Targeted cleavage of signaling proteins by caspase 3 inhibits T cell receptor signaling in anergic T cells

Targeting pyroptosis to treat ischemic stroke: From molecular pathways to treatment strategy

Ischemic stroke is the primary reason for human disability and death, but the available treatment options are limited. Hence, it is imperative to explore novel and efficient therapies. In recent years, pyroptosis (a pro-inflammatory cell death characterized by inflammation) has emerged as an important pathological mechanism in ischemic stroke that can cause cell death through plasma membrane rupture and release of inflammatory cytokines. Pyroptosis is closely associated with inflammation, which exacerbates the inflammatory response in ischemic stroke. The level of inflammasomes, GSDMD, Caspases, and inflammatory factors is increased after ischemic stroke, exacerbating brain injury by mediating pyroptosis. Hence, inhibition of pyroptosis can be a therapeutic strategy for ischemic stroke. In this review, we have summarized the relationship between pyroptosis and ischemic stroke, as well as a series of treatments to attenuate pyroptosis, intending to provide insights for new therapeutic targets on ischemic stroke.

VAV3 in human cancers: Mechanism and clinical implication

Guanine nucleotide exchange factors (GEFs) are primarily involved in signal transmission between cell membrane receptors and intracellular mediators. Upon replacing GDP with GTP, GEFs can alter their conformation, resulting in their binding to downstream effectors, such as GTPases like Ras homologous (Rho). VAV GEF family are versatile proteins working as an adaptor mediator and GEF for Rho GTPase. They act as a phosphorylation-dependent molecular switcher, fluctuating between active (tyrosine phosphorylated) and inactive (non-phosphorylated) conformation in cell signaling. Accumulating data showed that VAV3 is implicated in cancer progression. The higher levels of VAV3 in human cancers proposed that it may have an oncogenic role in cancer progression. Available studies demonstrated that VAV3 promoted cell proliferation, epithelial-mesenchymal transition (EMT), colony formation, cell cycle, survival, migration and invasion, and suppressed cell apoptosis. In addition, other studies indicated that VAV3 may have a prognostic value in cancer as well as it may act as a mediator in cancer chemoresistance. Here, we aimed to investigate the underlying molecular mechanism of VAV3 in cancer progression as well as to review its value as a prognostic biomarker and chemoresistance mediator in human cancers.

Flow cytometric detection of IFN-γ production and Caspase-3 activation in CD4+ T lymphocytes to discriminate between healthy and Mycobacterium bovis naturally infected water buffaloes

Tuberculosis has a negative economic impact on buffalo farming, and it poses a potential threat to human health. Interferon-gamma (IFN-γ) plays a central role in protection against mycobacterial diseases, illustrating the importance of T-cell mediated immune responses in tuberculosis infection. Recently, the expression of Caspase-3, a critical executor of apoptosis, in M. tuberculosis-specific IFN-γ⁺CD4⁺ T cells was used as a new marker to distinguish active from latent tuberculosis infection in humans. The aims of this work were to develop a whole blood flow cytometric assay to detect the production of IFN-γ and the activation of Caspase-3 by CD4⁺ T lymphocytes from water buffalo and to evaluate whether these parameters can discriminate between healthy and M. bovis naturally infected buffaloes. A total of 35 Italian Mediterranean buffaloes were grouped in two groups: uninfected and M. bovis infected (based on the results of antemortem diagnostic tests: single intradermal tuberculin (SIT) and ELISA IFN-γ tests). Whole blood was incubated for 6 h with tubercular antigens: PPD-B, PPD-A, ESAT-6/CFP-10 and a new mix of precocious secreted antigens (PA). Our results showed a significant increase in the percentage of IFN-γ⁺CD4⁺ T cells in infected compared to the uninfected animals after each stimulus. Improved sensitivity of the assay was obtained by including the stimulation with the new mix of PA. Interestingly, we observed a concomitant decrease in percentage of Caspase-3⁺CD4⁺ T cells in M. bovis infected animals compared to the control healthy ones, regardless of the stimulus used. Overall, these results showed that M. bovis infection activates CD4⁺ T lymphocytes to produce IFN-γ and at the same time causes a concomitant decrease of Caspase-3 activation in CD4⁺ T cells. This study for the first time in water buffalo describes the development of a whole blood flow cytometric assay for the detection of IFN-γ producing CD4⁺ T cells and proposes the expression of active Caspase-3 as an additional bovine TB biomarker. Although further studies are needed to better understand the mechanisms of Caspase-3-mediated cell death during tuberculosis, our data can help to better understand the cellular immune response to M. bovis infection in buffalo species.

Pyroptosis may play a crucial role in modifications of the immune microenvironment in periodontitis

BACKGROUND AND OBJECTIVE

Published studies proved that both pyroptosis and periodontitis owned a substantial relationship with immunity, and recent research revealed a solid correlation between periodontitis and pyroptosis. While abundant findings have confirmed pyroptosis has a strong impact on the tumor microenvironment, the function of pyroptosis in influencing the periodontitis immune microenvironment remains poorly understood. Thus, we aimed to identify pyroptosis-related genes whose expression signature can well discriminate periodontitis from healthy controls and to comprehend the role of pyroptosis in the periodontitis immune microenvironment.

MATERIALS AND METHODS

The periodontitis-related datasets were acquired from the Gene Expression Omnibus (GEO) database. A series of bioinformatics analyses were conducted to investigate the underlying mechanism of pyroptosis in the periodontitis immune microenvironment. Infiltrating immunocytes, immunological reaction gene sets, and the human leukocyte antigen (HLA) gene were all investigated as potential linkages between periodontitis immune microenvironment and pyroptosis.

RESULTS

Twenty-one pyroptosis-related genes were dysregulated. A four-mRNA combined classification model was constructed, and the receiver operating characteristic (ROC) curve analysis demonstrated its prominent classification capabilities. Subsequently, the mRNA levels of the four hub markers (CYCS, CASP3, NOD2, CHMP4B) were validated by quantitative real-time PCR (qRT-PCR). The correlation coefficients between each hub gene and immune characteristics were calculated, and CASP3 exhibited the most significant correlations with the immune characteristics. Furthermore, distinct pyroptosis-related expression patterns were revealed, along with immunological features of each pattern. Afterward, we discovered 1868 pyroptosis phenotype-related genes, 134 of which were related to immunity. According to the functional enrichment analysis, these 134 genes were closely related to cytokine signaling in immune system, and defense response. Finally, a co-expression network was constructed via the 1868 gene expression profiles.

CONCLUSION

Four hub mRNAs (CYCS, CASP3, NOD2, and CHMP4B) formed a classification model and concomitant results revealed the crucial role of pyroptosis in the periodontitis immune microenvironment, providing fresh insights into the etiopathogenesis of periodontitis and potential immunotherapy.

Balancing between apoptosis and survival biomarkers in the patients with tuberculous meningitis

BACKGROUND

The balancing factor of apoptosis, survival, inflammatory and oxidative stress biomarkers may determine the clinico-radiological severity and death in the patients with tuberculous meningitis (TBM).

AIM

We report the relationship of death [caspase-3, malondialdehyde (MDA), tumor necrosis factor-α (TNFα), interleukin 6 (IL6)] and survival biomarkers [X-linked inhibitory apoptotic protein (XIAP), IL10, glutathione (GSH) and catalase] in TBM, and its role in determining disease severity and death.

METHODS

The diagnosis of TBM was based on clinical, MRI and cerebrospinal fluid (CSF) findings. Their clinical and MRI findings were noted. The severity of TBM was categorized as stages I to III. Serum and CSF caspase-3 and XIAP were measured by ELISA, and TNFα, IL6 and IL10 gene expression in peripheral blood mononuclear cells using RT-PCR (reverse-transcriptase polymerase chain reaction). Plasma MDA, GSH and catalase were measured by spectrophotometer.

RESULTS

There were 40 patients with TBM whose mean age was 31.6 years and 50% were females. TBM patients had higher expression of death (caspase-3, TNFα, IL6, and MDA) and suppression of survival biomarkers (XIAP, catalase and GSH) compared to the healthy controls. Caspase-3 positively correlated with TNFα, IL6 and MDA, and negatively with XIAP, GSH and catalase. Patients with longer duration of illness and definite TBM had higher expression of caspase-3. Patients who died has higher expression of caspase-3 and suppression of XIAP compared to those who survived.

CONCLUSION

It can be concluded from this study that there is up-regulation of death signals and suppression of survival signals in TBM.

RhoG's Role in T Cell Activation and Function

The role of RhoG in T cell development is redundant with other Racs subfamily members, and this redundancy may be attributed to redundant signal transduction pathways. However, the absence of RhoG increases TCR signalling and proliferation, implying that RhoG activity is critical during late T cell activation following antigen–receptor interaction. Moreover, RhoG is required to halt signal transduction and prevent hyper-activated T cells. Despite increase in TCR signalling, cell proliferation is inhibited, implying that RhoG induces T cell anergy by promoting the activities of transcription factors, including nuclear factor of activated T cell (NFAT)/AP-1. The role of NFAT plays in T cell anergy is inducing the transcription of anergy-associated genes, such as IL-2, IL-5, and IFN-γ. Although information about RhoG in T cell-related diseases is limited, mutant forms of RhoG, Ala151Ser and Glu171Lys have been observed in thymoma and hemophagocytic lymphohistiocytosis (HLH), respectively. Current information only focuses on these two diseases, and thus the role of RhoG in normal and pathological circumstances should be further investigated. This approach is necessary because RhoG and its associated proteins represent prospective targets for attack particularly in the therapy of cancer and immune-mediated illnesses.

Anakoinosis: Correcting Aberrant Homeostasis of Cancer Tissue-Going Beyond Apoptosis Induction

The current approach to systemic therapy for metastatic cancer is aimed predominantly at inducing apoptosis of cancer cells by blocking tumor-promoting signaling pathways or by eradicating cell compartments within the tumor. In contrast, a systems view of therapy primarily considers the communication protocols that exist at multiple levels within the tumor complex, and the role of key regulators of such systems. Such regulators may have far-reaching influence on tumor response to therapy and therefore patient survival. This implies that neoplasia may be considered as a cell non-autonomous disease. The multi-scale activity ranges from intra-tumor cell compartments, to the tumor, to the tumor-harboring organ to the organism. In contrast to molecularly targeted therapies, a systems approach that identifies the complex communications networks driving tumor growth offers the prospect of disrupting or "normalizing" such aberrant communicative behaviors and therefore attenuating tumor growth. Communicative reprogramming, a treatment strategy referred to as anakoinosis, requires novel therapeutic instruments, so-called master modifiers to deliver concerted tumor growth-attenuating action. The diversity of biological outcomes following pro-anakoinotic tumor therapy, such as differentiation, trans-differentiation, control of tumor-associated inflammation, etc. demonstrates that long-term tumor control may occur in multiple forms, inducing even continuous complete remission. Accordingly, pro-anakoinotic therapies dramatically extend the repertoire for achieving tumor control and may activate apoptosis pathways for controlling resistant metastatic tumor disease and hematologic neoplasia.

Suboptimal stimulation by weak agonist epitope variants does not drive dysfunction of HIV-1-specific cytotoxic T lymphocyte clones

OBJECTIVE

To assess whether weakly recognized epitope variants induce anergy in HIV-1-specific CD8 T lymphocyte (CTL) clones as a mechanism of dysfunction.

DESIGN

HIV-1-specific CTL clones were exposed to suboptimally recognized epitope variants, and screened for anergy and other T-cell dysfunction markers, and subsequent capability to kill target cells bearing index epitope.

METHODS

In addition to the optimally recognized index epitope, two suboptimally recognized epitope variants were selected based on titration curves for killing of peptide-labeled target cells by three HIV-1-specific CTL clones targeting the epitopes SLYNTVATL (Gag 77-85, A02-restricted), RPAEPVPLQL (Rev 66-75, B07-restricted), and KRWIIMGLNK (Gag 263-272, B27-restricted). Consequences of suboptimal stimulation were assessed by cytokine secretion, gene expression, and capacity to kill index epitope-labeled target cells upon rechallenge.

RESULTS

Suboptimal recognition of epitope variants reduced cytokine production by CTL similarly to reduction in killing of target cells. Gene expression profiles after suboptimal stimulation demonstrated no patterns consistent with T-cell dysfunction due to anergy, exhaustion, or apoptosis. Preexposure of CTL to epitope variants had no discernable impact on their subsequent capacity to kill index epitope-bearing target cells.

CONCLUSION

Our data explore the hypothesis that poorly recognized epitope variants not only facilitate HIV-1 evasion of CTL recognition, but also induce CTL dysfunction through suboptimal signaling causing anergy. However, the results do not suggest that suboptimal signaling induces anergy (or exhaustion or apoptosis), indicating that the major role of CTL epitope variation is likely viral escape.

Bridging the Gap: Modulatory Roles of the Grb2-Family Adaptor, Gads, in Cellular and Allergic Immune Responses

Antigen receptor signaling pathways are organized by adaptor proteins. Three adaptors, LAT, Gads, and SLP-76, form a heterotrimeric complex that mediates signaling by the T cell antigen receptor (TCR) and by the mast cell high affinity receptor for IgE (FcεRI). In both pathways, antigen recognition triggers tyrosine phosphorylation of LAT and SLP-76. The recruitment of SLP-76 to phospho-LAT is bridged by Gads, a Grb2 family adaptor composed of two SH3 domains flanking a central SH2 domain and an unstructured linker region. The LAT-Gads-SLP-76 complex is further incorporated into larger microclusters that mediate antigen receptor signaling. Gads is positively regulated by dimerization, which promotes its cooperative binding to LAT. Negative regulation occurs via phosphorylation or caspase-mediated cleavage of the linker region of Gads. FcεRI-mediated mast cell activation is profoundly impaired in LAT- Gads- or SLP-76-deficient mice. Unexpectedly, the thymic developmental phenotype of Gads-deficient mice is much milder than the phenotype of LAT- or SLP-76-deficient mice. This distinction suggests that Gads is not absolutely required for TCR signaling, but may modulate its sensitivity, or regulate a particular branch of the TCR signaling pathway; indeed, the phenotypic similarity of Gads- and Itk-deficient mice suggests a functional connection between Gads and Itk. Additional Gads binding partners include costimulatory proteins such as CD28 and CD6, adaptors such as Shc, ubiquitin regulatory proteins such as USP8 and AMSH, and kinases such as HPK1 and BCR-ABL, but the functional implications of these interactions are not yet fully understood. No interacting proteins or function have been ascribed to the evolutionarily conserved N-terminal SH3 of Gads. Here we explore the biochemical and functional properties of Gads, and its role in regulating allergy, T cell development and T-cell mediated immunity.

The Vav GEF Family: An Evolutionary and Functional Perspective

Vav proteins play roles as guanosine nucleotide exchange factors for Rho GTPases and signaling adaptors downstream of protein tyrosine kinases. The recent sequencing of the genomes of many species has revealed that this protein family originated in choanozoans, a group of unicellular organisms from which animal metazoans are believed to have originated from. Since then, the Vav family underwent expansions and reductions in its members during the evolutionary transitions that originated the agnates, chondrichthyes, some teleost fish, and some neoaves. Exotic members of the family harboring atypical structural domains can be also found in some invertebrate species. In this review, we will provide a phylogenetic perspective of the evolution of the Vav family. We will also pay attention to the structure, signaling properties, regulatory layers, and functions of Vav proteins in both invertebrate and vertebrate species.

Bridging the Gap: Modulatory Roles of the Grb2-Family Adaptor, Gads, in Cellular and Allergic Immune Responses

Antigen receptor signaling pathways are organized by adaptor proteins. Three adaptors, LAT, Gads, and SLP-76, form a heterotrimeric complex that mediates signaling by the T cell antigen receptor (TCR) and by the mast cell high affinity receptor for IgE (FcεRI). In both pathways, antigen recognition triggers tyrosine phosphorylation of LAT and SLP-76. The recruitment of SLP-76 to phospho-LAT is bridged by Gads, a Grb2 family adaptor composed of two SH3 domains flanking a central SH2 domain and an unstructured linker region. The LAT-Gads-SLP-76 complex is further incorporated into larger microclusters that mediate antigen receptor signaling. Gads is positively regulated by dimerization, which promotes its cooperative binding to LAT. Negative regulation occurs via phosphorylation or caspase-mediated cleavage of the linker region of Gads. FcεRI-mediated mast cell activation is profoundly impaired in LAT- Gads- or SLP-76-deficient mice. Unexpectedly, the thymic developmental phenotype of Gads-deficient mice is much milder than the phenotype of LAT- or SLP-76-deficient mice. This distinction suggests that Gads is not absolutely required for TCR signaling, but may modulate its sensitivity, or regulate a particular branch of the TCR signaling pathway; indeed, the phenotypic similarity of Gads- and Itk-deficient mice suggests a functional connection between Gads and Itk. Additional Gads binding partners include costimulatory proteins such as CD28 and CD6, adaptors such as Shc, ubiquitin regulatory proteins such as USP8 and AMSH, and kinases such as HPK1 and BCR-ABL, but the functional implications of these interactions are not yet fully understood. No interacting proteins or function have been ascribed to the evolutionarily conserved N-terminal SH3 of Gads. Here we explore the biochemical and functional properties of Gads, and its role in regulating allergy, T cell development and T-cell mediated immunity.

High Frequencies of Caspase-3 Expressing Mycobacterium tuberculosis-Specific CD4+ T Cells Are Associated With Active Tuberculosis

Antigen-specific CD4⁺ T cell responses to Mycobacterium tuberculosis (Mtb) infection are important for host defense against tuberculosis (TB). However, Mtb-specific IFN-γ-producing T cells do not distinguish active tuberculosis (ATB) patients from individuals with asymptomatic latent Mtb infection (LTBI). We reasoned that the immune phenotype of Mtb-specific IFN-γ⁺CD4⁺ T cells could provide an indirect gauge of Mtb antigen load within individuals. We sought to identify immune markers in Mtb-specific IFN-γ⁺CD4⁺ T cells and hypothesized that expression of caspase-3 Mtb-specific CD4⁺ T cells would be associated with ATB. Using polychromatic flow cytometry, we evaluated the expression of caspase-3 in Mtb-specific CD4⁺ T cells from LTBI and ATB as well as from ATB patients undergoing anti-TB treatment. We found significantly higher frequencies of Mtb-specific caspase-3⁺IFN-γ⁺CD4⁺ T cells in ATB compared to LTBI. Caspase-3⁺IFN-γ⁺CD4⁺ T cells were also more activated compared to their caspase-3-negative counterparts. Furthermore, the frequencies of caspase-3⁺IFN-γ⁺CD4⁺ T cells decreased in response to anti-TB treatment. Our studies suggest that the frequencies of caspase-3-expressing antigen-specific CD4⁺ T cells may reflect mycobacterial burden in vivo and may be useful for distinguishing Mtb infection status along with other host biomarkers.

NFAT control of immune function: New Frontiers for an Abiding Trooper

Nuclear factor of activated T cells (NFAT) was first described almost three decades ago as a Ca ²⁺/calcineurin-regulated transcription factor in T cells. Since then, a large body of research uncovered the regulation and physiological function of different NFAT homologues in the immune system and many other tissues. In this review, we will discuss novel roles of NFAT in T cells, focusing mainly on its function in humoral immune responses, immunological tolerance, and the regulation of immune metabolism.

A critical appraisal of the role of intracellular Ca2+-signaling pathways in Kawasaki disease

Kawasaki disease is a multi-systemic vasculitis that generally occurs in children and that can lead to coronary artery lesions. Recent studies showed that Kawasaki disease has an important genetic component. In this review, we discuss the single-nucleotide polymorphisms in the genes encoding proteins with a role in intracellular Ca²⁺ signaling: inositol 1,4,5-trisphosphate 3-kinase C, caspase-3, the store-operated Ca²⁺-entry channel ORAI1, the type-3 inositol 1,4,5-trisphosphate receptor, the Na⁺/Ca²⁺ exchanger 1, and phospholipase Cß4 and Cß1. An increase of the free cytosolic Ca²⁺ concentration is proposed to be a major factor in susceptibility to Kawasaki disease and disease outcome, but only for polymorphisms in the genes encoding the inositol 1,4,5-trisphosphate 3-kinase C and the Na⁺/Ca²⁺ exchanger 1, the free cytosolic Ca²⁺ concentration was actually measured and shown to be increased. Excessive cytosolic Ca²⁺ signaling can result in hyperactive calcineurin in T cells with an overstimulated nuclear factor of activated T cells pathway, in hypersecretion of interleukin-1ß and tumor necrosis factor-α by monocytes/macrophages, in increased urotensin-2 signaling, and in an overactivation of vascular endothelial cells.

Glycolysis promotes caspase-3 activation in lipid rafts in T cells

Resting T cells undergo a rapid metabolic shift to glycolysis upon activation in the presence of interleukin (IL)-2, in contrast to oxidative mitochondrial respiration with IL-15. Paralleling these different metabolic states are striking differences in susceptibility to restimulation-induced cell death (RICD); glycolytic effector T cells are highly sensitive to RICD, whereas non-glycolytic T cells are resistant. It is unclear whether the metabolic state of a T cell is linked to its susceptibility to RICD. Our findings reveal that IL-2-driven glycolysis promotes caspase-3 activity and increases sensitivity to RICD. Neither caspase-7, caspase-8, nor caspase-9 activity is affected by these metabolic differences. Inhibition of glycolysis with 2-deoxyglucose reduces caspase-3 activity as well as sensitivity to RICD. By contrast, IL-15-driven oxidative phosphorylation actively inhibits caspase-3 activity through its glutathionylation. We further observe active caspase-3 in the lipid rafts of glycolytic but not non-glycolytic T cells, suggesting a proximity-induced model of self-activation. Finally, we observe that effector T cells during influenza infection manifest higher levels of active caspase-3 than naive T cells. Collectively, our findings demonstrate that glycolysis drives caspase-3 activity and susceptibility to cell death in effector T cells independently of upstream caspases. Linking metabolism, caspase-3 activity, and cell death provides an intrinsic mechanism for T cells to limit the duration of effector function.

Advantages of Extracellular Ubiquitin in Modulation of Immune Responses

T and B lymphocytes play a central role in protecting the human body from infectious pathogens but occasionally they can escape immune tolerance, become activated, and induce autoimmune diseases. All deregulated cellular processes are associated with improper functioning of the ubiquitin-proteasome system (UPS) in eukaryotic cells. The role of ubiquitin in regulation of immune responses and in autoimmune diseases is only beginning to emerge. Ubiquitin is found in intra- and extracellular fluids and is involved in regulation of numerous cellular processes. Extracellular ubiquitin ascribed a role in lymphocyte differentiation. It regulates differentiation and maturation of hematopoietic cell lines. Ubiquitination is involved in initiation, propagation, and termination of immune responses. Disrupted ubiquitination can lead to autoimmunity. Recent observations showed that it can suppress immune response and prevent inflammation. Exogenous ubiquitin may provide good potential as a new tool for targeted therapy for immune mediated disorders of various etiologies.

Extracellular 2'5'-oligoadenylate synthetase 2 mediates T-cell receptor CD3-ζ chain down-regulation via caspase-3 activation in oral cancer

Decreased expression of CD3-ζ chain, an adaptor protein associated with T-cell signalling, is well documented in patients with oral cancer, but the mechanistic justifications are fragmentary. Previous studies in patients with oral cancer have shown that decreased expression of CD3-ζ chain was associated with decreased responsiveness of T cells. Tumours are known to induce localized as well as systemic immune suppression. This study provides evidence that oral tumour-derived factors promote immune suppression by down-regulating CD3-ζ chain expression. 2'5'-Oligoadenylate synthetase 2 (OAS2) was identified by the proteomic approach and our results established a causative link between CD3-ζ chain down-regulation and OAS2 stimulation. The surrogate situation was established by over-expressing OAS2 in a HEK293 cell line and cell-free supernatant was collected. These supernatants when incubated with T cells resulted in down-regulation of CD3-ζ chain, which shows that the secreted OAS2 is capable of regulating CD3-ζ chain expression. Incubation of T cells with cell-free supernatants of oral tumours or recombinant human OAS2 (rh-OAS2) induced caspase-3 activation, which resulted in CD3-ζ chain down-regulation. Caspase-3 inhibition/down-regulation using pharmacological inhibitor or small interfering RNA restored down-regulated CD3-ζ chain expression in T cells induced by cell-free tumour supernatant or rh-OAS2. Collectively these results show that OAS2 leads to impairment in CD3-ζ chain expression, so offering an explanation that might be applicable to the CD3-ζ chain deficiency observed in cancer and diverse disease conditions.

Vav family exchange factors: an integrated regulatory and functional view

The Vav family is a group of tyrosine phosphorylation-regulated signal transduction molecules hierarchically located downstream of protein tyrosine kinases. The main function of these proteins is to work as guanosine nucleotide exchange factors (GEFs) for members of the Rho GTPase family. In addition, they can exhibit a variety of catalysis-independent roles in specific signaling contexts. Vav proteins play essential signaling roles for both the development and/or effector functions of a large variety of cell lineages, including those belonging to the immune, nervous, and cardiovascular systems. They also contribute to pathological states such as cancer, immune-related dysfunctions, and atherosclerosis. Here, I will provide an integrated view about the evolution, regulation, and effector properties of these signaling molecules. In addition, I will discuss the pros and cons for their potential consideration as therapeutic targets.

Lateral intracerebroventricular injection of Apelin-13 inhibits apoptosis after cerebral ischemia/reperfusion injury

Apelin-13 inhibits neuronal apoptosis caused by hydrogen peroxide, yet apoptosis following cerebral ischemia-reperfusion injury has rarely been studied. In this study, Apelin-13 (0.1 μg/g) was injected into the lateral ventricle of middle cerebral artery occlusion model rats. TTC, TUNEL, and immunohistochemical staining showed that compared with the cerebral ischemia/reperfusion group, infarct volume and apoptotic cell number at the ischemic penumbra region were decreased in the Apelin-13 treatment group. Additionally, Apelin-13 treatment increased Bcl-2 immunoreactivity and decreased caspase-3 immunoreactivity. Our findings suggest that Apelin-13 is neuroprotective against cerebral ischemia/reperfusion injury through inhibition of neuronal apoptosis.

Cutting Edge: Differential Regulation of PTEN by TCR, Akt, and FoxO1 Controls CD4+ T Cell Fate Decisions

Signaling via the Akt/mammalian target of rapamycin pathway influences CD4(+) T cell differentiation; low levels favor regulatory T cell induction and high levels favor Th induction. Although the lipid phosphatase phosphatase and tensin homolog (PTEN) suppresses Akt activity, the control of PTEN activity is poorly studied in T cells. In this study, we identify multiple mechanisms that regulate PTEN expression. During Th induction, PTEN function is suppressed via lower mRNA levels, lower protein levels, and an increase in C-terminal phosphorylation. Conversely, during regulatory T cell induction, PTEN function is maintained through the stabilization of PTEN mRNA transcription and sustained protein levels. We demonstrate that differential Akt/mammalian target of rapamycin signaling regulates PTEN transcription via the FoxO1 transcription factor. A mathematical model that includes multiple modes of PTEN regulation recapitulates our experimental findings and demonstrates how several feedback loops determine differentiation outcomes. Collectively, this work provides novel mechanistic insights into how differential regulation of PTEN controls alternate CD4(+) T cell fate outcomes.

Preclinical studies identify non-apoptotic low-level caspase-3 as therapeutic target in pemphigus vulgaris

The majority of pemphigus vulgaris (PV) patients suffer from a live-threatening loss of intercellular adhesion between keratinocytes (acantholysis). The disease is caused by auto-antibodies that bind to desmosomal cadherins desmoglein (Dsg) 3 or Dsg3 and Dsg1 in mucous membranes and skin. A currently unresolved controversy in PV is whether apoptosis is involved in the pathogenic process. The objective of this study was to perform preclinical studies to investigate apoptotic pathway activation in PV pathogenesis with the goal to assess its potential for clinical therapy. For this purpose, we investigated mouse and human skin keratinocyte cultures treated with PV antibodies (the experimental Dsg3 monospecific antibody AK23 or PV patients IgG), PV mouse models (passive transfer of AK23 or PVIgG into adult and neonatal mice) as well as PV patients' biopsies (n=6). A combination of TUNEL assay, analyses of membrane integrity, early apoptotic markers such as cleaved poly-ADP-ribose polymerase (PARP) and the collapse of actin cytoskeleton failed to provide evidence for apoptosis in PV pathogenesis. However, the in vitro and in vivo PV models, allowing to monitor progression of lesion formation, revealed an early, transient and low-level caspase-3 activation. Pharmacological inhibition confirmed the functional implication of caspase-3 in major events in PV such as shedding of Dsg3, keratin retraction, proliferation including c-Myc induction, p38MAPK activation and acantholysis. Together, these data identify low-level caspase-3 activation downstream of disrupted Dsg3 trans- or cis-adhesion as a major event in PV pathogenesis that is non-synonymous with apoptosis and represents, unlike apoptotic components, a promising target for clinical therapy. At a broader level, these results posit that an impairment of adhesive functions in concert with low-level, non-lethal caspase-3 activation can evoke profound cellular changes which may be of relevance for other diseases including cancer.

microRNA-mediated regulation of mTOR complex components facilitates discrimination between activation and anergy in CD4 T cells

Using Dicer-deficient CD4 T cells, Marcais et al. show that microRNAs regulate the expression of mTOR components that are needed to discriminate between activating and anergy-inducing stimuli.

T cell receptor (TCR) signals can elicit full activation with acquisition of effector functions or a state of anergy. Here, we ask whether microRNAs affect the interpretation of TCR signaling. We find that Dicer-deficient CD4 T cells fail to correctly discriminate between activating and anergy-inducing stimuli and produce IL-2 in the absence of co-stimulation. Excess IL-2 production by Dicer-deficient CD4 T cells was sufficient to override anergy induction in WT T cells and to restore inducible Foxp3 expression in Il2-deficient CD4 T cells. Phosphorylation of Akt on S473 and of S6 ribosomal protein was increased and sustained in Dicer-deficient CD4 T cells, indicating elevated mTOR activity. The mTOR components Mtor and Rictor were posttranscriptionally deregulated, and the microRNAs Let-7 and miR-16 targeted the Mtor and Rictor mRNAs. Remarkably, returning Mtor and Rictor to normal levels by deleting one allele of Mtor and one allele of Rictor was sufficient to reduce Akt S473 phosphorylation and to reduce co-stimulation–independent IL-2 production in Dicer-deficient CD4 T cells. These results show that microRNAs regulate the expression of mTOR components in T cells, and that this regulation is critical for the modulation of mTOR activity. Hence, microRNAs contribute to the discrimination between T cell activation and anergy.

ScreenCap3: Improving prediction of caspase-3 cleavage sites using experimentally verified noncleavage sites

Because of its wide range of substrates, caspase-3, a main executioner among apoptosis-related caspases, is thought to have many unknown substrates that have remained unidentified. This report describes our predictive method to facilitate the discovery of novel caspase-3 substrates. To develop a more reliable prediction method, we specifically examined improvement of the data quantity and quality of caspase-3 cleavage sites. The ScreenCap3 method is based on machine learning and on information not only of experimentally verified positive examples but also of negative examples, which were not cleaved by caspase-3. Using information of experimentally verified noncleavage sites, we elucidate novel patterns of amino acids around “actual” cleavage sites. Results show that ScreenCap3 provides substantial improvement in terms of precision, compared with existing methods. Therefore, ScreenCap3 is anticipated for use with proteomic screening and identification of novel caspase-3 substrates and their cleavage sites. ScreenCap3 is available at http://scap.cbrc.jp/ScreenCap3/.

Does antigen masking by ubiquitin chains protect from the development of autoimmune diseases?

Autoimmune diseases are characterized by the production of antibodies against self-antigens and generally arise from a failure of central or peripheral tolerance. However, these diseases may develop when newly appearing antigens are not recognized as self by the immune system. The mechanism by which some antigens are “invisible” to the immune system is not completely understood. Apoptotic and complement system defects or autophagy imbalance can generate this antigenic autoreactivity. Under particular circumstances, cellular debris containing autoreactive antigens can be recognized by innate immune receptors or other sensors and can eventually lead to autoimmunity. Ubiquitination may be one of the mechanisms protecting autoreactive antigens from the immune system that, if disrupted, can lead to autoimmunity. Ubiquitination is an essential post-translational modification used by cells to target proteins for degradation or to regulate other intracellular processes. The level of ubiquitination is regulated during T cell tolerance and apoptosis and E3 ligases have emerged as a crucial signaling pathway for the regulation of T cell tolerance toward self-antigens. I propose here that an unrecognized role of ubiquitin and ubiquitin-like proteins could be to render intracellular or foreign antigens (present in cellular debris resulting from apoptosis, complement system, or autophagy defects) invisible to the immune system in order to prevent the development of autoimmunity.

Induction and stability of the anergic phenotype in T cells

One of the mechanisms that are in place to control the activation of mature T cells that bear self-reactive antigen receptors is anergy, a long-term state of hyporesponsiveness that is established in T cells in response to suboptimal stimulation. T cells receive signals that result not only from antigen recognition and costimulation but also from other sources, including cytokine receptors, inhibitory receptors or metabolic sensors. Integration of those signals will determine T cell fate. Under conditions that induce anergy, T cells activate a program of gene expression that leads to the production of proteins that block T cell receptor signaling and inhibit cytokine gene expression. In this review we will examine those signals that determine functional outcome following antigen encounter, review current knowledge of the factors that ensure signaling inhibition and epigenetic gene silencing in anergic cells and explore the mechanisms that lead to the reversal of anergy and the reacquisition of effector functions.

Tle4 regulates epigenetic silencing of gamma interferon expression during effector T helper cell tolerance

In response to suboptimal activation, T cells become hyporesponsive, with a severely reduced capacity to proliferate and produce cytokines upon reencounter with antigen. Chromatin analysis of T cells made tolerant by use of different in vitro and in vivo approaches reveals that the expression of gamma interferon (IFN-γ) is epigenetically silenced in anergic effector TH1 cells. In those T cells, calcium signaling triggers the expression of Tle4, a member of the Groucho family of corepressors, which is then recruited to a distal regulatory element in the Ifng locus and causes the establishment of repressive epigenetic marks at the Ifng gene regulatory elements. Consequently, impaired Tle4 activity results in a markedly reduced capacity to inhibit IFN-γ production in tolerized T cells. We propose that Blimp1-dependent recruitment of Tle4 to the Ifng locus causes epigenetic silencing of the expression of the Ifng gene in anergic TH1 cells. These results define a novel function of Groucho family corepressors in peripheral T cells and demonstrate that specific mechanisms are activated in tolerant T helper cells to directly repress expression of effector cytokines, supporting the hypothesis that stable epigenetic imprinting contributes to the maintenance of the tolerance-associated hyporesponsive phenotype in T cells.

The membrane adaptor LAT is proteolytically cleaved following Fas engagement in a tyrosine phosphorylation-dependent fashion

Engagement of the TCR (T-cell receptor) induces tyrosine phosphorylation of the LAT (linker for the activation of T-cells) adaptor, and thereby it recruits several cytosolic mediators for downstream signalling pathways. The Fas protein is essential for T-lymphocyte apoptosis, and following Fas engagement, many proteins are proteolytically cleaved, including several molecules that are important for the transduction of TCR intracellular signals. In the present study, we demonstrate that the adaptor LAT is also subject to a proteolytic cleavage in mature T-lymphocytes and thymocytes in response to Fas engagement, and also on TCR stimulation, and we identify three aspartic acid residues at which LAT is cleaved. Interestingly, these aspartic acid residues are located in proximity to several functionally important tyrosine residues of LAT, raising the possibility that their phosphorylation could modulate LAT cleavage. Consistent with that hypothesis, we show that induction of phosphorylation by pervanadate or H2O2 in Jurkat cells and thymocytes inhibits Fas-mediated cleavage of LAT. Moreover, we show that LAT proteolysis is also enhanced during anergy induction of primary human T-cells, suggesting that LAT cleavage may act as a regulator of TCR-mediated activation of T-cells and not only as a transducer of cell death promoting stimuli.

Dynamin 2 potentiates invasive migration of pancreatic tumor cells through stabilization of the Rac1 GEF Vav1

The large GTPase Dynamin 2 (Dyn2) is markedly upregulated in pancreatic cancer, is a potent activator of metastatic migration, and is required for Rac1-mediated formation of lamellipodia. Here we demonstrate an unexpected mechanism of Dyn2 action in these contexts via direct binding to the Rac1 guanine nucleotide exchange factor (GEF) Vav1. Surprisingly, disruption of the Dyn2-Vav1 interaction targets Vav1 to the lysosome for degradation via an interaction with the cytoplasmic chaperone Hsc70, resulting in a dramatic reduction of Vav1 protein stability. Importantly, a specific mutation in Vav1 near its Dyn2-binding C-terminal Src homology 3 (SH3) domain prevents Hsc70 binding, resulting in a stabilization of Vav1 levels. Dyn2 binding regulates the interaction of Vav1 with Hsc70 to control the stability and subsequent activity of this oncogenic GEF. These findings elucidate how Dyn2 activates Rac1, lamellipod protrusion, and invasive cellular migration and provide insight into how this specific Vav is ectopically expressed in pancreatic tumors.

Assessment of caspase mediated degradation of linker for activation of T cells (LAT) at a single cell level

Caspase/Granzyme B mediated protein degradation is involved in elimination of activated T cell receptor (TCR) signaling molecules during processes of thymocyte selection and maintenance of peripheral homeostasis of T cells. Key components of TCR signaling cassette including LAT undergo biological inactivation in response to pro-apoptotic or anergy inducing environmental stimuli. Although available Western immunoblotting-based techniques are appropriate for detection of protein degradation in bulk populations of target cells, quantitative assessment of this process at a single cell level requires a different approach. Here we report on a novel, flow cytometry-based method for assessment of LAT integrity. This method exploits a loss of an anti-LAT antibody epitope recognition following proteolytic degradation of C-terminal domain of the LAT. We show that the LAT degradation precedes phosphatidylserine translocation to the outer leaflet of the plasma membrane and thus may constitute an early marker of T cell apoptosis. When used in conjunction with multi-parameter flow cytometry, our method revealed that FoxP3(+)CD4(+)CD8(low) thymocytes i.e. precursors of thymus derived CD4(+) regulatory T cells, in contrast to Foxp3(-)CD4(+)CD8(low) thymocytes are resistant to LAT degradation in response to CD3ε crosslinking. This finding can be used as an additional marker for T regulatory cell lineage.

Dissecting T cell contraction in vivo using a genetically encoded reporter of apoptosis

Contraction is a critical phase of immunity whereby the vast majority of effector T cells die by apoptosis, sparing a population of long-lived memory cells. Where, when, and why contraction occurs has been difficult to address directly due in large part to the rapid clearance of apoptotic T cells in vivo. To circumvent this issue, we introduced a genetically encoded reporter for caspase-3 activity into naive T cells to identify cells entering the contraction phase. Using two-photon imaging, we found that caspase-3 activity in T cells was maximal at the peak of the response and was associated with loss of motility followed minutes later by cell death. We demonstrated that contraction is a widespread process occurring uniformly in all organs tested and targeting phenotypically diverse T cells. Importantly, we identified a critical window of time during which antigen encounters act to antagonize T cell apoptosis, supporting a causal link between antigen clearance and T cell contraction. Our results offer insight into a poorly explored phase of immunity and provide a versatile methodology to study apoptosis during the development or function of a variety of immune cells in vivo.

The role of ubiquitin ligases in the control of organ specific autoimmunity

Diabetes mellitus is characterized by chronic hyperglycemia caused by a deficiency in insulin action, insulin secretion or both. Type 1 diabetes is classified as the destruction of beta cells leading to a deficiency in insulin production. Type1 diabetes accounts for 5-10% of patients with diabetes and most commonly is caused by the autoimmune destruction of the beta cells in the pancreas. The adaptive immune system is composed of antigen specific T and B lymphocytes which play a central role in protecting the human body from infectious pathogens but occasionally autoreactive T and B cells can escape immune tolerance, become activated and induce autoimmune diseases. Naïve T cells require two distinct signals one delivered via the antigen receptor and the second through the costimulatory receptor CD28 that leads to the induction of IL-2 gene transcription. IL-2 is an important T cell growth factor that can influence both immunity and tolerance. Given its pivotal role it is not surprising that the immune system places strict regulation over Il2 gene transcription that is controlled by a number of E3 ubiquitin ligases that modulate TCR and CD28 signaling. This review will examine how different E3 ligases function to control T effector cell differentiation and how studies in gene knockout animal models has been crucial in understanding how these proteins function in vivo to regulate immune tolerance in the peripheral circulation.

The transcription factors Egr2 and Egr3 are essential for the control of inflammation and antigen-induced proliferation of B and T cells

Lymphocytes provide optimal responses against pathogens with minimal inflammatory pathology. However, the intrinsic mechanisms regulating these responses are unknown. Here, we report that deletion of both transcription factors Egr2 and Egr3 in lymphocytes resulted in a lethal autoimmune syndrome with excessive serum proinflammatory cytokines but also impaired antigen receptor-induced proliferation of B and T cells. Egr2- and Egr3-defective B and T cells had hyperactive signal transducer and activator of transcription-1 (STAT1) and STAT3 while antigen receptor-induced activation of transcription factor AP-1 was severely impaired. We discovered that Egr2 and/or Egr3 directly induced expression of suppressor of cytokine signaling-1 (SOCS1) and SOCS3, inhibitors of STAT1 and STAT3, and also blocked the function of Batf, an AP-1 inhibitor, in B and T cells. Thus, Egr2 and Egr3 regulate B and T cell function in adaptive immune responses and homeostasis by promoting antigen receptor signaling and controlling inflammation.

Role of caspases in cytokine-induced barrier breakdown in human brain endothelial cells

During neuroinflammation, cytokines such as TNF-α and IFN-γ secreted by activated leukocytes and/or CNS resident cells have been shown to alter the phenotype and function of brain endothelial cells (BECs) leading to blood-brain barrier breakdown. In this study, we show that the human BEC line hCMEC/D3 expresses the receptors for TNF-α, TNF receptor 1 and TNF receptor 2, and for IFN-γ. BEC activation with TNF-α alone or in combination with IFN-γ induced endothelial leakage of paracellular tracers. At high cytokine concentrations (10 and 100 ng/ml), this effect was associated with caspase-3/7 activation and apoptotic cell death as evidenced by annexin V staining and DNA fragmentation (TUNEL) assays. In addition, inhibition of JNK and protein kinase C activation at these doses partially prevented activation of caspase-3/7, although only JNK inhibition was partially able to prevent the increase in BEC paracellular permeability induced by cytokines. By contrast, lower cytokine concentrations (1 ng/ml) also led to effector caspase activation, increased paracellular flux, and redistribution of zonula occludens-1 and VE-cadherin but failed to induce apoptosis. Under these conditions, specific caspase-3 and caspase-9, but not caspase-8, inhibitors partially blocked cytokine-induced disruption of tight and adherens junctions and BEC paracellular permeability. Our results suggest that the concentration of cytokines in the CNS endothelial microenvironment determines the extent of caspase-mediated barrier permeability changes, which may be generalized as a result of apoptosis or more subtle as a result of alterations in the organization of junctional complex molecules.

Magnitude of stimulation dictates the cannabinoid-mediated differential T cell response to HIVgp120

Approximately 25% of immunocompromised HIV patients smoke marijuana for its putative therapeutic benefit. The goal of these studies was to test the hypothesis that marijuana-derived cannabinoids have immunomodulatory effects on HIV antigen-specific T cell effector function. A surrogate mouse model to induce polyclonal T cell responses against HIV(gp120) was established. THC, a marijuana-derived cannabinoid, suppressed or enhanced mouse CD8(+) T cell proliferation and the gp120-specific CTL response depending on the magnitude of the IFN-γ response. To determine the molecular mechanisms by which cannabinoids differentially modulate T cell responses, P/I or anti-CD3/CD28 antibodies were used for stimulation, and another marijuana-derived cannabinoid, CBD, was also investigated. THC or CBD suppressed or enhanced IFN-γ and IL-2 production by mouse splenocytes under optimal or suboptimal stimulation, respectively. Similar differential effects of cannabinoids on cytokine production were also observed on nuclear translocation of NFAT and with human PBMCs in response to P/I stimulation. However, THC and CBD elevated intracellular calcium, regardless of the stimulation level with P/I, suggesting that the cannabinoid-induced calcium increase provides an appropriate signal for activation in suboptimally stimulated T cells but an anergic-like signal as a result of excessive calcium in optimally stimulated T cells. Overall, these data demonstrate differential modulation by cannabinoids of a HIV antigen-specific response and identify a possible mechanism responsible for this effect.

Silencing of the Il2 gene transcription is regulated by epigenetic changes in anergic T cells

Anergy is induced in T cells as a consequence of a partial or suboptimal stimulation. Anergic T cells become unresponsive and fail to proliferate and produce cytokines. We had previously shown that in anergic CD4(+) T cells, Ikaros participates in the transcriptional repression of the Il2 gene by recruiting histone deacetylases that cause core histone deacetylation at the Il2 promoter. Here we show that deacetylation at the Il2 promoter is the initial step in a process that leads to the stable silencing of the Il2 gene transcription in anergic T cells. We have found that anergy-induced deacetylation of the Il2 promoter permits binding of the histone methyl-transferase Suv39H1, which trimethylates lysine-9 of histone H3 (Me3H3-K9). Furthermore, the establishment of the Me3H3-K9 mark allows the recruitment of the heterochromatin protein HP1, allowing the silenced Il2 loci to reposition close to heterochromatin-rich regions. Our results indicate that silencing of Il2 transcription in anergic T cells is attained through a series of epigenetic changes that involve the establishment of repressive marks and the subsequent nuclear repositioning of the Il2 loci, which become juxtaposed to transcriptionally silent regions. This mechanism may account for the stable nature of the inhibition of IL-2 production in anergic cells.

PD-1-mediated attrition of polyfunctional memory CD8+ T cells in chronic toxoplasma infection

We reported earlier that during chronic toxoplasmosis CD8(+) T cells become functionally exhausted with concomitant PD-1 upregulation, leading to eventual host mortality. However, how immune exhaustion specifically mediates attrition of CD8 polyfunctionality, a hallmark of potent T-cell response, during persistent infections has not been addressed. In this study, we demonstrate that PD-1 is preferentially expressed on polyfunctional memory CD8(+) T cells, which renders them susceptible to apoptosis. In vitro blockade of the PD-1-PD-L1 pathway dramatically reduces apoptosis of polyfunctional and interferon γ(+)/granzyme B(-) memory but not effector CD8(+) T cells. In summary, the present report underscores the critical role of the PD-1-PD-L1 pathway in mediating attrition of this important CD8(+) T-cell subset and addresses the mechanistic basis of how αPD-L1 therapy reinvigorates polyfunctional CD8 response during chronic infections. The conclusions of this study can have profound immunotherapeutic implications in combating recrudescent toxoplasmosis as well other chronic infections.

Developing a powerful in silico tool for the discovery of novel caspase-3 substrates: a preliminary screening of the human proteome

Background

Caspases are a family of cysteinyl proteases that regulate apoptosis and other biological processes. Caspase-3 is considered the central executioner member of this family with a wide range of substrates. Identification of caspase-3 cellular targets is crucial to gain further insights into the cellular mechanisms that have been implicated in various diseases including: cancer, neurodegenerative, and immunodeficiency diseases. To date, over 200 caspase-3 substrates have been identified experimentally. However, many are still awaiting discovery.

Results

Here, we describe a powerful bioinformatics tool that can predict the presence of caspase-3 cleavage sites in a given protein sequence using a Position-Specific Scoring Matrix (PSSM) approach. The present tool, which we call CAT3, was built using 227 confirmed caspase-3 substrates that were carefully extracted from the literature. Assessing prediction accuracy using 10 fold cross validation, our method shows AUC (area under the ROC curve) of 0.94, sensitivity of 88.83%, and specificity of 89.50%. The ability of CAT3 in predicting the precise cleavage site was demonstrated in comparison to existing state-of-the-art tools. In contrast to other tools which were trained on cleavage sites of various caspases as well as other similar proteases, CAT3 showed a significant decrease in the false positive rate. This cost effective and powerful feature makes CAT3 an ideal tool for high-throughput screening to identify novel caspase-3 substrates.

The developed tool, CAT3, was used to screen 13,066 human proteins with assigned gene ontology terms. The analyses revealed the presence of many potential caspase-3 substrates that are not yet described. The majority of these proteins are involved in signal transduction, regulation of cell adhesion, cytoskeleton organization, integrity of the nucleus, and development of nerve cells.

Conclusions

CAT3 is a powerful tool that is a clear improvement over existing similar tools, especially in reducing the false positive rate. Human proteome screening, using CAT3, indicate the presence of a large number of possible caspase-3 substrates that exceed the anticipated figure. In addition to their involvement in various expected functions such as cytoskeleton organization, nuclear integrity and adhesion, a large number of the predicted substrates are remarkably associated with the development of nerve tissues.

Mechanisms that regulate peripheral immune responses to control organ-specific autoimmunity

The immune system must balance the need to maintain a diverse repertoire of lymphocytes to be able to fight infection with the need to maintain tolerance to self-proteins. The immune system places strict regulation over the ability of T cells to produce the major T cell growth factor interleukin 2 as this cytokine can influence a variety of immune outcomes. T cells require the delivery of two signals, one through the antigen receptor and a second through the costimulatory receptor CD28. The immune system uses a variety of E3 ubiquitin ligases to target signaling proteins that function downstream of the TCR and CD28 receptors. Mutations in these E3 ligases can lead to a breakdown in immune tolerance and development of autoimmunity. This paper will examine the role of a range of E3 ubiquitin ligases and signaling pathways that influence the development of T-cell effector responses and the development of organ-specific autoimmune diseases such as type 1 diabetes.

The role of cytokines in the regulation of drug disposition: extended functional pleiotropism?

INTRODUCTION

Drug disposition, metabolism and drug-drug interactions are important considerations for most drugs. Cytokines are integral to the successful resolution of many diseases. Data are emerging on a role for cytokines in regulation of the expression and activity of drug transporters and drug metabolising enzymes. Investigation of the interaction between pharmacological and immunological responses is key to understanding the complex relationships involved in patient response to therapy.

AREAS COVERED

Evidence detailing the ability of cytokines to regulate drug disposition and metabolism is reviewed in the context of different cell and tissue types. The literature search undertaken provides an overview of the current understanding of the interrelationship between pharmacological and immunological factors which may influence successful drug therapy.

EXPERT OPINION

Dysregulation of cytokines and cytokine networks is a hallmark of a number of diseases such as HIV and cancer. The mechanisms by which the immune system can influence drug disposition are relatively understudied but recent work has highlighted the necessity for examining its impact on pharmacokinetics and pharmacodynamics. A more comprehensive approach in clinical studies will allow better determination of the impact of cytokines on drug disposition. In addition, determining the mechanisms that underpin the differential effects of cytokines across different cell types will clarify the responses reported in these studies.

Caspase-3 is transiently activated without cell death during early antigen driven expansion of CD8(+) T cells in vivo

Background

CD8⁺ T cell responses develop rapidly during infection and are swiftly reduced during contraction, wherein >90% of primed CD8⁺ T cells are eliminated. The role of apoptotic mechanisms in controlling this rapid proliferation and contraction of CD8⁺ T cells remains unclear. Surprisingly, evidence has shown non-apoptotic activation of caspase-3 to occur during in vitro T-cell proliferation, but the relevance of these mechanisms to in vivo CD8⁺ T cell responses has yet to be examined.

Methods and Findings

We have evaluated the activity of caspase-3, a key downstream inducer of apoptosis, throughout the entirety of a CD8⁺ T cell response. We utilized two infection models that differ in the intensity, onset and duration of antigen-presentation and inflammation. Expression of cleaved caspase-3 in antigen specific CD8⁺ T cells was coupled to the timing and strength of antigen presentation in lymphoid organs. We also observed coordinated activation of additional canonical apoptotic markers, including phosphatidylserine exposure. Limiting dilution analysis directly showed that in the presence of IL7, very little cell death occurred in both caspase-3^hi and caspase-3^low CD8⁺ T cells. The expression of active caspase-3 peaked before effector phenotype (CD62L^low) CD8⁺ T cells emerged, and was undetectable in effector-phenotype cells. In addition, OVA-specific CD8⁺ cells remained active caspase-3^low throughout the contraction phase.

Conclusions

Our results specifically implicate antigen and not inflammation in driving activation of apoptotic mechanisms without cell death in proliferating CD8⁺ T cells. Furthermore, the contraction of CD8⁺ T cell response following expansion is likely not mediated by the key downstream apoptosis inducer, caspase-3.

NFAT, immunity and cancer: a transcription factor comes of age

Nuclear factor of activated T cells (NFAT) was first identified more than two decades ago as a major stimulation-responsive DNA-binding factor and transcriptional regulator in T cells. It is now clear that NFAT proteins have important functions in other cells of the immune system and regulate numerous developmental programmes in vertebrates. Dysregulation of these programmes can lead to malignant growth and cancer. This Review focuses on recent advances in our understanding of the transcriptional functions of NFAT proteins in the immune system and provides new insights into their potential roles in cancer development.

Effector T cells driving monophasic vs. relapsing/remitting experimental autoimmune uveitis show unique pathway signatures

Autoimmune diseases often show a relapsing-remitting course. Here we describe characteristics of the autoreactive T cell response in the Lewis rat model of experimental autoimmune uveitis (EAU), a model for the clinical heterogeneity seen in human uveitis. Depending on the autoantigen used, the experimental disease course can be either monophasic or relapsing/remitting. This appears to be dictated by subtle differences in the T cell effector phenotype elicited. Using transcriptomic profiling and pathway analysis, the molecular basis for the monophasic vs. relapsing/remitting effector T cell phenotype was investigated. CD4+ T cell lines specific for peptide R14 derived from interphotoreceptor retinoid-binding protein (IRBP), which mediate the relapsing disease, were compared to the monophasic disease-inducing lines responding to retinal S-antigen peptide PDSAg. Expression profiles from T cell lines representing each specificity were analyzed using Affymetrix microarrays. Differential gene expression was confirmed and extended by quantitative PCR and verified on the protein level. A set of genes was uniquely upregulated in the R14-specific T cells. Gene ontology analysis demonstrated that these genes were linked to regulatory pathways associated with antigen presentation, lymphocyte activation, regulation of apoptosis and WNT/Hedgehog signaling. R14-specific T cells were further demonstrated to have prolonged survival in vivo, and a Th1-dominated cytokine profile, while the PDSAg-specific T cells lines were more Th17-prone. Our findings suggest that the nature of specific antigens leads to subtle programming of the effector phenotype underlying recurrent inflammation.

Common variants in CASP3 confer susceptibility to Kawasaki disease

Kawasaki disease (KD; OMIM 611775) is an acute vasculitis syndrome which predominantly affects small- and medium-sized arteries of infants and children. Epidemiological data suggest that host genetics underlie the disease pathogenesis. Here we report that multiple variants in the caspase-3 gene (CASP3) that are in linkage disequilibrium confer susceptibility to KD in both Japanese and US subjects of European ancestry. We found that a G to A substitution of one commonly associated SNP located in the 5' untranslated region of CASP3 (rs72689236; P = 4.2 x 10(-8) in the Japanese and P = 3.7 x 10(-3) in the European Americans) abolished binding of nuclear factor of activated T cells to the DNA sequence surrounding the SNP. Our findings suggest that altered CASP3 expression in immune effecter cells influences susceptibility to KD.

Regulation of T-cell tolerance by calcium/NFAT signaling

Cells that escape negative selection in the thymus must be inactivated or eliminated in the periphery through a series of mechanisms that include the induction of anergy, dominant suppression by regulatory T cells, and peripheral deletion of self-reactive T cells. Calcium signaling plays a central role in the induction of anergy in T cells, which become functionally inactivated and incapable of proliferating and expressing cytokines following antigen re-encounter. Suboptimal stimulation of T cells results in the activation of a calcium/calcineurin/nuclear factor of activated T cells-dependent cell-intrinsic program of self-inactivation. The proteins encoded by those genes are required to impose a state of functional unresponsiveness through different mechanisms that include downregulation of T-cell receptor signaling and inhibition of cytokine transcription.

Rho family GTPases and their regulators in lymphocytes

Rho family GTPases, and the proteins that regulate them, have important roles in many cellular processes, including cell division, survival, migration and adhesion. Although most of our understanding of these proteins has come from studies using cell lines, more recent gene targeting studies in mice are providing insights into the in vivo function of these proteins. Here we review recent progress revealing crucial roles for these proteins in lymphocyte development, activation, differentiation and migration. The emerging picture shows that Rho family GTPases transduce signals from receptors for antigens, chemokines and cytokines, as well as adhesion molecules and pattern recognition receptors, and that they function as focal points for crosstalk between different signalling pathways.

New insights into the molecular basis of T cell anergy: anergy factors, avoidance sensors, and epigenetic imprinting

The vertebrate immune system has evolved to deal with invasive pathogens, but this adaptation comes at the expense of immunopathology. Among a number of mechanisms that coevolved to control adaptive immunity is anergy, the functional inactivation of T lymphocytes that respond to Ag in the absence of inflammation. In this review, I highlight a series of intracellular proteins in quiescent T cells that function to integrate signals from Ag, costimulatory, and growth factor receptors. These factors ensure that cells that fail to engage all three pathways are shunted into an alternative transcriptional program designed to dissuade them from participating in subsequent immune responses. Recent studies indicate that anergy is the combined result of factors that negatively regulate proximal TCR-coupled signal transduction, together with a program of active transcriptional silencing that is reinforced through epigenetic mechanisms.

Transcriptional complexes formed by NFAT dimers regulate the induction of T cell tolerance

In T cells, anergy can be induced after T cell receptor engagement in the absence of costimulation. Under these conditions, the expression of a specific set of anergy-associated genes is activated. Several lines of evidence suggest that nuclear factor of activated T cells (NFAT) proteins may regulate the expression of many of those genes; however, the nature of the complexes responsible for the induction of this new program of gene expression is unknown. Here, we show that transcriptional complexes formed by NFAT homodimers are directly responsible for the activation of at least two anergy-inducing genes, Grail and Caspase3. Our data shows that Grail expression is activated by direct binding of NFAT dimers to the Grail promoter at two different sites. Consequently, a mutant NFAT protein with impaired ability to dimerize is not able to induce an unresponsive state in T cells. Our results not only identify a new biological function for NFAT dimers but also reveal the different nature of NFAT-containing complexes that induce anergy versus those that are activated during a productive immune response. These data also establish a basis for the design of immunomodulatory strategies that specifically target each type of complex.