Protein kinase C theta is not essential for T-cell-mediated clearance of murine gammaherpesvirus 68

Discovery of a novel series of selective macrocyclic PKCTheta inhibitors

A series of macrocyclic PKCθ inhibitors based on a 1,3-dihydro-2H-imidazo[4,5-b]pyridin-2-one hinge binder has been studied. Different aromatic and heteroaromatic substituents have been explored in order to optimize potency, isoform selectivity as well as DMPK properties. The importance of the length of the macrocyclic linker has also been analyzed. In particular, it has been found that methyl substitutions on the linker can have a profound influence on both potency and metabolic stability. Several compounds showing very good profiles, suitable for in vivo testing, are disclosed.

T-cell receptor (TCR) signaling promotes the assembly of RanBP2/RanGAP1-SUMO1/Ubc9 nuclear pore subcomplex via PKC-θ-mediated phosphorylation of RanGAP1

The nuclear pore complex (NPC) is the sole and selective gateway for nuclear transport, and its dysfunction has been associated with many diseases. The metazoan NPC subcomplex RanBP2, which consists of RanBP2 (Nup358), RanGAP1-SUMO1, and Ubc9, regulates the assembly and function of the NPC. The roles of immune signaling in regulation of NPC remain poorly understood. Here, we show that in human and murine T cells, following T-cell receptor (TCR) stimulation, protein kinase C-θ (PKC-θ) directly phosphorylates RanGAP1 to facilitate RanBP2 subcomplex assembly and nuclear import and, thus, the nuclear translocation of AP-1 transcription factor. Mechanistically, TCR stimulation induces the translocation of activated PKC-θ to the NPC, where it interacts with and phosphorylates RanGAP1 on Ser⁵⁰⁴ and Ser⁵⁰⁶. RanGAP1 phosphorylation increases its binding affinity for Ubc9, thereby promoting sumoylation of RanGAP1 and, finally, assembly of the RanBP2 subcomplex. Our findings reveal an unexpected role of PKC-θ as a direct regulator of nuclear import and uncover a phosphorylation-dependent sumoylation of RanGAP1, delineating a novel link between TCR signaling and assembly of the RanBP2 NPC subcomplex.

Insights into CD8 T Cell Activation and Exhaustion from a Mouse Gammaherpesvirus Model

(S.R.S.) I was introduced to viral immunology while working in Peter Doherty's laboratory in the early stages of my research career, inspiring a lifelong interest in this area. During those early years under Peter's mentorship, we studied a mouse gammaherpesvirus model (murine gammaherpesvirus-68 [MHV-68]) that provided a useful small animal model for investigating the immunological control of gammaherpesvirus infection. Interestingly, while CD4 T cells were not required for acute control of MHV-68 in the lung, CD8 T cell-mediated control was progressively lost in the absence of CD4 T cell help, leading to viral recrudescence. This was one of several early studies showing that CD8 T cell control of persistent viral infections was lost in the absence of CD4 T cell help, preceding the concept of CD8 T cell exhaustion. Further studies showed that MHV-68 infection of mice offered a unique model for comparing the mechanisms of acute and long-term control of a persistent viral infection and developing strategies for reversing T cell exhaustion. Here, we provide a brief review of the literature on CD8 T cell activation and exhaustion in this model, focusing on the role of CD40 and B7 family members and including some previously unpublished data.

Discovery of Selective, Orally Bioavailable Pyrazolopyridine Inhibitors of Protein Kinase Cθ (PKCθ) That Ameliorate Symptoms of Experimental Autoimmune Encephalomyelitis

PKCθ plays an important role in T cell biology and is a validated target for a number of disease states. A series of potent and selective PKCθ inhibitors were designed and synthesized starting from a HTS hit compound. Cell activity, while initially a challenge to achieve, was built into the series by transforming the nitrile unit of the scaffold into a primary amine, the latter predicted to form a new hydrogen bond to Asp508 near the entrance of the ATP binding site of PKCθ. Significant improvements in physiochemical parameters were observed on introduction of an oxetane group proximal to a primary amine leading to compound 22, which demonstrated a reduction of symptoms in a mouse model of multiple sclerosis.

Phosphotyrosine-dependent interaction between the kinases PKCθ and Zap70 promotes proximal TCR signaling

Protein kinase C-θ (PKCθ) is an important component of proximal T cell receptor (TCR) signaling. We previously identified the amino-terminal C2 domain of PKCθ as a phosphotyrosine (pTyr)-binding domain. Using a mutant form of PKCθ that cannot bind pTyr (PKCθ^HR2A), we showed that pTyr binding by PKCθ was required for TCR-induced T cell activation, proliferation, and T_H2 cell differentiation but not for T cell development. Using tandem mass spectrometry and coimmunoprecipitation, we identified the kinase ζ-associated protein kinase of 70 kDa (Zap70) as a binding partner of the PKCθ pTyr-binding pocket. Tyr¹²⁶ of Zap70 directly bound to PKCθ, and the interdomain B residues Tyr³¹⁵ and Tyr³¹⁹ were indirectly required for binding to PKCθ, reflecting their role in promoting the open conformation of Zap70. PKCθ^HR2A-expressing CD4⁺ T cells displayed defects not only in known PKCθ-dependent signaling events, such as nuclear factor κB (NF-κB) activation and T_H2 cell differentiation, but also in full activation of Zap70 itself and in the activating phosphorylation of linker of activation of T cells (LAT) and phospholipase C-γ1 (PLCγ1), signaling proteins that are traditionally considered to be activated independently of PKC. These findings demonstrate that PKCθ plays an important role in a positive feedback regulatory loop that modulates TCR-proximal signaling and, moreover, provide a mechanistic explanation for earlier reports that documented an important role for PKCθ in T cell Ca²⁺ signaling. This PKCθ-Zap70 interaction could potentially serve as a promising and highly selective immunosuppressive drug target in autoimmunity and organ transplantation.

Structural determinants of phorbol ester binding activity of the C1a and C1b domains of protein kinase C theta

The PKC isozymes represent the most prominent family of signaling proteins mediating response to the ubiquitous second messenger diacylglycerol. Among them, PKCθ is critically involved in T-cell activation. Whereas all the other conventional and novel PKC isoforms have twin C1 domains with potent binding activity for phorbol esters, in PKCθ only the C1b domain possesses potent binding activity, with little or no activity reported for the C1a domain. In order to better understand the structural basis accounting for the very weak ligand binding of the PKCθ C1a domain, we assessed the effect on ligand binding of twelve amino acid residues which differed between the C1a and C1b domains of PKCθ. Mutation of Pro⁹ of the C1a domain of PKCθ to the corresponding Lys⁹ found in C1b restored in vitro binding activity for [³H]phorbol 12,13-dibutyrate to 3.6 nM, whereas none of the other residues had substantial effect. Interestingly, the converse mutation in the C1b domain of Lys⁹ to Pro⁹ only diminished binding affinity to 11.7 nM, compared to 254 nM in the unmutated C1a. In confocal experiments, deletion of the C1b domain from full length PKCθ diminished, whereas deletion of the C1a domain enhanced 5-fold (at 100 nM PMA) the translocation to the plasma membrane. We conclude that the Pro¹⁶⁸ residue in the C1a domain of full length PKCθ plays a critical role in the ligand and membrane binding, while exchanging the residue (Lys²⁴⁰) at the same position in C1b domain of full length PKCθ only modestly reduced the membrane interaction.

Pharmacological Inhibition of PKCθ Counteracts Muscle Disease in a Mouse Model of Duchenne Muscular Dystrophy

Inflammation plays a considerable role in the progression of Duchenne Muscular Dystrophy (DMD), a severe muscle disease caused by a mutation in the dystrophin gene. We previously showed that genetic ablation of Protein Kinase C θ (PKCθ) in mdx, the mouse model of DMD, improves muscle healing and regeneration, preventing massive inflammation. To establish whether pharmacological targeting of PKCθ in DMD can be proposed as a therapeutic option, in this study we treated young mdx mice with the PKCθ inhibitor Compound 20 (C20). We show that C20 treatment led to a significant reduction in muscle damage associated with reduced immune cells infiltration, reduced inflammatory pathways activation, and maintained muscle regeneration. Importantly, C20 treatment is efficient in recovering muscle performance in mdx mice, by preserving muscle integrity. Together, these results provide proof of principle that pharmacological inhibition of PKCθ in DMD can be considered an attractive strategy to modulate immune response and prevent the progression of the disease.

Research in context

Duchenne muscular dystrophy (DMD) is a severe muscle disease affecting 1:3500 male births. DMD is caused by a mutation in dystrophin gene, coding for a protein required for skeletal and cardiac muscle integrity. Lack of a functional dystrophin is primarily responsible for the muscle eccentric contraction-induced muscle damage, observed in dystrophic muscle. However, inflammation plays a considerable role in the progression of DMD. Glucocorticoids, which have anti-inflammatory properties, are being used to treat DMD with some success; however, long term treatment with these drugs induces muscle atrophy and wasting, outweighing their benefit. The identification of specific targets for anti-inflammatory therapies is one of the ongoing therapeutic options. Although blunting inflammation would not be a “cure” for the disease, the emerging clue is that multiple strategies, addressing different aspects of the pathology, which may eventually converge, may be successful. In this context, we previously showed that genetic ablation of Protein Kinase C θ (PKCθ), an enzyme known to be involved in immune response, in mdx, the mouse model of DMD, improves muscle healing and regeneration, preventing massive inflammation. To establish whether pharmacological targeting of PKCθ in DMD can be proposed as a therapeutic option, in this study we treated young mdx mice with the PKCθ inhibitor Compound 20 (C20). We show that C20 treatment led to a significant reduction in muscle damage associated with reduced immune cells infiltration, reduced inflammatory pathways activation, and maintained muscle regeneration. Importantly, C20 treatment is efficient in recovering muscle performance in mdx mice, by preserving muscle integrity. Together, these results provide proof of principle that pharmacological inhibition of PKCθ in DMD can be considered an attractive strategy to modulate immune response and prevent the progression of the disease.

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Immune-cell intrinsic PKCθ activity might play a hitherto unrecognized role of in the development of DMD.

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Mdx dystrophic mice were treated with the PKCθ inhibitor C20.

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C20 treatment prevents damage and inflammation in dystrophic muscle, while improving muscle regeneration.

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C20 treatment prevents drop in force and ameliorates fatigue resistance in dystrophic mice.

Protein Kinase C Enzymes in the Hematopoietic and Immune Systems

The protein kinase C (PKC) family, discovered in the late 1970s, is composed of at least 10 serine/threonine kinases, divided into three groups based on their molecular architecture and cofactor requirements. PKC enzymes have been conserved throughout evolution and are expressed in virtually all cell types; they represent critical signal transducers regulating cell activation, differentiation, proliferation, death, and effector functions. PKC family members play important roles in a diverse array of hematopoietic and immune responses. This review covers the discovery and history of this enzyme family, discusses the roles of PKC enzymes in the development and effector functions of major hematopoietic and immune cell types, and points out gaps in our knowledge, which should ignite interest and further exploration, ultimately leading to better understanding of this enzyme family and, above all, its role in the many facets of the immune system.

PKC-Theta in Regulatory and Effector T-cell Functions

One of the major goals in immunology research is to understand the regulatory mechanisms that underpin the rapid switch on/off of robust and efficient effector (Teffs) or regulatory (Tregs) T-cell responses. Understanding the molecular mechanisms underlying the regulation of such responses is critical for the development of effective therapies. T-cell activation involves the engagement of T-cell receptor and co-stimulatory signals, but the subsequent recruitment of serine/threonine-specific protein Kinase C-theta (PKC-θ) to the immunological synapse (IS) is instrumental for the formation of signaling complexes, which ultimately lead to a transcriptional network in T cells. Recent studies demonstrated that major differences between Teffs and Tregs occurred at the IS where its formation induces altered signaling pathways in Tregs. These pathways are characterized by reduced recruitment of PKC-θ, suggesting that PKC-θ inhibits Tregs suppressive function in a negative feedback loop. As the balance of Teffs and Tregs has been shown to be central in several diseases, it was not surprising that some studies revealed that PKC-θ plays a major role in the regulation of this balance. This review will examine recent knowledge on the role of PKC-θ in T-cell transcriptional responses and how this protein can impact on the function of both Tregs and Teffs.

The Role of PKC-θ in CD4+ T Cells and HIV Infection: To the Nucleus and Back Again

Protein kinase C (PKC)-θ is the only member of the PKC family that has the ability to translocate to the immunological synapse between T cells and antigen-presenting cells upon T cell receptor and MHC-II recognition. PKC-θ interacts functionally and physically with other downstream effector molecules to mediate T cell activation, differentiation, and migration. It plays a critical role in the generation of Th2 and Th17 responses and is less important in Th1 and CTL responses. PKC-θ has been recently shown to play a role in the nucleus, where it mediates inducible gene expression in the development of memory CD4+ T cells. This novel PKC (nPKC) can up-regulate HIV-1 transcription and PKC-θ activators such as Prostratin have been used in early HIV-1 reservoir eradication studies. The exact manner of the activation of virus by these compounds and the role of PKC-θ, particularly its nuclear form and its association with NF-κB in both the cytoplasmic and nuclear compartments, needs further precise elucidation especially given the very important role of NF-κB in regulating transcription from the integrated retrovirus. Continued studies of this nPKC isoform will give further insight into the complexity of T cell signaling kinases.

Selective protein kinase Cθ (PKCθ) inhibitors for the treatment of autoimmune diseases

Protein kinase Cθ (PKCθ) is a member of a large family of serine/threonine kinases that are involved in diverse cellular functions. PKCθ has roles in T-cell activation and survival, where the dependency of T-cell responses on this enzyme appears to be dictated by both the nature of the antigen and by the inflammatory environment. Studies in PKCθ-deficient mice have demonstrated that although anti-viral responses are PKCθ-independent, T-cell responses associated with autoimmune diseases are PKCθ-dependent. PKCθ-deficient mice are either resistant to or show markedly reduced symptoms in models of MS (multiple sclerosis), IBD (inflammatory bowel disease), arthritis and asthma. Thus potent and selective inhibition of PKCθ has the potential to block T-cell-mediated autoimmunity without compromising anti-viral responses. The present review describes the design and optimization of potent and selective PKCθ inhibitors and their efficacy in both in vitro and in vivo studies. First, our compounds confirm the critical role for PKCθ in T-cell activation and proliferation and secondly they help to demonstrate that murine and human memory T-cell function continues to be dependent on this enzyme. In addition, these inhibitors demonstrate impressive efficacy in treating established autoimmune disease in murine models of IBD and MS.

Analysis of protein kinase C theta inhibitors for the control of HIV-1 replication in human CD4+ T cells reveals an effect on retrotranscription in addition to viral transcription

HIV-1 infection cannot be cured due to reservoirs formed early after infection. Decreasing the massive CD4+ T cell activation that occurs at the beginning of the disease would delay reservoir seeding, providing a better prognosis for patients. CD4+ T cell activation is mediated by protein kinase C (PKC) theta (θ), which is involved in T-cell proliferation, as well as NF-κB, NF-AT, and AP-1 activation. We found that PKCθ activity increased viral replication, but also that HIV-1 induced higher activation of PKCθ in infected CD4+ T cells, creating a feedback loop. Therefore, specific inhibition of PKCθ activity could contribute to control HIV-1 replication. We tested the efficacy of seven PKCθ specific inhibitors to control HIV-1 replication in CD4+ T cells and selected two of the more potent and safer: CGX1079 and CGX0471. They reduced PKCθ phosphorylation at T538 and its translocation to the plasma membrane, which correlated with decreased HIV-1 retrotranscription through partial inhibition of SAMHD1 antiviral activity, rendering lower proviral integration. CGX1079 and CGX0471 also interfered with viral transcription, which would reduce the production of new virions, as well as the subsequent spread and infection of new targets that would increase the reservoir size. CGX1079 and CGX0471 did not completely abrogate T-cell functions such as proliferation and CD8-mediated release of IFN-γ in PBMCs from HIV-infected patients, thereby avoiding general immunosuppresion. Consequently, using PKCθ inhibitors as adjuvant of antiretroviral therapy in recently infected patients would decrease the pool of activated CD4+ T cells, thwarting proviral integration and reducing the reservoir size.

Identification of the activator-binding residues in the second cysteine-rich regulatory domain of protein kinase Cθ (PKCθ)

PKC (protein kinase C) θ is predominantly expressed in T-cells and is critically involved in immunity. Design of PKCθ-selective molecules to manage autoimmune disorders by targeting its activator-binding C1 domain requires the knowledge of its structure and the activator-binding residues. The C1 domain consists of twin C1 domains, C1A and C1B, of which C1B plays a critical role in the membrane translocation and activation of PKCθ. In the present study we determined the crystal structure of PKCθC1B to 1.63 Å (1 Å=0.1 nm) resolution, which showed that Trp(253) at the rim of the activator-binding pocket was orientated towards the membrane, whereas in PKCδC1B the homologous tryptophan residue was orientated away from the membrane. This particular orientation of Trp(253) affects the size of the activator-binding pocket and the membrane affinity. To further probe the structural constraints on activator-binding, five residues lining the activator-binding site were mutated (Y239A, T243A, W253G, L255G and Q258G) and the binding affinities of the PKCθC1B mutants were measured. These mutants showed reduced binding affinities for phorbol ester [PDBu (phorbol 12,13-dibutyrate)] and diacylglycerol [DOG (sn-1,2-dioctanoylglycerol), SAG (sn-1-stearoyl 2-arachidonyl glycerol)]. All five full-length PKCθ mutants exhibited reduced phorbol-ester-induced membrane translocation compared with the wild-type. These results provide insights into the PKCθ activator-binding domain, which will aid in future design of PKCθ-selective molecules.

Protein kinase C signaling and cell cycle regulation

A link between T cell proliferation and the protein kinase C (PKC) family of serine/threonine kinases has been recognized for about 30 years. However, despite the wealth of information on PKC-mediated control of, T cell activation, understanding of the effects of PKCs on the cell cycle machinery in this cell type remains limited. Studies in other systems have revealed important cell cycle-specific effects of PKC signaling that can either positively or negatively impact proliferation. The outcome of PKC activation is highly context-dependent, with the precise cell cycle target(s) and overall effects determined by the specific isozyme involved, the timing of PKC activation, the cell type, and the signaling environment. Although PKCs can regulate all stages of the cell cycle, they appear to predominantly affect G0/G1 and G2. PKCs can modulate multiple cell cycle regulatory molecules, including cyclins, cyclin-dependent kinases (cdks), cdk inhibitors and cdc25 phosphatases; however, evidence points to Cip/Kip cdk inhibitors and D-type cyclins as key mediators of PKC-regulated cell cycle-specific effects. Several PKC isozymes can target Cip/Kip proteins to control G0/G1 → S and/or G2 → M transit, while effects on D-type cyclins regulate entry into and progression through G1. Analysis of PKC signaling in T cells has largely focused on its roles in T cell activation; thus, observed cell cycle effects are mainly positive. A prominent role is emerging for PKCθ, with non-redundant functions of other isozymes also described. Additional evidence points to PKCδ as a negative regulator of the cell cycle in these cells. As in other cell types, context-dependent effects of individual isozymes have been noted in T cells, and Cip/Kip cdk inhibitors and D-type cyclins appear to be major PKC targets. Future studies are anticipated to take advantage of the similarities between these various systems to enhance understanding of PKC-mediated cell cycle regulation in T cells.

PKCθ/β and CYLD are antagonistic partners in the NFκB and NFAT transactivation pathways in primary mouse CD3+ T lymphocytes

In T cells PKCθ mediates the activation of critical signals downstream of TCR/CD28 stimulation. We investigated the molecular mechanisms by which PKCθ regulates NFκB transactivation by examining PKCθ/β single and double knockout mice and observed a redundant involvement of PKCθ and PKCβ in this signaling pathway. Mechanistically, we define a PKCθ-CYLD protein complex and an interaction between the positive PKCθ/β and the negative CYLD signaling pathways that both converge at the level of TAK1/IKK/I-κBα/NFκB and NFAT transactivation. In Jurkat leukemic T cells, CYLD is endoproteolytically processed in the initial minutes of stimulation by the paracaspase MALT1 in a PKC-dependent fashion, which is required for robust IL-2 transcription. However, in primary T cells, CYLD processing occurs with different kinetics and an altered dependence on PKC. The formation of a direct PKCθ/CYLD complex appears to regulate the short-term spatial distribution of CYLD, subsequently affecting NFκB and NFAT repressional activity of CYLD prior to its MALT1-dependent inactivation. Taken together, our study establishes CYLD as a new and critical PKCθ interactor in T cells and reveals that antagonistic PKCθ/β-CYLD crosstalk is crucial for the adjustment of immune thresholds in primary mouse CD3⁺ 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.

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.

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.

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.

Protein kinase C-theta is required for development of experimental cerebral malaria

Cerebral malaria is the most severe neurologic complication in children and young adults infected with Plasmodium falciparum. T-cell activation is required for development of Plasmodium berghei ANKA (PbA)-induced experimental cerebral malaria (CM). To characterize the T-cell activation pathway involved, the role of protein kinase C-theta (PKC-θ) in experimental CM development was examined. PKC-θ-deficient mice are resistant to CM development. In the absence of PKC-θ, no neurologic sign of CM developed after blood stage PbA infection. Resistance of PKC-θ-deficient mice correlated with unaltered cerebral microcirculation and absence of ischemia, as documented by magnetic resonance imaging and magnetic resonance angiography, whereas wild-type mice developed distinct microvascular pathology. Recruitment and activation of CD8(+) T cells, and ICAM-1 and CD69 expression were reduced in the brain of resistant mice; however, the pulmonary inflammation and edema associated with PbA infection were still present in the absence of functional PKC-θ. Resistant PKC-θ-deficient mice developed high parasitemia, and died at 3 weeks with severe anemia. Therefore, PKC-θ signaling is crucial for recruitment of CD8(+) T cells and development of brain microvascular pathology resulting in fatal experimental CM, and may represent a novel therapeutic target of CM.

Protective Toxoplasma gondii-specific T-cell responses require T-cell-specific expression of protein kinase C-theta

Protein kinase C-theta (PKC-theta) is important for the activation of autoreactive T cells but is thought to be of minor importance for T-cell responses in infectious diseases, suggesting that PKC-theta may be a target for the treatment of T-cell-mediated autoimmune diseases. To explore the function of PKC-theta in a chronic persisting infection in which T cells are crucial for pathogen control, we infected BALB/c PKC-theta(-/-) and PKC-theta(+/+) wild-type mice with Toxoplasma gondii. The PKC-theta(-/-) mice succumbed to necrotizing Toxoplasma encephalitis due to an insufficient parasite control up to day 40, whereas the wild-type mice survived. The number of T. gondii-specific CD4 and CD8 T cells was significantly reduced in the PKC-theta(-/-) mice, resulting in the impaired production of protective cytokines (gamma interferon, tumor necrosis factor) and antiparasitic effector molecules (inducible nitric oxide, gamma interferon-induced GTPase) in the spleen and brain. In addition, Th2-cell numbers were reduced in infected the PKC-theta(-/-) mice, paralleled by the diminished GATA3 expression of PKC-theta(-/-) CD4 T cells and reduced T. gondii-specific IgG production in serum and cerebrospinal fluid. Western blot analysis of splenic CD4 and CD8 T cells revealed an impaired activation of the NF-kappaB, AP-1, and MAPK pathways in T. gondii-infected PKC-theta(-/-) mice. Adoptive transfer of wild-type CD4 plus CD8 T cells significantly protected PKC-theta(-/-) mice from death by increasing the numbers of gamma interferon-producing T. gondii-specific CD4 and CD8 T cells, illustrating a cell-autonomous, protective function of PKC-theta in T cells. These findings imply that PKC-theta inhibition drastically impairs T. gondii-specific T-cell responses with fatal consequences for intracerebral parasite control and survival.

Primary clearance of murine gammaherpesvirus 68 by PKCtheta-/- CD8 T cells is compromised in the absence of help from CD4 T cells

CD4 T cells are dispensable for acute control of murine gammaherpesvirus 68 (MHV-68) but are necessary for effective long-term control of the virus by CD8 T cells. In contrast, protein kinase C theta (PKCtheta) is not essential for either acute or long-term viral control. However, we found that while either CD4 or CD8 T cells could mediate the clearance of MHV-68 from the lungs of PKCtheta(+/+) mice, PKCtheta(-/-) mice depleted of either subset failed to clear the virus. These data suggest that there are two alternative pathways for MHV-68 clearance, one dependent on CD4 T cells and the other on PKCtheta. Protection mediated by the latter appears to be short-lived. These observations may help to explain the differential requirement for PKCtheta in various models of CD8 T-cell activation and differences in the costimulatory requirements for acute and long-term viral control.

Identification, characterization and initial hit-to-lead optimization of a series of 4-arylamino-3-pyridinecarbonitrile as protein kinase C theta (PKCtheta) inhibitors

The protein kinase C (PKC) family of serine/threonine kinases is implicated in a wide variety of cellular processes. The PKC theta (PKCtheta) isoform is involved in TCR signal transduction and T cell activation and regulates T cell mediated diseases, including lung inflammation and airway hyperresponsiveness. Thus inhibition of PKCtheta enzyme activity by a small molecule represents an attractive strategy for the treatment of asthma. A PKCtheta high-throughput screening (HTS) campaign led to the identification of 4-(3-bromophenylamino)-5-(3,4-dimethoxyphenyl)-3-pyridinecarbonitrile 4a, a low microM ATP competitive PKCtheta inhibitor. Structure based hit-to-lead optimization led to the identification of 5-(3,4-dimethoxyphenyl)-4-(1H-indol-5-ylamino)-3-pyridinecarbonitrile 4p, a 70 nM PKCtheta inhibitor. Compound 4p was selective for inhibition of novel PKC isoforms over a panel of 21 serine/threonine, tyrosine, and phosphoinositol kinases, in addition to the conventional and atypical PKCs, PKCbeta, and PKCzeta, respectively. Compound 4p also inhibited IL-2 production in antiCD3/anti-CD28 activated T cells enriched from splenocytes.

Protein kinase C isozymes as potential therapeutic targets in immune disorders

BACKGROUND

Members of the protein kinase C (PKC) family are key signalling mediators in immune responses, and pharmacological inhibition of PKCs may be useful for treating immune-mediated diseases.

OBJECTIVE

To review and discuss the insights gained so far into various PKC isozymes and the therapeutic potential and challenges of developing PKC inhibitors for immune disorder therapy.

METHODS

A literature review of the role of PKCs in immune cell signalling and recent studies describing immune functions associated with PKC isozyme deficiency in relevant mouse disease models, followed by specific case studies of current and potential therapeutic strategies targeting PKCs.

RESULTS/CONCLUSION

There is vast amount of data supporting PKC isozymes as attractive drug targets for certain immune disorders. Although the development of specific PKC isozyme inhibitors has been challenging, some progress has been made. It remains to be seen if broad-scale or isozyme-selective inhibition of PKC will have clinical efficacy.

T-cell fate and function: PKC-theta and beyond

The serine/threonine-specific protein kinase C-theta (PKC-theta) is a core component of the immunological synapse that was shown in vitro to play a central role in the activation of T cells after T cell receptor (TCR) and co-stimulatory molecule engagement. In recent years, a series of in vivo studies have shown that the situation is far more complex; specifically, PKC-theta signaling is differentially required for Th1, Th2, Th17 and CD8+ cytotoxic T-cell responses. These studies highlight the combination of signals that directly regulate T-cell differentiation and effector responses. In this review, we highlight recent in vivo studies investigating PKC-theta function and discuss this in the context of how the integration of extrinsic signals determines T cell fate and function.

Natural killer cells from protein kinase C theta-/- mice stimulated with interleukin-12 are deficient in production of interferon-gamma

Protein kinase C theta (PKCtheta) is expressed in NK cells, but its functional role has not been defined. Here, we demonstrate involvement of PKCtheta in IL-12-induced NK cell IFN-gamma production. NK cells from PKCtheta(-/-) mice produced less IFN-gamma in response to IL-12 than those from wild-type (WT) mice. IL-12-induced NK cell cytotoxicity was unaffected, and NK cells from PKCtheta(-/-) mice did not display reduced IFN-gamma production in response to IL-18, indicating a specific role for PKCtheta in IL-12-induced IFN-gamma production. Under the conditions tested, T cells did not produce IFN-gamma in response to IL-12 or affect the ability of NK cells to produce the cytokine. PKCtheta deficiency did not affect NK cell numbers, granularity, viability, or cytotoxic activity in response to polyinosinic:polycytydylic acid. NK cells from PKCtheta(-/-) mice exhibited normal expression of IL-12Rbeta1 and STAT4 proteins and normal induction of STAT4 phosphorylation in response to IL-12. Phosphorylation of threonine 538 within the catalytic domain of PKCtheta was detectable in NK cells from WT mice but was not enhanced by IL-12. Transcription of IFN-gamma increased similarly in NK cells from WT and PKCtheta(-/-) mice in response to IL-12, and there was no difference in IFN-gamma mRNA stability. Taken together, these findings indicate a role for PKCtheta in the post-transcriptional regulation of IL-12-induced IFN-gamma production.

Structure-based optimization of PKCtheta inhibitors

PKCtheta (protein kinase Ctheta) is a central signalling molecule in the T-cell receptor activation pathway and is a target for treatment of a number of diseases. Several PKC inhibitors are in the drug-discovery pharmaceutical programmes today for the treatment of cancer, diabetes and arthritis. CD4(+) T-lymphocytes also play a critical role in the initiation and progression of allergic airway inflammation. Our goal is the development of PKCtheta antagonists as a means to control asthma and autoimmune diseases, using the strategy based on developing small-molecule agents that would block the enzyme's catalytic activity. Here, we discuss our work on the discovery of lead chemical series and review our X-ray structural and modelling approaches, including a structure-surrogate strategy that helped guide us in the lead compound optimizations.

Protein kinase C theta (PKCtheta): a key player in T cell life and death

Protein kinase C theta (PKCtheta) is a member of the novel, Ca(2+)-independent PKC subfamily, which plays an important and non-redundant role in several aspects of T cell biology. Much progress has been accomplished in understanding the function of PKCtheta in the immune system and its unique translocation to the immunological synapse in Ag-stimulated T lymphocytes. Biochemical and genetic approaches revealed that PKCtheta is required for the activation of mature T cells as well as for their survival. Mutation of the PKCtheta gene leads to impaired receptor-induced stimulation of the transcription factors AP-1, NF-kappaB and NFAT, which results in defective T cell activation, and to aberrant expression of apoptosis-related proteins, resulting in poor T cell survival. Furthermore, PKCtheta-deficient mice display defects in the differentiation of T helper subsets, particularly in Th2- and Th17-mediated inflammatory responses. Therefore, PKCtheta is a critical enzyme that regulates T cell function at multiple stages, and it represents an attractive drug target for allergic and autoimmune diseases.

Differential requirement for CD28 and CD80/86 pathways of costimulation in the long-term control of murine gammaherpesvirus-68

The costimulatory molecules CD80 and CD86 (B7-1 and B7-2) are upregulated on mature antigen-presenting cells and interact with positive and negative regulators of CD8 T cell function, CD28 and CD152 (CTLA4) respectively. In this study, we examined the role of CD80 and CD86 in the immune response to murine gammaherpesvirus-68 (MHV-68) using CD80/86-/- mice. As we had previously shown that CD28 (the only known activating receptor for CD80 and 86) is not essential for long-term control of MHV-68, we predicted that CD80 and 86 would also be dispensable for an effective response to this virus. However, surprisingly, we observed that CD80/86-/- mice failed to maintain effective long-term control of MHV-68 and showed viral reactivation in the lungs. We did not observe viral reactivation in mice deficient in either CD80 or CD86 alone, indicating that these molecules play overlapping roles in the long-term control of MHV-68. Antiviral antibody responses were dramatically reduced in CD80/86-/- mice, while CD8 T cell expansion and recruitment to the lungs were not significantly affected. The unexpected disparity in the requirement for CD28 and CD80/86 in the response to MHV-68 suggests that CD28 is not the only positive regulatory receptor for CD80/86.

The role of serine/threonine kinases in T-cell activation

Signalling through serine/threonine kinases is a key mechanism that regulates immune cell development, activation and effector functions. An emerging theme is that serine kinases do not act in isolation, but function in a complex overlapping network. Understanding the molecular targets of serine kinases as well as their links to other serine kinases is key to advancing our understanding of the intracellular signalling pathways that link immune receptors with the gene transcriptional programs that control the immune system in vivo.

Resistance to experimental autoimmune encephalomyelitis and impaired IL-17 production in protein kinase C theta-deficient mice

The protein kinase C theta (PKC theta) serine/threonine kinase has been implicated in signaling of T cell activation, proliferation, and cytokine production. However, the in vivo consequences of ablation of PKC theta on T cell function in inflammatory autoimmune disease have not been thoroughly examined. In this study we used PKC theta-deficient mice to investigate the potential involvement of PKC theta in the development of experimental autoimmune encephalomyelitis, a prototypic T cell-mediated autoimmune disease model of the CNS. We found that PKC theta-/- mice immunized with the myelin oligodendrocyte glycoprotein (MOG) peptide MOG(35-55) were completely resistant to the development of clinical experimental autoimmune encephalomyelitis compared with wild-type control mice. Flow cytometric and histopathological analysis of the CNS revealed profound reduction of both T cell and macrophage infiltration and demyelination. Ex vivo MOG(35-55) stimulation of splenic T lymphocytes from immunized PKC theta-/- mice revealed significantly reduced production of the Th1 cytokine IFN-gamma as well as the T cell effector cytokine IL-17 despite comparable levels of IL-2 and IL-4 and similar cell proliferative responses. Furthermore, IL-17 expression was dramatically reduced in the CNS of PKC theta-/- mice compared with wild-type mice during the disease course. In addition, PKC theta-/- T cells failed to up-regulate LFA-1 expression in response to TCR activation, and LFA-1 expression was also significantly reduced in the spleens of MOG(35-55)-immunized PKC theta-/- mice as well as in in vitro-stimulated CD4+ T cells compared with wild-type mice. These results underscore the importance of PKC theta in the regulation of multiple T cell functions necessary for the development of autoimmune disease.