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

The yin and yang of protein kinase C-theta (PKCθ): a novel drug target for selective immunosuppression

Protein kinase C-theta (PKCθ) is a protein kinase C (PKC) family member expressed predominantly in T lymphocytes, and extensive studies addressing its function have been conducted. PKCθ is the only T cell-expressed PKC that localizes selectively to the center of the immunological synapse (IS) following conventional T cell antigen stimulation, and this unique localization is essential for PKCθ-mediated downstream signaling. While playing a minor role in T cell development, early in vitro studies relying, among others, on the use of PKCθ-deficient (Prkcq(-/-)) T cells revealed that PKCθ is required for the activation and proliferation of mature T cells, reflecting its importance in activating the transcription factors nuclear factor kappa B, activator protein-1, and nuclear factor of activated T cells, as well as for the survival of activated T cells. Upon subsequent analysis of in vivo immune responses in Prkcq(-/-) mice, it became clear that PKCθ has a selective role in the immune system: it is required for experimental Th2- and Th17-mediated allergic and autoimmune diseases, respectively, and for alloimmune responses, but is dispensable for protective responses against pathogens and for graft-versus-leukemia responses. Surprisingly, PKCθ was recently found to be excluded from the IS of regulatory T cells and to negatively regulate their suppressive function. These attributes of PKCθ make it an attractive target for catalytic or allosteric inhibitors that are expected to selectively suppress harmful inflammatory and alloimmune responses without interfering with beneficial immunity to infections. Early progress in developing such drugs is being made, but additional studies on the role of PKCθ in the human immune system are urgently needed.

The protein kinase C inhibitor AEB071 (sotrastaurin) modulates migration and superoxide anion production by human neutrophils in vitro

We examined the effect of the protein kinase C-selective inhibitor AEB071 (sotrastaurin) on neutrophil functions in vitro. Pre-incubation with AEB071 at concentrations similar to those reached during in vivo therapy significantly reduced cell capacity to migrate toward three different chemo-attractants and to produce superoxide anions (O₂⁻) in response to phorbol myristate acetate (PMA) or to N-formyl-methionyl-leucyl-phenylalanine (fMLP). AEB071 also significantly inhibited the O₂⁻ overproduction induced by fMLP in neutrophils primed with tumor necrosis factor alpha (TNF-α) or granulocyte/macrophage-colony stimulating factor (GM-CSF). This inhibition was not linked to fMLP-receptor down-regulation since the drug had no effect on either fMLP-receptors or fMLP-induced CD11b membrane expression. When the activity of AEB071 was compared to that of the conventional protein kinase C (PKC) inhibitor Gö6850 (which, like sotrastaurin, inhibits classical and novel PKC isoforms), Gö6976 (an inhibitor of α and α PKC isoforms) and rottlerin (a prevailing δ PKC isoform inhibitor), AEB071 at an equimolar concentration of 3 μM (close to the maximum drug concentration reached in patients treated with AEB071) caused significantly more inhibition on both chemotactic response and superoxide production. These in vitro findings suggest that neutrophils may offer a cellular target for AEB071 activity in vivo.

The emerging role of protein kinase Cθ in cytoskeletal signaling

Cytoskeletal rearrangements often occur as the result of transduction of signals from the extracellular environment. Efficient awakening of this powerful machinery requires multiple activation and deactivation steps, which usually involve phosphorylation or dephosphorylation of different signaling units by kinases and phosphatases, respectively. In this review, we discuss the signaling characteristics of one of the nPKC isoforms, PKCθ, focusing on PKCθ-mediated signal transduction to cytoskeletal elements, which results in cellular rearrangements critical for cell type-specific responses to stimuli. PKCθ is the major PKC isoform present in hematopoietic and skeletal muscle cells. PKCθ plays roles in T cell signaling through the IS, survival responses in adult T cells, and T cell FasL-mediated apoptosis, all of which involve cytoskeletal rearrangements and relocation of this enzyme. PKCθ has been linked to the regulation of cell migration, lymphoid cell motility, and insulin signaling and resistance in skeletal muscle cells. Additional roles were suggested for PKCθ in mitosis and cell-cycle regulation. Comprehensive understanding of cytoskeletal regulation and the cellular "modus operandi" of PKCθ holds promise for improving current therapeutic applications aimed at autoimmune diseases.

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.

Involvement of distinct PKC gene products in T cell functions

It is well established that members of the protein kinase C (PKC) family seem to have important roles in T cells. Focusing on the physiological and non-redundant PKC functions established in primary mouse T cells via germline gene-targeting approaches, our current knowledge defines two particularly critical PKC gene products, PKCθ and PKCα, as the "flavor of PKC" in T cells that appear to have a positive role in signaling pathways that are necessary for full antigen receptor-mediated T cell activation ex vivo and T cell-mediated immunity in vivo. Consistently, in spite of the current dogma that PKCθ inhibition might be sufficient to achieve complete immunosuppressive effects, more recent results have indicated that the pharmacological inhibition of PKCθ, and additionally, at least PKCα, appears to be needed to provide a successful approach for the prevention of allograft rejection and treatment of autoimmune diseases.

Intervention of PKC-θ as an immunosuppressive regimen

PKC-θ is selectively enriched in T cells and specifically translocates to immunological synapse where it mediates critical T cell receptor signals required for T cell activation, differentiation, and survival. T cells deficient in PKC-θ are defective in their ability to differentiate into inflammatory effector cells that mediate actual immune responses whereas, their differentiation into regulatory T cells (Treg) that inhibits the inflammatory T cells is enhanced. Therefore, the manipulation of PKC-θ activity can shift the ratio between inflammatory effector T cells and inhibitory Tregs, to control T cell-mediated immune responses that are responsible for autoimmunity and allograft rejection. Indeed, PKC-θ-deficient mice are resistant to the development of several Th2 and Th17-dependent autoimmune diseases and are defective in mounting alloimmune responses required for rejection of transplanted allografts and graft-versus-host disease. Selective inhibition of PKC-θ is therefore considered as a potential treatment for prevention of autoimmune diseases and allograft rejection.

Protein kinase Cθ C2 domain is a phosphotyrosine binding module that plays a key role in its activation

Protein kinase Cθ (PKCθ) is a novel PKC that plays a key role in T lymphocyte activation. To understand how PKCθ is regulated in T cells, we investigated the properties of its N-terminal C2 domain that functions as an autoinhibitory domain. Our measurements show that a Tyr(P)-containing peptide derived from CDCP1 binds the C2 domain of PKCθ with high affinity and activates the enzyme activity of the intact protein. The Tyr(P) peptide also binds the C2 domain of PKCδ tightly, but no enzyme activation was observed with PKCδ. Mutations of PKCθ-C2 residues involved in Tyr(P) binding abrogated the enzyme activation and association of PKCθ with Tyr-phosphorylated full-length CDCP1 and severely inhibited the T cell receptor/CD28-mediated activation of a PKCθ-dependent reporter gene in T cells. Collectively, these studies establish the C2 domain of PKCθ as a Tyr(P)-binding domain and suggest that the domain may play a major role in PKCθ activation via its Tyr(P) binding.

Protein Kinase C-θ (PKC-θ) in Natural Killer Cell Function and Anti-Tumor Immunity

The protein kinase C-θ (PKCθ), which is essential for T cell function and survival, is also required for efficient anti-tumor immune surveillance. Natural killer (NK) cells, which express PKCθ, play a prominent role in this process, mainly by elimination of tumor cells with reduced or absent major histocompatibility complex class-I (MHC-I) expression. This justifies the increased interest of the use of activated NK cells in anti-tumor immunotherapy in the clinic. The in vivo development of MHC-I-deficient tumors is much favored in PKCθ^−/− mice compared with wild-type mice. Recent data offer some clues on the mechanism that could explain the important role of PKCθ in NK cell-mediated anti-tumor immune surveillance: some studies show that PKCθ is implicated in signal transduction and anti-tumoral activity of NK cells elicited by interleukin (IL)-12 or IL-15, while others show that it is implicated in NK cell functional activation mediated by certain killer-activating receptors. Alternatively, the possibility that PKCθ is involved in NK cell degranulation is discussed, since recent data indicate that it is implicated in microtubule-organizing center polarization to the immune synapse in CD4⁺ T cells. The implication of PKC isoforms in degranulation has been more extensively studied in cytotoxic T lymphocyte, and these studies will be also summarized.

The role of protein kinase cη in T cell biology

Protein kinase Cη (PKCη) is a member of the novel PKC subfamily, which also includes δ, ε, and θ isoforms. Compared to the other novel PKCs, the function of PKCη in the immune system is largely unknown. Several studies have started to reveal the role of PKCη, particularly in T cells. PKCη is highly expressed in T cells, and is upregulated during thymocyte positive selection. Interestingly, like the θ isoform, PKCη is also recruited to the immunological synapse that is formed between a T cell and an antigen-presenting cell. However, unlike PKCθ, which becomes concentrated to the central region of the synapse, PKCη remains in a diffuse pattern over the whole area of the synapse, suggesting distinctive roles of these two isoforms in signal transduction. Although PKCη is dispensable for thymocyte development, further analysis of PKCη- or PKCθ-deficient and double-knockout mice revealed the redundancy of these two isoforms in thymocyte development. In contrast, PKCη rather than PKCθ, plays an important role for T cell homeostatic proliferation, which requires recognition of self-antigen. Another piece of evidence demonstrating that PKCη and PKCθ have isoform-specific as well as redundant roles come from the analysis of CD4 to CD8 T cell ratios in the periphery of these knockout mice. Deficiency in PKCη or PKCθ had opposing effects as PKCη knockout mice had a higher ratio of CD4 to CD8 T cells compared to that of wild-type mice, whereas PKCθ-deficient mice had a lower ratio. Biochemical studies showed that calcium flux and NFκB translocation is impaired in PKCη-deficient T cells upon TCR crosslinking stimulation, a character shared with PKCθ-deficient T cells. However, unlike the case with PKCθ, the mechanistic study of PKCη is at early stage and the signaling pathways involving PKCη, at least in T cells, are essentially unknown. In this review, we will cover the topics mentioned above as well as provide some perspectives for further investigations regarding PKCη.

Protein kinase C-θ promotes Th17 differentiation via upregulation of Stat3

Although protein kinase C-θ (PKC-θ)-deficient mice are resistant to the induction of Th17-dependent experimental autoimmune encephalomyelitis, the function of PKC-θ in Th17 differentiation remains unknown. In this article, we show that purified, naive CD4 PKC-θ(-/-) T cells were defective in Th17 differentiation, whereas Th1 and Th2 differentiation appeared normal. Activation of PKC-θ with PMA promoted Th17 differentiation in wild type (WT) but not PKC-θ(-/-) T cells. Furthermore, PKC-θ(-/-) T cells had notably lower levels of Stat3, a transcription factor required for Th17 differentiation, and PMA markedly stimulated the expression of Stat3 in WT but not PKC-θ(-/-) T cells. In contrast, activation of Stat4 and Stat6, which are critical for Th1 and Th2 differentiation, was normal in PKC-θ(-/-) T cells. Forced expression of Stat3 significantly increased Th17 differentiation in PKC-θ(-/-) T cells, suggesting that reduced Stat3 levels were responsible for impaired Th17 differentiation, and that Stat3 lies downstream of PKC-θ. Constitutively active PKC-θ, or WT PKC-θ activated by either PMA or TCR cross-linking, stimulated expression of a luciferase reporter gene driven by the Stat3 promoter. PKC-θ-mediated activation of the Stat3 promoter was inhibited by dominant-negative AP-1 and IκB kinase-β, but stimulated by WT AP-1 and IκB kinase-β, suggesting that PKC-θ stimulates Stat3 transcription via the AP-1 and NF-κB pathways. Lastly, conditions favoring Th17 differentiation induced the highest activation level of PKC-θ. Altogether, the data indicate that PKC-θ integrates the signals from TCR signaling and Th17 priming cytokines to upregulate Stat3 via NF-κB and AP-1, resulting in the stimulation of Th17 differentiation.

Protein kinase C-θ inhibits inducible regulatory T cell differentiation via an AKT-Foxo1/3a-dependent pathway

Protein kinase C (PKC)-θ has been shown to be a critical TCR signaling molecule that promotes the activation and differentiation of naive T cells into inflammatory effector T cells. In this study, we demonstrate that PKC-θ-mediated signals inhibit inducible regulatory T cell (iTreg) differentiation via an AKT-Foxo1/3A pathway. TGF-β-induced iTreg differentiation was enhanced in PKC-θ(-/-) T cells or wild-type cells treated with a specific PKC-θ inhibitor, but was inhibited by the PKC-θ activator PMA, or by CD28 crosslinking, which enhances PKC-θ activation. PKC-θ(-/-) T cells had reduced activity of the AKT kinase, and the expression of a constitutively active form of AKT in PKC-θ(-/-) T cells restored the ability to inhibit iTreg differentiation. Furthermore, knockdown or overexpression of the AKT downstream targets Foxo1 and Foxo3a was found to inhibit or promote iTreg differentiation in PKC-θ(-/-) T cells accordingly, indicating that the AKT-Foxo1/3A pathway is responsible for the inhibition of iTreg differentiation of iTregs downstream of PKC-θ. We conclude that PKC-θ is able to control T cell-mediated immune responses by shifting the balance between the differentiation of effector T cells and inhibitory Tregs.

Molecular mechanisms of treg-mediated T cell suppression

CD4(+)CD25(high)Foxp3(+) regulatory T cells (Tregs) can suppress other immune cells and, thus, are critical mediators of peripheral self-tolerance. On the one hand, Tregs avert autoimmune disease and allergies. On the other hand, Tregs can prevent immune reactions against tumors and pathogens. Despite the importance of Tregs, the molecular mechanisms of suppression remain incompletely understood and controversial. Proliferation and cytokine production of CD4(+)CD25(-) conventional T cells (Tcons) can be inhibited directly by Tregs. In addition, Tregs can indirectly suppress Tcon activation via inhibition of the stimulatory capacity of antigen presenting cells. Direct suppression of Tcons by Tregs can involve immunosuppressive soluble factors or cell contact. Different mechanisms of suppression have been described, so far with no consensus on one universal mechanism. Controversies might be explained by the fact that different mechanisms may operate depending on the site of the immune reaction, on the type and activation state of the suppressed target cell as well as on the Treg activation status. Further, inhibition of T cell effector function can occur independently of suppression of proliferation. In this review, we summarize the described molecular mechanisms of suppression with a particular focus on suppression of Tcons and rapid suppression of T cell receptor-induced calcium (Ca(2+)), NFAT, and NF-κB signaling in Tcons by Tregs.

Syk kinase-coupled C-type lectin receptors engage protein kinase C-δ to elicit Card9 adaptor-mediated innate immunity

C-type lectin receptors (CLRs) that couple with the kinase Syk are major pattern recognition receptors for the activation of innate immunity and host defense. CLRs recognize fungi and other forms of microbial or sterile danger, and they induce inflammatory responses through the adaptor protein Card9. The mechanisms relaying CLR proximal signals to the core Card9 module are unknown. Here we demonstrated that protein kinase C-δ (PKCδ) was activated upon Dectin-1-Syk signaling, mediated phosphorylation of Card9 at Thr231, and was responsible for Card9-Bcl10 complex assembly and canonical NF-κB control. Prkcd(-/-) dendritic cells, but not those lacking PKCα, PKCβ, or PKCθ, were defective in innate responses to Dectin-1, Dectin-2, or Mincle stimulation. Moreover, Candida albicans-induced cytokine production was blocked in Prkcd(-/-) cells, and Prkcd(-/-) mice were highly susceptible to fungal infection. Thus, PKCδ is an essential link between Syk activation and Card9 signaling for CLR-mediated innate immunity and host protection.

Protein kinase C-θ is required for murine neutrophil recruitment and adhesion strengthening under flow

Protein kinase C (PKC)-θ is involved in T cell activation via regulating the avidity of the β(2) integrin LFA-1 in the immunological synapse. LFA-1 also mediates leukocyte adhesion. To investigate the role of PKC-θ in neutrophil adhesion, we performed intravital microscopy in cremaster venules of mice reconstituted with bone marrow from LysM-GFP(+) (wild-type [WT]) and PKC-θ gene-deficient (Prkcq(-/-)) mice. Following stimulation with CXCL1, both WT and Prkcq(-/-) cells became adherent. Although most WT neutrophils remained adherent for at least 180 s, 50% of Prkcq(-/-) neutrophils were detached after 105 s and most by 180 s. Upon CXCL1 injection, rolling of all WT neutrophils stopped for 90 s, but rolling of Prkcq(-/-) neutrophils started 30 s after CXCL1 stimulation. A similar neutrophil adhesion defect was seen in vitro, and spreading of Prkcq(-/-) neutrophils was delayed. Prkcq(-/-) neutrophil recruitment was impaired in fMLP-induced transmigration into the cremaster muscle, thioglycollate-induced peritonitis, and LPS-induced lung injury. We conclude that PKC-θ mediates integrin-dependent neutrophil functions and is required to sustain neutrophil adhesion in postcapillary venules in vivo. These findings suggest that the role of PKC-θ in outside-in signaling following engagement of neutrophil integrins is relevant for inflammation in vivo.

Ameliorated ConA-induced hepatitis in the absence of PKC-theta

Severe liver injury that occurs when immune cells mistakenly attack an individual's own liver cells leads to autoimmune hepatitis. In mice, acute hepatitis can be induced by concanavalin A (ConA) treatment, which causes rapid activation of CD1d-positive natural killer (NK) T cells. These activated NKT cells produce large amounts of cytokines, which induce strong inflammation that damages liver tissues. Here we show that PKC-θ^−/− mice were resistant to ConA-induced hepatitis due to essential function of PKC-θ in NKT cell development and activation. A dosage of ConA (25 mg/kg) that was lethal to wild-type (WT) mice failed to induce death resulting from liver injury in PKC-θ^−/− mice. Correspondingly, ConA-induced production of cytokines such as IFNγ, IL-6, and TNFα, which mediate the inflammation responsible for liver injury, were significantly lower in PKC-θ^−/− mice. Peripheral NKT cells had developmental defects at early stages in the thymus in PKC-θ^−/− mice, and as a result their frequency and number were greatly reduced. Furthermore, PKC-θ^−/− bone marrow adoptively transferred to WT mice displayed similar defects in NKT cell development, suggesting an intrinsic requirement for PKC-θ in NKT cell development. In addition, upon stimulation with NKT cell-specific lipid ligand, peripheral PKC-θ^−/− NKT cells produced lower levels of inflammatory cytokines than that of WT NKT cells, suggesting that activation of NKT cells also requires PKC-θ. Our results suggest PKC-θ is an essential molecule required for activation of NKT cell to induce hepatitis, and thus, is a potential drug target for prevention of autoimmune hepatitis.

AMP-activated protein kinase mediates T cell activation-induced expression of FasL and COX-2 via protein kinase C theta-dependent pathway in human Jurkat T leukemia cells

AMP-activated protein kinase (AMPK), an important regulator of energy homeostasis, is known to be activated during T cell activation. T cell activation by T cell receptor (TCR) engagement or its pharmacological mimics, PMA plus ionomycin (PMA/Io), induces immunomodulatory FasL and cyclooxygenase-2 (COX-2) expression. In this study, we examined the role and mechanisms of AMPK in PMA/Io-induced expression of FasL and COX-2 in Jurkat T human leukemic cells. Inhibition of AMPK by a pharmacological agent, compound C, or AMPKα1 siRNA suppressed expression of FasL and COX-2 mRNAs and proteins in PMA/Io-activated Jurkat cells. It also reduced secretion of FasL protein and prostaglandin E2, a main product of COX-2, in Jurkat cells and peripheral blood lymphocytes activated with PMA/Io or monoclonal anti-CD3 plus anti-CD28. Consistently, inhibition of AMPK blocked promoter activities of FasL and COX-2 in activated Jurkat cells. As protein kinase C theta (PKCθ) is a central molecule for TCR signaling, we examined any possible cross-talk between AMPK and PKCθ in activated T cells. Of particular importance, we found that inhibition of AMPK blocked phosphorylation and activation of PKCθ, suggesting that AMPK is an upstream kinase of PKCθ. Moreover, we showed that AMPK was directly associated with PKCθ and phosphorylated Thr538 of PKCθ in PMA/Io-stimulated Jurkat cells. We also showed that inhibition of PKCθ by rottlerin or dominant negative PKCθ reduced AMPK-mediated transcriptional activation of NF-AT and AP-1 in activated Jurkat cells. Taken together, these results suggest that AMPK regulates expression of FasL and COX-2 via the PKCθ and NF-AT and AP-1 pathways in activated Jurkat cells.

Anergic CD8+ T lymphocytes have impaired NF-κB activation with defects in p65 phosphorylation and acetylation

Because of the cytotoxic potential of CD8(+) T cells, maintenance of CD8(+) peripheral tolerance is extremely important. A major peripheral tolerance mechanism is the induction of anergy, a refractory state in which proliferation and IL-2 production are inhibited. We used a TCR transgenic mouse model to investigate the signaling defects in CD8(+) T cells rendered anergic in vivo. In addition to a previously reported alteration in calcium/NFAT signaling, we also found a defect in NF-κB-mediated gene transcription. This was not due to blockade of early NF-κB activation events, including IκB degradation and NF-κB nuclear translocation, as these occurred normally in tolerant T cells. However, we discovered that anergic cells failed to phosphorylate the NF-κB p65 subunit at Ser(311) and also failed to acetylate p65 at Lys(310). Both of these modifications have been implicated as critical for NF-κB transactivation capacity, and thus, our results suggest that defects in key phosphorylation and acetylation events are important for the inhibition of NF-κB activity (and subsequent T cell function) in anergic CD8(+) T cells.

Developing and activated T cell survival depends on differential signaling pathways to regulate anti-apoptotic Bcl-x(L)

Survival of T cells in both the central and peripheral immune system determines its ultimate function in the regulation of immune responses. In the thymus, developing T cells undergo positive and negative selection to generate a T cell repertoire that responds to foreign, but not self, antigens. During T cell development, the T cell receptor α chain is rearranged. However, the first round of rearrangement may fail, which triggers another round of α chain rearrangement until either successful positive selection or cell death occurs. Thus, the lifespan of double positive (CD4(+)CD8(+); DP) thymocytes determines how many rounds of α chain rearrangement can be carried out and influences the likelihood of completing positive selection. The anti-apoptotic protein Bcl-x(L) is the ultimate effector regulating the survival of CD4(+)CD8(+) thymocytes subject to the selection process, and the deletion of Bcl-x(L) leads to premature apoptosis of thymocytes prior to the completion of the developmental process. In addition to its critical function in the thymus, Bcl-x(L) also regulates the survival of peripheral T cells. Upon engagement with antigens, T cells are activated and differentiated into effectors. Activated T cells upregulate Bcl-x(L) to enhance their own survival. Bcl-x(L)-mediated survival is required for the generation of effectors that carry out the actual immune responses. In the absence of Bcl-x(L), mature T cells undergo apoptosis prior to the completion of the differentiation process to become effector cells. Therefore, Bcl-x(L) ensures the survival of both developing and peripheral T cells, which is essential for a functional immune system.

Protein kinase C η is required for T cell activation and homeostatic proliferation

Protein kinase C η (PKCη) is abundant in T cells and is recruited to the immunological synapse that is formed between a T cell and an antigen-presenting cell; however, its function in T cells is unknown. We showed that PKCη was required for the activation of mature CD8+ T cells through the T cell receptor. Compared with wild-type T cells, PKCη-/- T cells showed poor proliferation in response to antigen stimulation, a trait shared with T cells deficient in PKCθ, which is the most abundant PKC isoform in T cells and was thought to be the only PKC isoform with a specific role in T cell activation. In contrast, only PKCη-deficient T cells showed defective homeostatic proliferation, which requires self-antigen recognition. PKCη was dispensable for thymocyte development; however, thymocytes from mice doubly deficient in PKCη and PKCθ exhibited poor development, indicating some redundancy between the PKC isoforms. Deficiency in PKCη or PKCθ had opposing effects on the relative numbers of CD4+ and CD8+ T cells. PKCη-/- mice had a higher ratio of CD4+ to CD8+ T cells compared to that of wild-type mice, whereas PKCθ-/- mice had a lower ratio. Mice deficient in both isoforms exhibited normal cell ratios. Together, these data suggest that PKCη shares some redundant roles with PKCθ in T cell biology and also performs nonredundant functions that are required for T cell homeostasis and activation.

Phytocomponent p-Hydroxycinnamic acid inhibits T-cell activation by modulation of protein kinase C-θ-dependent pathway

The phytocomponent p-hydoxycinnamic acid (HCA) has been shown to have many beneficial effects in terms of antioxidant activity, inhibition of melanogenesis, bone resorption, and platelet activity, and stimulation of mineralization. However, effects of HCA in immune functions have not been investigated. Here, we show that HCA has a profound effect on IL-2 production in Jurkat T cells as well as in human peripheral blood leukocytes. HCA, at a concentration that optimally inhibits IL-2 production, had little effect on apoptotic or necrotic cell death of Jurkat T cells, suggesting that apoptosis is not a mechanism for HCA-induced T-cell suppression. On the contrary, HCA dramatically inhibited PKC-θ accumulation and further phosphorylation at the immunological synapse which formed at the contact site between T cells and superantigen SEE-loaded antigen presenting cells. In addition, HCA significantly inhibited ERK and p38 kinase phosphorylation in both anti-CD3/28- and PMA/A23187-stimulated T cells. Consequently, HCA inhibited both AP-1 and NF-κB promoter activities in Jurkat T cells. Collectively, our results provide evidence for the immunosuppressive effect of HCA on activated T cells, through modulation of PKC-θ pathway.

Role of T cell-nuclear factor κB in transplantation

Nuclear factor (NF) κB is a pleiotropic transcription factor that is ubiquitously expressed. After transplantation of solid organs, NF-κB in the graft is activated within a few hours as a consequence of ischemia/reperfusion and then again after a few days in intragraft infiltrating cells during the process of acute allograft rejection. In the present article, we review the components of the NF-κB pathway, their mechanisms of activation, and their role in T cell and antigen-presenting cell activation and differentiation and in solid organ allograft rejection. Targeted inhibition of NF-κB in selected cell types may promote graft survival with fewer adverse effects compared with global immunosuppressive therapies.

SAP-mediated inhibition of diacylglycerol kinase α regulates TCR-induced diacylglycerol signaling

Diacylglycerol kinases (DGKs) metabolize diacylglycerol to phosphatidic acid. In T lymphocytes, DGKα acts as a negative regulator of TCR signaling by decreasing diacylglycerol levels and inducing anergy. In this study, we show that upon costimulation of the TCR with CD28 or signaling lymphocyte activation molecule (SLAM), DGKα, but not DGKζ, exits from the nucleus and undergoes rapid negative regulation of its enzymatic activity. Inhibition of DGKα is dependent on the expression of SAP, an adaptor protein mutated in X-linked lymphoproliferative disease, which is essential for SLAM-mediated signaling and contributes to TCR/CD28-induced signaling and T cell activation. Accordingly, overexpression of SAP is sufficient to inhibit DGKα, whereas SAP mutants unable to bind either phospho-tyrosine residues or SH3 domain are ineffective. Moreover, phospholipase C activity and calcium, but not Src-family tyrosine kinases, are also required for negative regulation of DGKα. Finally, inhibition of DGKα in SAP-deficient cells partially rescues defective TCR/CD28 signaling, including Ras and ERK1/2 activation, protein kinase C membrane recruitment, induction of NF-AT transcriptional activity, and IL-2 production. Thus SAP-mediated inhibition of DGKα sustains diacylglycerol signaling, thereby regulating T cell activation, and it may represent a novel pharmacological strategy for X-linked lymphoproliferative disease treatment.

2,6-Naphthyridines as potent and selective inhibitors of the novel protein kinase C isozymes

The present study describes a novel series of ATP-competitive PKC inhibitors based on the 2,6-naphthyridine template. Example compounds potently inhibit the novel Protein Kinase C (PKC) isotypes δ, ε, η, θ (in particular PKCε/η, and display a 10-100-fold selectivity over the classical PKC isotypes. The prototype compound 11 was found to inhibit PKCθ-dependent pathways in vitro and in vivo. In vitro, a-CD3/a-CD28-induced lymphocyte proliferation could be effectively blocked in 10% rat whole blood. In mice, 11 dose-dependently inhibited Staphylococcus aureus enterotoxin B-triggered IL-2 serum levels after oral dosing.

A motif in the V3 domain of the kinase PKC-θ determines its localization in the immunological synapse and functions in T cells via association with CD28

Protein kinase C-θ (PKC-θ) translocates to the center of the immunological synapse, but the underlying mechanism and its importance in T cell activation are unknown. Here we found that the V3 domain of PKC-θ was necessary and sufficient for localization to the immunological synapse mediated by association with the coreceptor CD28 and dependent on the kinase Lck. We identified a conserved proline-rich motif in V3 required for association with CD28 and immunological synapse localization. We found association with CD28 to be essential for PKC-θ-mediated downstream signaling and the differentiation of T helper type 2 cells (T(H)2 cells) and interleukin 17-producing helper T cells (T(H)17 cells) but not of T helper type 1 cells (T(H)1 cells). Ectopic expression of V3 sequestered PKC-θ from the immunological synapse and interfered with its functions. Our results identify a unique mode of CD28 signaling, establish a molecular basis for the immunological synapse localization of PKC-θ and indicate V3-based 'decoys' may be therapeutic modalities for T cell-mediated inflammatory diseases.

Overexpression of Ste20-related proline/alanine-rich kinase exacerbates experimental colitis in mice

Inflammatory bowel disease, mainly Crohn's disease and ulcerative colitis, are characterized by epithelial barrier disruption and altered immune regulation. Colonic Ste20-like proline/alanine-rich kinase (SPAK) plays a role in intestinal inflammation, but its underlying mechanisms need to be defined. Both SPAK-transfected Caco2-BBE cells and villin-SPAK transgenic (TG) FVB/6 mice exhibited loss of intestinal barrier function. Further studies demonstrated that SPAK significantly increased paracellular intestinal permeability to FITC-dextran. In vivo studies using the mouse models of colitis induced by dextran sulfate sodium (DSS) and trinitrobenzene sulfonic acid showed that TG FVB/6 mice were more susceptible to DSS and trinitrobenzene sulfonic acid treatment than wild-type FVB/6 mice, as demonstrated by clinical and histological characteristics and enzymatic activities. Consistent with this notion, we found that SPAK increased intestinal epithelial permeability, which likely facilitated the production of inflammatory cytokines in vitro and in vivo, aggravated bacterial translocation in TG mice under DSS treatment, and consequently established a context favorable for the triggering of intestinal inflammation cascades. In conclusion, overexpression of SPAK inhibits maintenance of intestinal mucosal innate immune homeostasis, which makes regulation of SPAK important to attenuate pathological responses in inflammatory bowel disease.

PKC-θ function at the immunological synapse: prospects for therapeutic targeting

Protein kinase C (PKC)-θ regulates conventional effector T (Teff) cell function. Since this initial finding, it has become clear that the role of PKC-θ in T cells is complex. PKC-θ plays a central role in Teff cell activation and survival, and negatively regulates stability of the immunological synapse (IS). Recent studies demonstrated that PKC-θ is required for the development of natural CD4(+)Foxp3(+) regulatory T (Treg) cells, and mediates negative regulation of Treg cell function. Here, we examine the role of PKC-θ in the IS, evidence for its distinct localization in Treg cells and the therapeutic implications of inhibiting PKC-θ in Teff cells, to reduce effector function, and in Treg cells, to increase suppressor function, for the prevention and treatment of autoimmune and alloimmune disease states.

Activation of a nuclear factor of activated T-lymphocyte-3 (NFAT3) by oxidative stress in carboplatin-mediated renal apoptosis

BACKGROUND AND PURPOSE

Although carboplatin is currently used as a therapeutic drug for ovarian, breast, and non-small cell lung cancers, it has serious side effects including renal and cardiac toxicity. Herein, we examined the effect of carboplatin on murine renal tubular cell (RTC) apoptosis both in vivo and in vitro and the underlying molecular mechanisms associated with its activation of the nuclear factor of activated T-lymphocytes-3 (NFAT3).

EXPERIMENTAL APPROACH

Mechanisms of carboplatin-mediated renal apoptosis were examined using NFAT-reporter transgenic mice and RTCs with NFAT3 overexpression or knockdown.

KEY RESULTS

We demonstrated that carboplatin initiated an intrinsic apoptotic pathway of activating caspase-3 and -9, accompanied by a decrease in the ratio of Bcl-XL/Bax and a significant increase in Bcl-XS. Carboplatin increased NFAT activation in NFAT-luciferase reporter transgenic mice, RTCs and cells exogenously overexpressing NFAT3 that exacerbated cell death. Furthermore, the addition of either N-acetylcysteine (NAC, an antioxidant) or NFAT inhibitors, including FK-506 (tacrolimus), cyclosporin A (CsA, a calcineurin inhibitor), and BAPTA-AM (a calcium chelator) successfully reversed carboplatin-mediated cell apoptosis, which was further confirmed using siNFAT3. Additionally, NAC blocked NFAT3 activation by inhibition of NADPH oxidase activation, and ERK/JNK and PKC pathways, resulting in a decrease in cell apoptosis; the therapeutic effect of NAC was verified in vivo.

CONCLUSION AND IMPLICATIONS

The results presented herein show that carboplatin-mediated reactive oxygen species might signal calcineurin and NFAT3 activation in RTCs, whereas NAC and NFAT inhibitors reversed carboplatin-mediated RTC apoptosis, suggesting that oxidative stress-mediated NFAT3 activation is essential for carboplatin-mediated RTC apoptosis.

Itk: the rheostat of the T cell response

The nonreceptor tyrosine kinase Itk plays a key role in TCR-initiated signaling that directly and significantly affects the regulation of PLCγ1 and the consequent mobilization of Ca²⁺. Itk also participates in the regulation of cytoskeletal reorganization as well as cellular adhesion, which is necessary for a productive T cell response. The functional cellular outcome of these molecular regulations by Itk renders it an important mediator of T cell development and differentiation. This paper encompasses the structure of Itk, the signaling parameters leading to Itk activation, and Itk effects on molecular pathways resulting in functional cellular outcomes. The incorporation of these factors persuades one to believe that Itk serves as a modulator, or rheostat, critically fine-tuning the T cell response.

PKCθ signaling is required for myoblast fusion by regulating the expression of caveolin-3 and β1D integrin upstream focal adhesion kinase

Using both in vivo and in vitro protein kinase C (PKC) θ mutant models, we found that PKCθ, the PKC isoform predominantly expressed in skeletal muscle, is required for myoblast fusion and myofiber growth, by regulating focal adhesion kinase activity and, in turn, the expression of the pro-fusion genes caveolin-3 and β1D-integrin.

Fusion of mononucleated myoblasts to form multinucleated myofibers is an essential phase of skeletal myogenesis, which occurs during muscle development as well as during postnatal life for muscle growth, turnover, and regeneration. Many cell adhesion proteins, including integrins, have been shown to be important for myoblast fusion in vertebrates, and recently focal adhesion kinase (FAK), has been proposed as a key mediator of myoblast fusion. Here we focused on the possible role of PKCθ, the PKC isoform predominantly expressed in skeletal muscle, in myoblast fusion. We found that the expression of PKCθ is strongly up-regulated following freeze injury–induced muscle regeneration, as well as during in vitro differentiation of satellite cells (SCs; the muscle stem cells). Using both PKCθ knockout and muscle-specific PKCθ dominant-negative mutant mouse models, we observed delayed body and muscle fiber growth during the first weeks of postnatal life, when compared with wild-type (WT) mice. We also found that myofiber formation, during muscle regeneration after freeze injury, was markedly impaired in PKCθ mutant mice, as compared with WT. This phenotype was associated with reduced expression of the myogenic differentiation program executor, myogenin, but not with that of the SC marker Pax7. Indeed in vitro differentiation of primary muscle-derived SCs from PKCθ mutants resulted in the formation of thinner myotubes with reduced numbers of myonuclei and reduced fusion rate, when compared with WT cells. These effects were associated to reduced expression of the profusion genes caveolin-3 and β1D integrin and to reduced activation/phosphorylation of their up-stream regulator FAK. Indeed the exogenous expression of a constitutively active mutant form of PKCθ in muscle cells induced FAK phosphorylation. Moreover pharmacologically mediated full inhibition of FAK activity led to similar fusion defects in both WT and PKCθ-null myoblasts. We thus propose that PKCθ signaling regulates myoblast fusion by regulating, at least in part, FAK activity, essential for profusion gene expression.

Decreased T-cell receptor signaling through CARD11 differentially compromises forkhead box protein 3-positive regulatory versus T(H)2 effector cells to cause allergy

BACKGROUND

Allergy, the most common disease of immune dysregulation, has a substantial genetic component that is poorly understood. Although complete disruption of T-cell receptor (TCR) signaling causes profound immunodeficiency, little is known about the consequences of inherited genetic variants that cause partial quantitative decreases in particular TCR-signaling pathways, despite their potential to dysregulate immune responses and cause immunopathology.

OBJECTIVE

We sought to elucidate how an inherited decrease in TCR signaling through CARD11, a critical scaffold protein that signals to nuclear factor κB (NF-κB) transcription factors, results in spontaneous selective accumulation of large numbers of T(H)2 cells.

METHODS

"Unmodulated" mice carry a Card11 single nucleotide variant that decreases but does not abolish TCR/CD28 signaling to induce targets of NF-κB. The consequences of this mutation on T-cell subset formation in vivo were examined, and its effects within effector versus regulatory T-cell subsets were dissected by the adoptive transfer of wild-type cells and by the examination of forkhead box protein 3 (Foxp3)-deficient unmodulated mice.

RESULTS

Unlike the pathology-free boundary points of complete Card11 sufficiency or deficiency, unmodulated mice have a specific allergic condition characterized by increased IgE levels and dermatitis. The single nucleotide variant partially decreases both the frequency of Foxp3(+) regulatory T cells and the efficiency of effector T-cell formation in vivo. These intermediate effects combine to cause a gradual and selective expansion of T(H)2 cells.

CONCLUSIONS

Inherited reduction in the efficiency of TCR-NF-κB signaling has graded effects on T-cell activation and Foxp3(+) regulatory T-cell suppression that result in selective T(H)2 dysregulation and allergic disease.

Evaluation of the novel protein kinase C inhibitor sotrastaurin as immunosuppressive therapy after renal transplantation

IMPORTANCE OF THE FIELD

The prevalence of acute renal allograft rejection has decreased substantially in past decades due to new and more specific immunosuppressive compounds but improvements in long-term graft function have not been achieved. There is a large need for new immunosuppressive agents that lack toxicity of current agents such as calcineurin inhibitors but show high synergistic efficiency in preventing rejection processes.

AREAS COVERED IN THIS REVIEW

This review summarizes data concerning the pharmacokinetics, pharmacodynamics and clinical efficacy of the new PKC inhibitor sotrastaurin with a focus on renal transplantation. The article contains information that has been presented at international transplant meetings and congresses and that has been published between 2006 and 2010. Additionally, current ongoing trials are described in detail.

WHAT THE READER WILL GAIN

Immunosuppressive regimens after kidney transplantation consist of a combination of several agents in order to minimize drug toxicity. Therefore, the reader is presented with the most up-to-date/current developments in sotrastaurin applications in Phase I and II trials with emphasis on data maintained from studies that combined sotrastaurin with established agents such as mycophenolic acid and tacrolimus.

TAKE HOME MESSAGE

Several trials are ongoing and planned to determine the optimal immunosuppressive regimen to benefit from sotrastaurin's distinct mechanism of action.

Unique functions for protein kinase D1 and protein kinase D2 in mammalian cells

Mammalian PKD (protein kinase D) isoforms have been implicated in the regulation of diverse biological processes in response to diacylglycerol and PKC (protein kinase C) signalling. To compare the functions of PKD1 and PKD2 in vivo, we generated mice deficient in either PKD1 or PKD2 enzymatic activity, via homozygous expression of PKD1^S744A/S748A or PKD2^S707A/S711A ‘knockin’ alleles. We also examined PKD2-deficient mice generated using ‘gene-trap’ technology. We demonstrate that, unlike PKD1, PKD2 catalytic activity is dispensable for normal embryogenesis. We also show that PKD2 is the major PKD isoform expressed in lymphoid tissues, but that PKD2 catalytic activity is not essential for the development of mature peripheral T- and B-lymphocytes. PKD2 catalytic activity is, however, required for efficient antigen receptor-induced cytokine production in T-lymphocytes and for optimal T-cell-dependent antibody responses in vivo. Our results reveal a key in vivo role for PKD2 in regulating the function of mature peripheral lymphocytes during adaptive immune responses. They also confirm the functional importance of PKC-mediated serine phosphorylation of the PKD catalytic domain for PKD activation and downstream signalling and reveal that different PKD family members have unique and non-redundant roles in vivo.

Oxidized ATP inhibits T-cell-mediated autoimmunity

T cells directed against self antigens play an important role in several autoimmune diseases. The available immunosuppressive compounds used to treat autoimmune diseases are limited, and often they have side effects that limit their application. T cells express ATP receptors, which could be new target molecules to treat autoimmune disease. Here we analyzed the effect of oxidized ATP (oxATP), an inhibitor of the ATP receptor P2rx7, in different murine models of T-cell-mediated autoimmune diseases. Treatment with oxATP inhibited proliferation and effector function of T cells. In the systems we used, oxATP did not obviously interfere with the innate immune response, but strongly reduced antigen-specific T-cell responses. This treatment ameliorated T-cell-mediated autoimmune type I diabetes and autoimmune encephalitis in mice. In conclusion, oxATP was found to strongly inhibit activated T cells and could thus be used to target T-cell-mediated autoimmune disease.

Taurine and proliferation of lymphocytes in physically restrained rats

Background

Taurine is present in lymphocytes and seems to modulate certain immune cell functions. Among the effects of taurine on these cells are protection against antioxidants and regulation of inflammatory aspects of the immune response. Stress affects antigen presentation, traffic and proliferation of leukocytes, as well as antibody and cytokine secretion. The purposes of this study were to explore the possible direct effects of taurine concentrations on lymphoproliferation and interleukins levels in control and in physical restrained rats.

Methods

Lymphocytes of male Sprague-Dawley rats, stressed by physical restrain and controls (5 h per day for 5 days) were isolated from blood by Histopaque (1077 g/l) and differential adhesion to plastic, and then cultured (72 h) in the presence of different concentrations of taurine (0.5 – 50 mM), β-alanine (0.5 – 50 mM), or both, without or with the T cells mitogen, concanavalin A. Plasma and lymphocytes levels of pro-inflammatory interleukin-1β and anti-inflammatory interleukin-10 were respectively measured by Pierce Endogen rat ELISA Kits. Taurine in plasma and in lymphocytes were determined by HPLC.

Results

Lymphoproliferation of resting cells significantly decreased in the presence of 3 and 6 mM taurine and increased up to control level at 12 mM taurine. In concanavalin A-activated lymphocytes, the effect of taurine was greater. β-alanine increased lymphoproliferation in a bell shaped dose-dependent manner and decreased it in activated lymphocytes but in a lower magnitude. In combination, β-alanine impaired the effect of taurine at 3 and 6 mM. After restriction, no change in lymphoproliferation was observed at different concentrations of the amino acids without or with concanavalin A, although pro-inflammatory interleukin and taurine in plasma and in lymphocytes significantly increased.

Conclusions

Taurine affects lymphoproliferation in control rats, following a dose-dependent manner, an effect that might involve its transport into the cells. Elevation of interleukin-1β produced in stressed rats by physical restrain could seriously affect the immune balance, whereas taurine increase might be protective. These results suggest that taurine and taurine transport play a role in lymphoproliferation. In addition, modifications of taurine system in lymphocytes take place during restriction stress.

Protein kinase C θ deficiency increases resistance of C57BL/6J mice to Plasmodium berghei infection-induced cerebral malaria

Protein kinase C θ (PKCθ) functions as a core component of the immunological synapse and serves as a key protein in the integrated T-cell antigen receptor (TCR)/CD28-induced signaling cascade leading to T-cell activation. However, the involvement of PKCθ in host-mediated immune responses to pathogens has not been thoroughly investigated. We tested the consequences of PKCθ ablation on the host response to infection by Plasmodium berghei ANKA (PbA). We found that both PKCθ(+/+) and PKCθ(-/-) C57BL/6J mice are susceptible to infection with PbA. However, despite a similar parasite burden, PKCθ(+/+) mice had an earlier onset of neurological signs, characteristics of experimental cerebral malaria (ECM), resulting in an earlier death. These mice suffered from an early and pronounced splenomegaly with a concomitant increase in the total number of CD4(+) splenic T cells. In contrast, a large proportion of PbA-infected PKCθ(-/-) mice overcame the acute phase characterized by neurological symptoms and survived longer than PKCθ(+/+) mice. The partial resistance of PKCθ(-/-) mice to ECM was associated with an impaired production of Th1-type cytokines, including gamma interferon and tumor necrosis factor alpha/lymphotoxin-α, which are known to exacerbate symptoms leading to ECM. In addition, PbA infection-induced LFA-1 expression in CD8(+) T cells was suppressed in PKCθ-deficient T cells, suggesting a diminished ability to adhere to endothelial cells and sequester in brain microvasculature, which may explain the decrease in neurological symptoms. These data implicate PKCθ in CD4(+) Th1(+) and CD8(+) T-cell-mediated immune responses during PbA infection that contribute to the development of ECM.

T cell activation leads to protein kinase C theta-dependent inhibition of TGF-beta signaling

TGF-beta is an ubiquitous cytokine that plays a pivotal role in the maintenance of self-tolerance and prevention of immunopathologies. Under steady-state conditions, TGF-beta keeps naive T cells in a resting state and inhibits Th1 and Th2 cell differentiation. Because rapid generation of Th1 and Th2 effector cells is needed in response to pathogen invasion, how do naive T cells escape from the quiescent state maintained by TGF-beta? We hypothesized that stimulation by strong TCR agonists might interfere with TGF-beta signaling. Using both primary mouse CD4(+) T cells and human Jurkat cells, we observed that strong TCR agonists swiftly suppress TGF-beta signaling. TCR engagement leads to a rapid increase in SMAD7 levels and decreased SMAD3 phosphorylation. We present evidence that TCR signaling hinders SMAD3 activation by inducing recruitment of TGF-betaRs in lipid rafts together with inhibitory SMAD7. This effect is dependent on protein kinase C, a downstream TCR signaling intermediary, as revealed by both pharmacological inhibition and expression of dominant-negative and constitutively active protein kinase C mutants. This work broadens our understanding of the cross-talk occurring between the TCR and TGF-beta signaling pathways and reveals that strong TCR agonists can release CD4 T cells from constitutive TGF-beta signaling. We propose that this process may be of vital importance upon confrontation with microbial pathogens.

Alteration of the PKC theta-Vav1 complex and phosphorylation of Vav1 in TCDD-induced apoptosis in the lymphoblastic T cell line, L-MAT

We have previously reported that protein kinase C (PKC) theta (theta) and protein tyrosine kinase are involved in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced apoptosis of L-MAT, a human lymphoblastic T cell line. In the current report, we show that Vav1, a GDP/GTP exchange factor for Rho-like small GTPases, could be detected by Western blotting in the membrane fraction of L-MAT cells after TCDD treatment and was precipitated by incubating with an antibody against PKC theta. Furthermore, the degree of phosphorylation of Vav1, which can be detected using the phosphotyrosine-specific antibody PY-20 or 4G10, is significantly increased after treatment with TCDD. In addition, pretreatment of the cells with genistein, a protein tyrosine kinase inhibitor, abolished the phosphorylation of Vav1 and inhibited the apoptosis. These results suggest that TCDD treatment may activate an unidentified protein tyrosine kinase. Accordingly we hypothesize that this kinase phosphorylates Vav1, following which phosphorylated Vav1 may translocate to the membrane with PKC theta. Finally, PKC theta may mediate the transfer of the apoptotic signal to downstream components.

PKCtheta is necessary for efficient activation of NFkappaB, NFAT, and AP-1 during positive selection of thymocytes

While it has been shown in several publications that the serine-threonine kinase PKCθ is required for efficient activation of mature T lymphocytes, the role of PKCθ in T cell development in the thymus is somewhat controversial. In this study, using knockout mice, we show that PKCθ is important in positive selection. The thymus of PKCθ^−/− animals contains significantly less mature single positive T cells compared to wild-type controls. Biochemically, PKCθ deficient thymocytes show defective activation of the transcription factors AP-1, NFAT and NFκB as well as impaired phosphorylation of the MAP kinase ERK after T cell receptor stimulation in vitro. Together, these results reveal a crucial role of PKCθ in positive selection of thymocytes in a pathway leading to the activation of ERK, AP-1, NFAT, and NFκB.

c-Rel phenocopies PKCtheta but not Bcl-10 in regulating CD8+ T-cell activation versus tolerance

Elucidating the signaling events that promote T-cell tolerance versus activation provides important insights for manipulating immunity in vivo. Previous studies have suggested that the absence of PKCtheta results in the induction of anergy and that the balance between the induction of the transcription factors NFAT, AP1 and NF-kappaB plays a key role in determining whether T-cell anergy or activation is induced. Here, we examine whether Bcl-10 and specific family members of NF-kappaB act downstream of PKCtheta to alter CD8(+) T-cell activation and/or anergy. We showed that T cells from mice deficient in c-Rel but not NF-kappaB1 (p50) have increased susceptibility to the induction of anergy, similar to T cells from PKCtheta-deficient mice. Surprisingly T cells from Bcl-10-deficient mice showed a strikingly different phenotype to the PKCtheta-deficient T cells, with a severe block in TCR-mediated activation. Furthermore, we have also shown that survival signals downstream of NF-kappaB, are uncoupled from signals that mediate T-cell anergy. These results suggest that c-Rel plays a critical role downstream of PKCtheta in controlling CD8(+) T-cell anergy induction.

c-Rel but not NF-kappaB1 is important for T regulatory cell development

Regulatory T (Treg) cells are crucial for maintaining peripheral tolerance and controlling T-cell responses. The generation of Treg in the thymus requires TCR triggering and CD28 costimulation. Engagement of these receptors induces a number of signalling pathways, including the activation of NF-kappaB via PKCtheta and the Bcl-10/CARMA1/MALT complex. Previous studies have shown that PKCtheta, Bcl-10 and CARMA1 are important for Treg development. It is unclear, however, whether different members of the NF-kappaB family contribute to Treg development or homeostasis. In this study, we show that Treg numbers are reduced in the absence of c-Rel but not NF-kappaB1 (p50). Furthermore, using mixed bone marrow chimeras from WT and KO animals, we demonstrate that the requirement for PKCtheta, Bcl-10 and c-Rel is T-cell intrinsic, and cannot be rescued by the presence of WT cells. Therefore, c-Rel and NF-kappaB1 have differential roles in Treg development.

Regulation of interferon-γ, interleukin-4 and interleukin-2 by Schizonepeta tenuifolia through differential effects on nuclear factor-κB, NFATc2 and STAT4/6

Contact with antigen on T-cells is made via the T-cell receptor/CD3 complex plus CD28, resulting in the production of cytokines including interleukin (IL)-2, IL-4 and interferon (IFN)-γ. In particular, dysregulation of IFN-γ and IL-4 accounts in part for organ-specific autoimmune diseases, allergic inflammation and other chronic inflammatory disorders. The dried above-ground parts of Schizonepeta tenuifolia Briq are used for the treatment of common cold and skin rashes observed in allergic dermatitis, psoriasis and other dermatological disorders in oriental medicine. In the present study, we investigated whether S. tenuifolia water extract (STE) may modulate systemic levels of IFN-γ, IL-4 and IL-2 in anti-CD3-stimulated mice and the production of those cytokines in anti-CD3-stimulated peripheral blood mononuclear cells (PBMCs). In addition, the effects of STE on anti-CD3-induced activation of several transcription factors were examined. Oral administration of STE significantly reduced the serum levels of IFN-γ and IL-4 from anti-CD3-treated mice but enhanced those of IL-2. Similar patterns were demonstrated in anti-CD3-stimulated splenocytes and PBMCs in vitro. Further analysis showed that STE enhanced the nuclear translocation of nuclear factor of activated T cells (NFAT)c2 but reduced that of the nuclear factor (NF)-κB. The downregulation of IFN-γ and IL-4 was not mediated by its effects on signal transducer and activator of transcription (STAT)4 and STAT6 activation. These results suggest that the differential regulation of STE on IFN-γ, IL-4 and IL-2 may be due to its suppression of NF-κB, concomitant with its enhancement of NFATc2. Further mechanistic work is required to investigate the role of STE on its modulation of anti-CD3-induced cytokines.

Effects of the new immunosuppressive agent AEB071 on human immune cells

BACKGROUND. The novel immunosuppressive agent AEB071 is currently being evaluated for its capability to prevent rejection after kidney transplantation as a potential adjunct to calcineurin inhibitor-based regimen. AEB071 is a selective protein kinase C inhibitor and has been shown to be well tolerated in humans. We here present extensive in vitro studies that contribute to the understanding of AEB071 effects on human lymphocyte, natural killer (NK) cell and dendritic cell (DC) action.

METHODS

The impact of AEB071 on several T-cell activation and costimulatory markers was assessed. Furthermore, assays were performed to study the effect on T-cell proliferation and intracellular cytokine production. Additionally, the effect of AEB071 on DC maturation and their capacity to stimulate allogeneic T-cells was examined. Also, an evaluation of AEB071 effects on the lytic activity of human NK cells was performed.

RESULTS

We were able to show that T-cell proliferation and cytokine production rates are significantly reduced after AEB071 administration. Also, mitogen-induced T-cell activation characterized by expression levels of surface markers could be significantly inhibited. In contrast, the T-cell stimulatory capacity of AEB071-treated mature monocyte-derived DC (Mo-DC) is not reduced, and AEB071 administration does not prevent lipopolysaccharide (LPS)-induced Mo-DC maturation. It could be demonstrated that AEB071 significantly inhibited the cytotoxic activity of NK cells.

CONCLUSIONS

The promising immunosuppressive agent AEB071 has a strong impact on T-cell activation, proliferation and cytokine production as well as NK cell activity, but not DC maturation in vitro, and therefore, seems to function T-cell and NK cell specific via protein kinase C (PKC) inhibition.

The immunological synapse, TCR microclusters, and T cell activation

T cell activation begins with the interaction between an antigen-specific T cell and an antigen-presenting cell (APC). This interaction results in the formation of the immunological synapse, which had been considered to be responsible for antigen recognition and T cell activation. Recent advances in imaging analysis have provided new insights into T cell activation. The T cell receptor (TCR) microclusters, TCRs, kinases, and adaptors are generated upon antigen recognition at the interfaces between the T cells and the APCs and serve as a fundamental signaling unit for T cell activation. CD28-mediated costimulation is also found to be regulated by the formation of microclusters. Therefore, the dynamic regulations of TCR and CD28 microcluster formation, migration, and interaction are the key events for the initiation of T cell-mediated immune responses. Comprehensive analyses of the composition and characteristics of the TCR microcluster have identified its dynamic features. This review will outline new discoveries of the microclusters and its related concept in T cell activation.

Effects of the novel protein kinase C inhibitor AEB071 (Sotrastaurin) on rat cardiac allograft survival using single agent treatment or combination therapy with cyclosporine, everolimus or FTY720

NVP-AEB071 (AEB, sotrastaurin), an oral inhibitor of protein kinase C (PKC), effectively blocks T-cell activation. The immunosuppressive effects of oral AEB were demonstrated in a rat local graft versus host (GvH) reaction and rat cardiac transplantation models. T-cell activation was suppressed by 95% in blood from AEB-treated rats, with a positive correlation between T-cell inhibition and AEB blood concentration. In GvH studies, AEB inhibited lymph node swelling dose-dependently (3-30 mg/kg). BN and DA cardiac allografts were acutely rejected within 6-10 days post-transplantation in untreated LEW rats. AEB at 10 and 30 mg/kg b.i.d. prolonged BN graft survival to a mean survival time of 15 and >28 days, and DA grafts to 6.5 and 17.5 days, respectively. In the DA to LEW model, combining a nonefficacious dose of AEB (10 mg/kg b.i.d.) with a nonefficacious dose of cyclosporine, everolimus or FTY720 led to prolonged median survival times (26 days, >68 days and >68 days, respectively). Pharmacokinetic monitoring excluded drug-drug interactions, suggesting synergy. In conclusion, these studies are the first to demonstrate that AEB prolongs rat heart allograft survival safely as monotherapy and in combination with nonefficacious doses of cyclosporine, everolimus or FTY720. Thus, AEB may have the potential to offer an alternative to calcineurin inhibitor-based therapies.