 binding site. The IMiDs enhanced AP-1-driven transcriptional activity 2- to 4-fold after 6 h of T cell stimulation, and their relative potencies for AP-1 activation correlated with their potencies for increased IL-2 production in Jurkat T cells and in CD4+ or CD8+ human peripheral blood T cells. The most potent of these IMiDs, CC-4047, had no effect on nuclear factor of activated T cells transcriptional activity, calcium signaling, or phosphorylation of extracellular signal-regulated kinase 1/2, c-Jun NH2-terminal kinase 1/2, p38 mitogen-activated protein kinase, or c-Jun/Jun D in Jurkat T cells. These data suggest that IMiDs increase T cell cytokine production by potentiating AP-1 transcriptional activity.

A novel ERK-dependent signaling process that regulates interleukin-2 expression in a late phase of T cell activation

Engagement of the T cell antigen receptor (TCR) rapidly induces multiple signal transduction pathways, including ERK activation. Here, we report a critical role for ERK at a late stage of T cell activation. Inhibition of the ERK pathway 2-6 h after the start of TCR stimulation significantly impaired interleukin-2 (IL-2) production, whereas the same treatment during the first 2 h had no effect. ERK inhibition significantly impaired nuclear translocation of c-Rel with a minimum reduction of NF-AT activity. Requirement for sustained ERK activation was also confirmed using primary T cells. To induce sustained activation of ERK, T cells required continuous engagement of TCR. Stimulation of T cells with soluble anti-TCR antibody resulted in activation of ERK lasting for 60 min, but failed to induce IL-2 production. In contrast, plate-bound anti-TCR antibody activated ERK over 4 h and induced IL-2. Furthermore, T cells treated with soluble anti-TCR antibody produced IL-2 when phorbol 12-myristate 13-acetate, which activates ERK, was present in the culture medium 2-6 h after the start of stimulation. Together, the data demonstrate the presence of a novel activation process following TCR stimulation that requires ERK-dependent regulation of c-Rel, a member of the NF-kappaB family.

AKT1/PKBalpha is recruited to lipid rafts and activated downstream of PKC isotypes in CD3-induced T cell signaling

Protein kinase (PK) Ctheta and Akt/PKBalpha cooperate in T cell receptor/CD28-induced T cell signaling. We here demonstrate the recruitment of endogenous Akt1 and PKCtheta to lipid rafts in CD3-stimulated T cells. Further we show that Myr-PKCtheta mediates translocation of endogenous Akt1 to the plasma membrane as well as to lipid rafts, most likely explained by the observed complex formation of both protein kinases. In addition, in peripheral mouse T cells, the PKC inhibitor Gö6850 could partially block Akt1 activation in CD3-induced signaling, placing PKC isotype(s) upstream of Akt1. However, T cells derived from PKCtheta knockout mice were not impaired in CD3- or phorbol ester-induced Akt1 activity. Taken together, the results of this study give new insights into the functional link of Akt1 and PKCtheta in T cell signaling, demonstrating the co-recruitment of the two kinases and showing a novel pathway leading to Akt1 transactivation where PKC isotype(s) are involved but PKCtheta is not essential.

Activation of Raf-1 signaling by protein kinase C through a mechanism involving Raf kinase inhibitory protein

Protein kinase C (PKC) regulates activation of the Raf-1 signaling cascade by growth factors, but the mechanism by which this occurs has not been elucidated. Here we report that one mechanism involves dissociation of Raf kinase inhibitory protein (RKIP) from Raf-1. Classic and atypical but not novel PKC isoforms phosphorylate RKIP at serine 153 (Ser-153). RKIP Ser-153 phosphorylation by PKC either in vitro or in response to 12-O-tetradecanoylphorbol-13-acetate or epidermal growth factor causes release of RKIP from Raf-1, whereas mutant RKIP (S153V or S153E) remains bound. Increased expression of PKC can rescue inhibition of the mitogen-activated protein (MAP) kinase signaling cascade by wild-type but not mutant S153V RKIP. Taken together, these results constitute the first model showing how phosphorylation by PKC relieves a key inhibitor of the Raf/MAP kinase signaling cascade and may represent a general mechanism for the regulation of MAP kinase pathways.

Protein kinase C-theta (PKCtheta): it's all about location, location, location

Much progress has been made in understanding the function of protein kinase C-theta (PKCtheta) in the immune system since this Ca2+-independent PKC isotype was isolated in 1993 as an enzyme that is highly expressed in T lymphocytes and in muscle cells. Biochemical and genetic approaches revealed that, while dispensable for T-cell development, PKCtheta is required for the activation of mature T cells and for interleukin (IL)-2 production. This deficiency results from impaired receptor-induced stimulation of the transcription factors AP-1 and NF-kappaB. PKCtheta integrates T-cell receptor (TCR)/CD28 costimulatory signals, which are essential for productive T-cell activation and, most likely, for prevention of T-cell anergy. A unique property of PKCtheta is its highly selective recruitment to the central supramolecular activation complex (cSMAC) region of the immunological synapse (IS) in antigen-stimulated T cells. Our work revealed that this highly selective localization is not entirely dependent on phospholipase C (PLC) activity and diacylglycerol (DAG) production. Instead, a novel signaling pathway that requires functional Vav1, phosphatidylinositol 3-kinase (PI3-K), the small GTPase Rac and actin cytoskeleton reorganization regulates the localization and, perhaps, activation of PKCtheta. PKCtheta also provides a survival signal, which protects T cells from apoptosis. Additional work is required to identify the immediate targets of PKCtheta and its immune functions in vivo. This work is likely to validate PKCtheta as an attractive drug target.

Vav-1 and the IKK alpha subunit of I kappa B kinase functionally associate to induce NF-kappa B activation in response to CD28 engagement

We have recently observed that CD28 engagement initiates a signaling pathway leading to the activation of I kappa B kinase (IKK) complex and, consequently, to NF-kappa B activation, and we identified Vav-1 as an important mediator of this function. Here we report for the first time that Vav-1 constitutively associates with IKK alpha in both Jurkat and primary CD4(+) T cells. Vav-1/IKK alpha association is mediated by their helix-loop-helix domains, does not involve IKK beta, and is functionally relevant in that Vav-1-associated IKK alpha kinase activity is increased following CD28 engagement by B7. Moreover, we demonstrate that CD28-induced NF-kappa B activation is augmented by both IKK alpha and Vav-1, but not IKK beta. Confocal microscopy showed that endogenous Vav-1 and IKK alpha, but not IKK beta, were recruited to the membrane and colocalized in response to CD28 stimulation. Taken together, these data evidence that Vav-1 plays a key role in the control of NF-kappa B pathway by targeting IKK alpha in the T cell membrane and favoring its activation in response to CD28 stimulation.

T cell receptor can be recruited to a subset of plasma membrane rafts, independently of cell signaling and attendantly to raft clustering

The constitutive/inducible association of the T cell receptor (TCR) with isolated detergent-resistant, lipid raft-derived membranes has been studied in Jurkat T lymphocytes. Membranes resistant to 1% Triton X-100 contained virtually no CD3epsilon, part of the TCR complex, irrespective of cell stimulation. On the other hand, membranes resistant either to a lower Triton X-100 concentration (i.e. 0.2%) or to the less hydrophobic detergent Brij 58 (1%) contained (i) a low CD3epsilon amount (approximate 2.7% of total) in resting cells and (ii) a several times higher amount of the TCR component, after T cell stimulation with either antigen-presenting cells or with phytohemagglutinin. It appeared that CD3/TCR was constitutively associated with and recruited to a raft-derived membrane subset because (i) all three membrane preparations contained a similar amount of the raft marker tyrosine kinase Lck but no detectable amounts of the conventional membrane markers, CD45 phosphatase and transferrin receptor; (ii) a larger amount of particulate membranes were resistant to solubilization with 0.2% Triton X-100 and Brij 58 than to solubilization with 1% Triton X-100; and (iii) higher cholesterol levels were present in membranes resistant to either the lower Triton X-100 concentration or to Brij 58, as compared with those resistant to 1% Triton X-100. The recruitment of CD3 to the raft-derived membrane subset appeared (i) to occur independently of cell signaling events, such as protein-tyrosine phosphorylation and Ca(2+) mobilization/influx, and (ii) to be associated with clustering of plasma membrane rafts induced by multiple cross-linking of either TCR or the raft component, ganglioside GM(1). We suggest that during T cell stimulation a lateral reorganization of rafts into polarized larger domains can determine the recruitment of TCR into these domains, which favors a polarization of the signaling cascade.

Melanoma cell migration to type IV collagen requires activation of NF-kappaB

Chemotaxis is the consequence of environmental factors engaging their receptors to initiate signaling cascades. However, the biochemical mechanisms controlling this phenomenon are not clear. We employed an in vitro modified Boyden 48-well chemotaxis migration system to characterize the role of signal transducers in type IV collagen (CIV) induced A2058 human melanoma cell migration. Using specific pharmacological inhibitors and a series of dominant-negative and constitutively active signaling proteins, we show that Ras and Rac GTPases, PI-3K, and PKC participate in cell migration mediated by beta1 integrins. Collagen also induces a time- dependent degradation of IkappaB-alpha and an increase in nuclear translocation of NF-kappaB which is dependent on PKC pathway. More importantly, collagen-stimulated melanoma cell migration directly correlated with an increase in NF-kappaB transactivation. Furthermore, CIV induced an increase in beta1 integrin mRNA levels. Specific NF-kappaB inhibitors Helenalin and SN-50 inhibited melanoma cell migration to collagen, indicating a novel requirement for NF-kappaB transactivation in cell chemotaxis mediated by beta1 integrin signals. These results describe signal transduction events that are initiated by type IV collagen through beta1 integrins and demonstrate an important role for NF-kappaB in regulating melanoma chemotaxis.

Induction of the NF-kappaB cascade by recruitment of the scaffold molecule NEMO to the T cell receptor

The mechanism by which TCR signaling activates NF-kappaB is poorly understood. We demonstrate here that the IKK kinase complex is recruited to the immunological synapse and can be coprecipitated with the TCR after T cell activation. Using ZAP-70-deficient T cells expressing a hybrid molecule between the SH2 domain of ZAP-70 and NEMO/IKKgamma, we showed that targeting NEMO to the immunological synapse, and more specifically its 120 N-terminal amino acids, was sufficient to selectively restore NF-kappaB activation in response to TCR ligation. Finally, we demonstrated that targeting of NEMO to the membrane of T cells was sufficient to induce constitutive NF-kappaB activation. This study shows that the localization of NEMO to the immunological synapse is important for TCR-induced NF-kappaB activation and offers a powerful system to dissect the NF-kappaB cascade in T cells.

Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa and phospholipase C gamma 1 are required for NF-kappa B activation and lipid raft recruitment of protein kinase C theta induced by T cell costimulation

The NF-kappaB activation pathway induced by T cell costimulation uses various molecules including Vav1 and protein kinase C (PKC)theta. Because Vav1 inducibly associates with further proteins including phospholipase C (PLC)gamma1 and Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76), we investigated their role for NF-kappaB activation in Jurkat leukemia T cell lines deficient for expression of these two proteins. Cells lacking SLP-76 or PLCgamma1 failed to activate NF-kappaB in response to T cell costimulation. In contrast, replenishment of SLP-76 or PLCgamma1 expression restored CD3/CD28-induced IkappaB kinase (IKK) activity as well as NF-kappaB DNA binding and transactivation. PKCtheta activated NF-kappaB in SLP-76- and PLCgamma1-deficient cells, showing that PKCtheta is acting further downstream. In contrast, Vav1-induced NF-kappaB activation was normal in SLP-76(-) cells, but absent in PLCgamma1(-) cells. CD3/CD28-stimulated recruitment of PKCtheta and IKKgamma to lipid rafts was lost in SLP-76- or PLCgamma1-negative cells, while translocation of Vav1 remained unaffected. Accordingly, recruitment of PKCtheta to the immunological synapse strictly relied on the presence of SLP-76 and PLCgamma1, but synapse translocation of Vav1 identified in this study was independent from both proteins. These results show the importance of SLP-76 and PLCgamma1 for NF-kappaB activation and raft translocation of PKCtheta and IKKgamma.

The Wiskott-Aldrich syndrome protein acts downstream of CD2 and the CD2AP and PSTPIP1 adaptors to promote formation of the immunological synapse

The Wiskott-Aldrich syndrome protein (WASp) couples actin cytoskeletal rearrangement to T cell activation, but the mechanisms involved are unknown. Here, we show that antigen-induced formation of T cell:APC conjugates and synapses is abrogated in WASp-deficient T cells and that CD2 engagement evokes interactions between the proline-rich region required for WASp translocation to the synapse and the PSTPIP1 adaptor SH3 domain and between the PSTPIp1 coiled-coil domain and both CD2 and another CD2-binding adaptor, CD2AP. The induced colocalization of these proteins at the synapse is disrupted by expression of coiled-coil domain-deleted PSTPIP1. These data, together with the impairment in CD2-induced actin polymerization observed in WASp-deficient cells, suggest that PSTPIP1 acts downstream of CD2/CD2AP to link CD2 engagement to the WASp-evoked actin polymerization required for synapse formation and T cell activation.

Molecular Mechanism of the Activation-Induced Cell Death Inhibition Mediated by a p70 Inhibitory Killer Cell Ig-Like Receptor in Jurkat T Cells

In this study we investigated the molecular mechanism of the activation-induced cell death (AICD) inhibition mediated by a p70 inhibitory killer cell Ig-like receptor (KIR3DL1, also called NKB1) in Jurkat T cells. Using stable Jurkat transfectants that express KIR or CD8-KIR fusion proteins we have shown for the first time that KIR inhibits, in a ligation-independent manner, the AICD induced by PHA, PMA/ionomycin, or anti-CD3 Ab. The AICD inhibition mediated by KIR appears to result from the blockade of Fas ligand induction upon activation of the Jurkat transfectants. Moreover, the membrane-proximal 20 aa of the KIR cytoplasmic tail were determined to play a crucial role in this process. Since the membrane-proximal portion of the KIR cytoplasmic tail contains a putative protein kinase C (PKC) substrate site, we investigated the molecular interaction between KIR and PKC. Immunoprecipitation analysis demonstrated that KIR constitutively bound both to PKCalpha, a conventional Ca(2+)-dependent PKC, and to PKCtheta, a novel Ca(2+)-independent PKC. Furthermore, an in vitro kinase assay revealed that PKC activation was blocked after PHA stimulation in Jurkat transfectants expressing KIR. These observations were supported by the finding that a recombinant KIR cytoplasmic tail also appeared to inhibit PKCalpha activation in vitro. Taken together these data strongly suggest that KIR inhibits the AICD of T cells by blocking Fas ligand induction upon stimulation, in a process that seems to be accomplished by PKC recruitment to the membrane-proximal PKC binding site and subsequent inhibition of PKC activation against the activating stimuli.

CARMA1 is a critical lipid raft-associated regulator of TCR-induced NF-kappa B activation

CARMA1 is a lymphocyte-specific member of the membrane-associated guanylate kinase (MAGUK) family of scaffolding proteins, which coordinate signaling pathways emanating from the plasma membrane. CARMA1 interacts with Bcl10 via its caspase-recruitment domain (CARD). Here we investigated the role of CARMA1 in T cell activation and found that T cell receptor (TCR) stimulation induced a physical association of CARMA1 with the TCR and Bcl10. We found that CARMA1 was constitutively associated with lipid rafts, whereas cytoplasmic Bcl10 translocated into lipid rafts upon TCR engagement. A CARMA1 mutant, defective for Bcl10 binding, had a dominant-negative (DN) effect on TCR-induced NF-kappa B activation and IL-2 production and on the c-Jun NH(2)-terminal kinase (Jnk) pathway when the TCR was coengaged with CD28. Together, our data show that CARMA1 is a critical lipid raft-associated regulator of TCR-induced NF-kappa B activation and CD28 costimulation-dependent Jnk activation.

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.

PKC-beta controls I kappa B kinase lipid raft recruitment and activation in response to BCR signaling

NF-kappa B signaling is required for the maintenance of normal B lymphocytes, whereas dysregulated NF-kappa B activation contributes to B cell lymphomas. The events that regulate NF-kappa B signaling in B lymphocytes are poorly defined. Here, we demonstrate that PKC-beta is specifically required for B cell receptor (BCR)-mediated NF-kappa B activation. B cells from protein kinase C-beta (PKC-beta)-deficient mice failed to recruit the I kappa B kinase (IKK) complex into lipid rafts, activate IKK, degrade I kappa B or up-regulate NF-kappa B-dependent survival signals. Inhibition of PKC-beta promoted cell death in B lymphomas characterized by exaggerated NF-kappa B activity. Together, these data define an essential role for PKC-beta in BCR survival signaling and highlight PKC-beta as a key therapeutic target for B-lineage malignancies.

Protein kinase Czeta phosphorylates nuclear factor of activated T cells and regulates its transactivating activity

Although several isoforms of protein kinase C (PKC) have been implicated in T lymphocyte activation events, little is known about their mode of action. To address the role of PKCzeta in T cell activation, we have generated Jurkat T cell transfectants expressing either the wild type (J-PKCzeta) or "kinase-dead" mutant (J-PKCzeta(mut)) versions of this protein. Expression of PKCzeta but not PKCzeta(mut) increased transcriptional activation mediated by the NF-kappaB or nuclear factor of activated T cells (NFAT). PKCzeta cooperates with calcium ionophore and with NFAT1 or NFAT2 proteins to enhance transcriptional activation of a NFAT reporter construct. However, neither NFAT nuclear translocation nor DNA binding were in J-PKCzeta cells. Our results show that PKCzeta enhanced transcriptional activity mediated by Gal4-NFAT1 fusion proteins containing the N-terminal transactivation domain of human NFAT1. Interestingly, PKCzeta synergizes with calcineurin to induce transcriptional activation driven by the NFAT1 transactivation domain. Co-precipitation experiments showed physical interaction between PKCzeta and NFAT1 or NFAT2 isoforms. Even more, PKCzeta was able to phosphorylate recombinant glutathione S-transferase-NFAT1 (1-385) protein. These data reveal a new role of PKCzeta in T cells through the control of NFAT function by modulating the activity of its transactivation domain.

Cystic fibrosis and the use of pharmacogenomics to determine surrogate endpoints for drug discovery

Cystic fibrosis (CF) is caused by a mutation in the CFTR gene, encoding a chloride channel. For the most common mutation, Delta F508, the basis of the deficit is the failure of the mutant CFTR channel protein to traffic properly to the apical plasma membrane of the affected epithelial cell. The trafficking failure results in loss of the cyclic adenosine monophosphate (cAMP)-activated chloride channel function of the CFTR protein in the plasma membrane. The lung is the principal site affecting patient morbidity and mortality in CF. The main reason is that the CF airway epithelial cells also secrete high levels of the proinflammatory cytokine interleukin (IL)-8, resulting in massive cellular inflammation, infection, tissue damage and lung destruction. The relationship between the trafficking defect, the loss of chloride channel activity, and inflammation is not known. However, gene therapy of CF lung epithelial cells with the wild-type CFTR gene can repair the chloride channel defect, as well as suppress the intrinsic hypersecretion of IL-8. Repair of both defective channels and high IL-8 secretion can also be effected by treatment with the candidate CF drug CPX, which is in clinical trials in CF patients. CPX acts by binding to the mutant CFTR protein, and helps the protein to mature and gain access to the plasma membrane. CPX also suppresses the synthesis and secretion of IL-8 from CF epithelial cells, presumably by virtue of its repair of the trafficking defect of mutant CFTR. To guide pharmacogenomic experiments we have therefore hypothesized that the genomic signature of CF epithelial cells treated with CPX should resemble the signature of the same cells repaired by gene therapy. We have developed two algorithms for identifying genes modified by repair of CFTR defects. The GRASP algorithm uses a statistical test to identify the most profoundly changing genes. The GENESAVER algorithm allows us to identify those genes whose pattern of expression changes in-phase or out-of-phase with IL-8 secretion by CF cells. For the latter algorithm we modified IL-8 secretion from CF cells by treatment with wild-type CFTR, with CPX, or by exposure to bacteria. The results have supported the hypothesis, and have provided a basis for considering the common pharmacogenomic expression signature as a surrogate endpoint for CF drug discovery. Significantly, the nature of the hypothesis, as well as the algorithm developed for this study, can be easily applied to pharmacogenomic studies with other goals.

Regulation of retinoid X receptor responsive element-dependent transcription in T lymphocytes by Ser/Thr phosphatases: functional divergence of protein kinase C (PKC)theta; and PKC alpha in mediating calcineurin-induced transactivation

T lymphocyte activation signals regulate the expression and transactivation function of retinoid X receptor (RXR) alpha through an interplay of complex signaling cascades that are not yet fully understood. We show that cellular Ser/Thr protein phosphatases (PPs) play an important role in mediating these processes. Inhibitors specific for PP1 and PP2A decreased basal expression of RXR alpha RNA and protein in T lymphocyte leukemia Jurkat cells and prevented activation-induced RXR alpha accumulation in these cells. In addition, these inhibitors attenuated the RXR responsive element (RXRE)-dependent transcriptional activation in transient transfection assays. Inhibitors of calcineurin (CN), by contrast, did not have any effect on the basal RXR alpha expression and even augmented activation-induced RXR alpha expression. Expression of a dominant-active (DA) mutant of CN together with a DA mutant of protein kinase C (PKC)theta;, a novel PKC isoform, significantly increased RXRE-dependent transcription. Expression of catalytically inactive PKC theta; or a dominant-negative mutant of PKC theta; failed to synergize with CN and did not increase RXRE-dependent transcription. Expression of a DA mutant of PKC alpha or treatment with PMA was found to attenuate PKC theta; and CN synergism. We conclude that PP1, PP2A, and CN regulate levels and transcriptional activation function of RXR alpha in T cells. In addition, CN synergizes with PKC theta; to induce RXRE-dependent activation, a cooperative function that is antagonized by the activation of the conventional PKC alpha isoform. Thus, PKC theta; and PKC alpha may function as positive and negative modulators, respectively, of CN-regulated RXRE-dependent transcription during T cell activation.

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.

Protein kinase C-theta;: signaling from the center of the T-cell synapse

The hypothesis that protein kinase C (PKC)-theta; plays an important role in T-lymphocyte activation, as indicated by numerous studies in cell lines, was recently confirmed in mice deficient in the expression of this enzyme. In response to TCR stimulation, peripheral T cells lacking PKC-theta; failed to activate NF-kappaB and AP-1, and to express IL-2. This revealed a critical function for this PKC family member in linking membrane-proximal activation cascades to transcriptional responses governing T-cell activation. Although the molecular interactions in which PKC-theta; engages have not been fully delineated, insights from a variety of recent studies have permitted new models to be formulated regarding the mechanisms through which it achieves its unique effector functions.

T-cell activation through the antigen receptor. Part 2: role of signaling cascades in T-cell differentiation, anergy, immune senescence, and development of immunotherapy

Part 2 of this review on cellular activation by the T-cell antigen receptor (TCR) will highlight how TCR signaling pathways are adapted to achieve specific biologic outcomes, including different states of T-cell differentiation and the induction of T-cell tolerance. We will also explore how treatment with altered peptide ligands affects TCR signaling to change T-cell differentiation or to induce an anergy state. These changes are accomplished through alteration of protein tyrosine kinase activity, the stoichiometry of phosphorylation of immunoreceptor tyrosine-based activation motifs, intracellular free ionized calcium flux, mitogen-activated protein kinase activity, and transcriptional activation of key cytokine promoters. The CTLA-4 plays an important role in the induction and maintenance of anergy. The second theme will highlight how altered TCR signal transduction, including changes in the compartmentalization of signaling components at the TCR synapse, contributes to decreased T-cell activation during immune senescence. Finally, we will illustrate how the molecular details of TCR activation can be used to modify the function of the immune system. This includes a description of the mechanism of action of altered peptide ligands, CTLA-4Ig, and pharmacologic inhibitors of mitogen-activated protein kinases, nuclear factor kappaB, and protein kinase C cascades.

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.