MEK activity regulates negative selection of immature CD4+CD8+ thymocytes

Protein Kinase-Mediated Decision Between the Life and Death

Protein kinases are intracellular signaling enzymes that catalyze the phosphorylation of specific residues in their target substrate proteins. They play important role for regulation of life and death decisions. The complexity of the relationship between death receptors and protein kinases' cell death decision-making mechanisms create many difficulties in the treatment of various diseases. The most of fifteen different cell death pathways, which are reported by Nomenclature Committee on Cell Death (NCCD) are protein kinase signal transduction-mediated negative or positive selections. Tumor necrosis factor (TNF) as a main player of death pathways is a dual-functioning molecule in that it can promote both cell survival or cell death. All apoptotic and necrotic signal transductions are conveyed through death domain-containing death receptors, which are expressed on the surface of nearly all human cells. In humans, eight members of the death receptor family have been identified. While the interaction of TNF with TNF Receptor 1 (TNFR1) activates various signal transduction pathways, different death receptors activate three main signal transduction pathways: nuclear factor kappa B (NF-ĸB)-mediated differentiation or pro-inflammatory cytokine synthesis, mitogen-activated protein kinase (MAPK)-mediated stress response and caspase-mediated apoptosis. The link between the NF-ĸB and the c-Jun NH₂-terminal kinase (JNK) pathways comprise another check-point to regulate cell death. TNF-α also promotes the "receptor-interacting serine/threonine protein kinase 1" (RIPK1)/RIPK3/ mixed lineage kinase domain-like pseudokinase (MLKL)-dependent necrosis. Thus, necrosome is mainly comprised of MLKL, RIPK3 and, in some cases, RIPK1. In fact, RIPK1 is at the crossroad between life and death, downstream of various receptors as a regulator of endoplasmic reticulum stress-induced death. TNFR1 signaling complex (TNF-RSC), which contains multiple kinase activities, promotes phosphorylation of transforming growth factor β-activated kinase 1 (TAK1), inhibitor of nuclear transcription factor κB (IκB) kinase (IKK) α/IKKβ, IκBα, and NF-κB. IKKs affect cell-survival pathways in NF-κB-independent manner. Toll-like receptor (TLR) stimulation triggers various signaling pathways dependent on myeloid differentiation factor-88 (MyD88), Interleukin-1 receptor (IL-1R)-associated kinase (IRAK1), IRAK2 and IRAK4, lead to post-translational activation of nucleotide and oligomerization domain (NLRP3). Thereby, cell fate decisions following TLR signaling is parallel with death receptor signaling. Inhibition of IKKα/IKKβ or its upstream activators sensitize cells to death by inducing RIPK1-dependent apoptosis or necroptosis. During apoptosis, several kinases of the NF-κB pathway, including IKK1 and NF-κB essential modulator (NEMO), are cleaved by cellular caspases. This event can terminate the NF-κB-derived survival signals. In both canonical and non-canonical pathways, IKK is key to NF-κB activation. Whereas, the activation process of IKK, the functions of NEMO ubiquitination, IKK-related non-canonical pathway and the nuclear transportation of NEMO and functions of IKKα are still debated in cell death. In addition, cluster of differentiation 95 (CD95)-mediated non-apoptotic signaling and CD95- death-inducing signaling complex (DISC) interactions are waiting for clarification.

'Nur'turing tumor T cell tolerance and exhaustion: novel function for Nuclear Receptor Nur77 in immunity

The nuclear receptor Nur77 is expressed in a multitude of tissues, regulating cell differentiation and homeostasis. Dysregulation of Nur77 signaling is associated with cancer, cardiovascular disease, and disorders of the CNS. The role of Nur77 in T cells has been studied for almost 30 years now. There is a clear appreciation that Nur77 is crucial for apoptosis of self‐reactive T cells. However, the regulation and function of Nur77 in mature T cells remains largely unclear. In an exciting development, Nur77 has been recently demonstrated to impinge on cancer immunotherapy involving chimeric antigen receptor (CAR) T cells and tumor infiltrating lymphocytes (TILs). These studies indicated that Nur77 deficiency reduced T cell tolerance and exhaustion, thus raising the effectiveness of immune therapy in mice. Based on these novel insights, it may be proposed that regulation of Nur77 activity holds promise for innovative drug development in the field of cellular immunotherapy in cancer. In this review, we therefore summarize the role of Nur77 in T cell selection and maturation; and further develop the idea of targeting its activity in these cells as a potential strategy to augment current cancer immunotherapy treatments.

Alteration of Bcl11b upon stimulation of both the MAP kinase- and Gsk3-dependent signaling pathways in double-negative thymocytes

B-cell lymphoma/leukemia 11B (Bcl11b) is a transcription factor critical for thymocyte development. We have previously characterized the kinetic post-translational modifications (PTMs) of Bcl11b in double-positive (DP) thymocytes during stimulation of the T cell receptor-activated MAP kinase pathway. However, the PTMs of Bcl11b in thymocytes from other developmental stages in the thymus, primarily double-negative (DN) cells, have not been previously identified. We found that kinetic modifications of Bcl11b in DN cells are somewhat different than the patterns observed in DP cells. Distinct from DP thymocytes, phosphorylation and sumoylation of Bcl11b in DN cells were not oppositely regulated in response to activation of MAP kinase, even though hyper-phosphorylation of Bcl11b coincided with near complete desumoylation. Additionally, prolonged stimulation of the MAP kinase pathway in DN cells, unlike DP thymocytes, did not alter Bcl11b levels of sumoylation or ubiquitinylation, or stability. On the other hand, activation of Wnt-Gsk3-dependent signaling in DN cells resulted in composite dephosphorylation and sumoylation of Bcl11b. Moreover, stimulation of MAP kinase and (or) Wnt signaling pathways differentially affects gene expression of some Bcl11b target and maturation-associated genes. Defining the signaling pathways and regulation of sequence-specific transcription factors by PTMs at various stages of thymopoiesis may improve our understanding of leukemogenesis.

B-Raf is required for positive selection and survival of DP cells, but not for negative selection of SP cells

The duration of signaling through the MAP kinase (or ERK pathway) cascade has been implicated in thymic development, particularly positive and negative selection. In T cells, two isoforms of the MAP kinase kinase kinase Raf function to transmit signals from the T-cell receptor to ERK: C-Raf and B-Raf. In this study, we conditionally ablated B-Raf expression within thymocytes to assess the effects on ERK activation and thymocyte development. The complete loss of B-Raf is accompanied by a dramatic loss of ERK activation in both the double positive (DP) and single positive (SP) thymocytes, as well as peripheral splenocytes. There was a significant decrease in the cellularity of KO thymi, largely due to a loss of pre-selected DP cells, a decrease in DP cells undergoing positive selection, and a defect in SP maturation. B-Raf plays significant roles in survival of DP thymocytes and function of SP cells in the periphery. Surprisingly, we saw no effect of B-Raf deficiency on negative selection of autoreactive SP thymocytes, despite the greatly reduced ERK activation in these cells.

How the TCR balances sensitivity and specificity for the recognition of self and pathogens

The T cell repertoire is generated during thymic development in preparation for the response to antigens from pathogens. The T cell repertoire is shaped by positive selection, which requires recognition by the T cell antigen receptor (TCR) of complexes of self peptide and major histocompatibility complex proteins (self-pMHC) with low affinity, and negative selection, which eliminates T cells with TCRs that recognize self-pMHC with high affinity. This generates a repertoire with low affinity for self-pMHC but high affinity for foreign antigens. The TCR must successfully engage both of these ligands for development, homeostasis and immune responses. This review discusses mechanisms underlying the interaction of the TCR with peptide-major histocompatibility complex ligands of varying affinity and highlights signaling mechanisms that enable the TCR to generate different responses to very distinct ligands.

Role for Ets-2(Thr-72) transcription factor in stage-specific thymocyte development and survival

Interference of Ras signaling deregulates thymocyte development in mouse models. However, the role of Ets-2, a transcription factor that is phosphorylated on a critical threonine residue (Thr-72) by the Ras/MAPK pathway in thymocyte development, has not been defined. Transgenic mice overexpressing a phosphomutant Ets-2 (T72A) in the thymus displayed reduced thymus size associated with a 60-80% reduction in thymocyte populations. The transgenic mice exhibited a 20-fold increase in a c-Kit(+) CD4(+) CD8(+) CD3(-) population and a 5-fold increase in a unique CD5(low) population associated with a partial developmental block at the DN2-DN3 stage of thymocytes. Transgenic thymocytes exhibited increased apoptosis, and overexpression of Bcl-2 rescued the hypocellularity and associated thymocyte developmental block in double transgenic mice. The observed defects in these mice are not dependent on Ets-1 expression. These studies implicate for the first time a stage-specific Ets-1-independent regulatory role for Ets-2 in early thymocyte development and survival.

Role of Ras/Raf/MEK/ERK signaling in physiological hematopoiesis and leukemia development

Recent research on hematological malignancies has shown that malignant cells often co-opt physiological pathways to promote their growth and development. Bone marrow homeostasis requires a fine balance between cellular differentiation and self-renewal; cell survival and apoptosis; and cellular proliferation and senescence. The Ras/Raf/MEK/ERK pathway has been shown to be important in regulating these biological functions. Moreover, the Ras/Raf/MEK/ERK pathway has been estimated to be mutated in 30% of all cancers, thus making it the focus of many scientific studies which have lead to a deeper understanding of cancer development and help to elucidate potential weaknesses that can be targeted by pharmacological agents [1]. In this review, we specifically focus on the role of this pathway in physiological hematopoiesis and how augmentation of the pathway may lead to hematopoietic malignancies. We also discuss the challenges and success of targeting this pathway.

A role for p120 RasGAP in thymocyte positive selection and survival of naive T cells

Activation of the Ras small GTP-binding protein is necessary for normal T cell development and function. However, it is unknown which Ras GTPase-activating proteins (RasGAPs) inactivate Ras in T cells. We used a T cell-specific RASA1-deficient mouse model to investigate the role of the p120 RasGAP (RASA1) in T cells. Death of CD4(+)CD8(+) double-positive thymocytes was increased in RASA1-deficient mice. Despite this finding, on an MHC class II-restricted TCR transgenic background, evidence was obtained for increased positive selection of thymocytes associated with augmented activation of the Ras-MAPK pathway. In the periphery, RASA1 was found to be dispensable as a regulator of Ras-MAPK activation and T cell functional responses induced by full agonist peptides. However, numbers of naive T cells were substantially reduced in RASA1-deficient mice. Loss of naive T cells in the absence of RASA1 could be attributed in part to impaired responsiveness to the IL-7 prosurvival cytokine. These findings reveal an important role for RASA1 as a regulator of double-positive survival and positive selection in the thymus as well as naive T cell survival in the periphery.

Kinase suppressor of Ras 1 is required for full ERK activation in thymocytes but not for thymocyte selection

The scaffold protein kinase suppressor of Ras 1 (KSR1) is critical for efficient activation of ERK in a number of cell types. Consistent with this, we observed a defect in ERK activation in thymocytes that lack KSR1. Interestingly, we found that the defect was much greater after PMA stimulation than by CD3 activation. Since ERK activation is believed to be important for thymocyte development, we analyzed thymocyte selection in KSR1-deficient (KSR1(-/-) ) mice. We found that positive selection in two different TCR transgenic models, HY and AND, was normal. On the other hand, negative selection in the HY model was slightly impaired in KSR1(-/-) mice. However, a defect in negative selection was not apparent in the AND TCR model system or in an endogenous superantigen-mediated model of negative selection. These results suggest that, despite a requirement for KSR1 for full ERK activation in thymocytes, full and efficient ERK activation is not essential for the majority of thymocyte selection events.

Prosurvival and proapoptotic functions of ERK1/2 activation in murine thymocytes in vitro

The extracellular signal-regulated kinases 1/2 (ERK1/2) are serine/threonine-selective protein kinases involved in proliferation and differentiation of cells, including thymocytes. The requirement of ERK1/2 for thymocyte differentiation and maturation has been well established; however, their role in regulating thymocyte survival and apoptosis has not been resolved. Here, we asked whether ERK1/2 affected thymocyte survival in vitro in response to apoptotic stimuli. The results show that phorbol 12-myristate 13-acetate (PMA) treatment (with or without ionomycin) and serum starvation (s/s) induced sustained ERK1/2 activation in murine thymocytes. Importantly, pharmacological treatment of thymocytes with the MEK inhibitor UO126 revealed that PMA-induced ERK1/2 activation was proapoptotic, whereas serum starvation-induced ERK1/2 activation inhibited apoptosis and promoted cell survival. While basal MEK activity was required for both s/s- and PMA-induced ERK1/2 activation, MEK activity increased only in response to PMA. The results show that the suppression of ERK1/2 phosphatases was responsible for s/s-induced sustained ERK1/2 activation. Unexpectedly, neither s/s-induced proapoptotic nor PMA-induced anti-apoptotic functions of ERK1/2 depended on the Bcl-2 family phosphoprotein Bim(EL), which was previously implicated in thymocyte apoptosis. Lastly, etoposide treatment of immature thymocytes induced both p53 and ERK1/2 activation, but ERK1/2 activity did not affect the phosphorylation and stabilization of p53. Thus, ERK1/2 has a dual role in promoting cell survival and cell death in thymocytes in the context of different stimuli.

Cutting edge: Extracellular signal-related kinase is not required for negative selection of developing T cells

Signals initiated through the TCR during development can result in either survival and differentiation or cell death. High affinity signals that induce death elicit a robust yet transient activation of signaling pathways, including Erk, whereas low affinity ligands, which promote survival, generate a gradual and weaker activation of the same pathways. It was recently demonstrated that Erk localizes to distinct cellular locations in response to high and low affinity ligands. Although a requirement for Erk in positive selection is well established, its role in negative selection is controversial and, thus, the importance of Erk relocalization during development is not understood. In this study, we examined the role of Erk in negative selection using mice that are genetically deficient in both Erk1 and Erk2 in T cells. Results from three different models reveal that thymocyte deletion remains intact in the absence of Erk.

Doxycycline enhances the Ras-MAPK signaling and proliferation of mouse thymic epithelial cells

Depletion of T-cell-dependent immunity is a major consideration for patients suffering from infections of human immunodeficiency virus (HIV), those undergoing organ transplantation, and those receiving anti-cancer chemotherapy and/or radiotherapy. In general, T-cell regeneration occurs in the thymus through thymopoiesis. We have found that doxycycline (Dox), a tetracycline derivative, enhances the proliferation of mouse thymic epithelial cells, which are unique in their capacity to support positive selection and are essential throughout the development of thymocytes. Cell cycle analysis indicates that the increased cell proliferation is due to a shortened G(0)/G(1) phase. To reveal the underlying mechanisms, we examined the expression of an array of molecules that regulate the cell cycle. The results show that in mouse thymic medullary-type epithelial cell line 1 (MTEC1) Dox leads to elevated levels of H-Ras, phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2), cyclin E, cyclin dependent kinase 4/2 (CDK4/CDK2), E2F3, and c-myc. These data, and the observation that the proliferation-enhancing effect is largely abolished following treatment with an ERK inhibitor support an active role of the Ras-ERK/mitogen-activated protein kinase (MAPK) signaling pathway. In conclusion, the present study reveals a new activity of an old family of antibiotics. The in vivo effect of Dox on immune reconstitution warrants further exploration.

T-cell receptor signaling is mediated by transient Lck activity, which is inhibited by inorganic mercury

Genetically susceptible rodents exposed to low nontoxic levels of inorganic mercury (Hg(2+)) develop idiosyncratic autoimmune disease associated with defective T-cell function. However, the molecular mechanisms underlying this phenomenon remain mostly unexplained. Brief exposure of T cells to micromolar concentrations of Hg(2+) leads to physiologically relevant nontoxic cellular mercury burdens, and as we have previously reported, attenuates T-cell receptor (TCR) signal strength by approximately 50%. We have found this to be the result of an inadequate activation of the tyrosine kinase ZAP-70, which is hypophosphorylated following TCR stimulation in Hg(2+) burdened cells when compared to untreated controls. In T cells, ZAP-70 phosphorylation is dependent on lymphocyte-specific protein tyrosine kinase (Lck) activity, which in turn is either positively or negatively regulated by the phosphorylation of specific Lck tyrosine residues. In particular, the general belief is that Lck is negatively regulated by phosphorylation of tyrosine 192 (Y192). We now demonstrate by Western blotting that, in Jurkat T cells, TCR signal transduction (and ZAP-70 phosphorylation) was positively associated with a rapid transient phosphorylation of Y192, which was inhibited in cells that were briefly (5 min) exposed to 5 microM Hg(2+). Thus, Hg(2+) inhibits a critical activating role played by Lck Y192 during the most proximal events of the TCR-induced cell signaling.

TGF-beta 1 inhibition of IFN-gamma-induced signaling and Th1 gene expression in CD4+ T cells is Smad3 independent but MAP kinase dependent

In addition to classic Smad signaling pathways, the pleiotropic immunoregulatory cytokine TGF-beta1 can activate MAP kinases, but a role for TGF-beta1-MAP kinase pathways in T cells has not been defined heretofore. We have shown previously that TGF-beta1 inhibits Th1 development by inhibiting IFN-gamma's induction of T-bet and other Th1 differentiation genes, and that TGF-beta1 inhibits receptor-proximal IFN-gamma-Jak-Stat signaling responses. We now show that these effects of TGF-beta1 are independent of the canonical TGF-beta1 signaling module Smad3, but involve a specific MAP kinase pathway. In primary T cells, TGF-beta1 activated the MEK/ERK and p38 MAP kinase pathways, but not the JNK pathway. Inhibition of the MEK/ERK pathway completely eliminated the inhibitory effects of TGF-beta1 on IFN-gamma responses in T cells, whereas inhibition of the p38 pathway had no effect. Thus, TGF-beta1's inhibition of IFN-gamma signaling in T cells is mediated through a highly specific Smad3 independent, MEK/ERK-dependent signaling pathway.

Thymic selection threshold defined by compartmentalization of Ras/MAPK signalling

A healthy individual can mount an immune response to exogenous pathogens while avoiding an autoimmune attack on normal tissues. The ability to distinguish between self and non-self is called 'immunological tolerance' and, for T lymphocytes, involves the generation of a diverse pool of functional T cells through positive selection and the removal of overtly self-reactive thymocytes by negative selection during T-cell ontogeny. To elucidate how thymocytes arrive at these cell fate decisions, here we have identified ligands that define an extremely narrow gap spanning the threshold that distinguishes positive from negative selection. We show that, at the selection threshold, a small increase in ligand affinity for the T-cell antigen receptor leads to a marked change in the activation and subcellular localization of Ras and mitogen-activated protein kinase (MAPK) signalling intermediates and the induction of negative selection. The ability to compartmentalize signalling molecules differentially in the cell endows the thymocyte with the ability to convert a small change in analogue input (affinity) into a digital output (positive versus negative selection) and provides the basis for establishing central tolerance.

The role of erk1 and erk2 in multiple stages of T cell development

Activation of extracellular-signal-regulated protein kinase (Erk) is central to growth-factor-receptor-mediated signaling including that originating from the T cell antigen receptor. It integrates cytoplasmic signals to effect changes in transcription associated with differentiation, proliferation, and survival. In this report, we present an analysis of mice with targeted deletions in Erk1 and Erk2 to assess the relationship between Erk activity and cell-cycle progression, thymocyte development, and lineage commitment. These studies show that Erk is selectively retained during beta selection-driven proliferation, and yet Erk1/2 are not required to complete differentiation to CD4+CD8+ preselection stage of development. Erk activity is essential for the process of positive selection, and it differentially affects CD4 and CD8 T cell maturation; yet, diminished expression itself is not sufficient to alter lineage commitment.

The molecular scaffold kinase suppressor of Ras 1 (KSR1) regulates adipogenesis

Mitogen-activated protein kinase pathways are implicated in the regulation of cell differentiation, although their precise roles in many differentiation programs remain elusive. The Raf/MEK/extracellular signal-regulated kinase (ERK) kinase cascade has been proposed to both promote and inhibit adipogenesis. Here, we titrate expression of the molecular scaffold kinase suppressor of Ras 1 (KSR1) to regulate signaling through the Raf/MEK/ERK/p90 ribosomal S6 kinase (RSK) kinase cascade and show how it determines adipogenic potential. Deletion of KSR1 prevents adipogenesis in vitro, which can be rescued by introduction of low levels of KSR1. Appropriate levels of KSR1 coordinate ERK and RSK activation with C/EBPbeta synthesis leading to the phosphorylation and stabilization of C/EBPbeta at the precise moment it is required within the adipogenic program. Elevated levels of KSR1 expression, previously shown to enhance cell proliferation, promote high, sustained ERK activation that phosphorylates and inhibits peroxisome proliferator-activated receptor gamma, inhibiting adipogenesis. Titration of KSR1 expression reveals how a molecular scaffold can modulate the intensity and duration of signaling emanating from a single pathway to dictate cell fate.

The transmembrane adapter protein SIT regulates thymic development and peripheral T-cell functions

SIT is a transmembrane adapter protein that modulates signals emanating from the T-cell receptor (TCR). Here, we have used gene-targeted mice to assess the role of SIT for T-cell development and peripheral T-cell functions. SIT(-/-) double-positive thymocytes show an upregulation of the activation markers CD5 and CD69, suggesting that SIT negatively regulates TCR-mediated signals at the CD4(+) CD8(+) stage of thymic development. This assumption is further supported by the observation that in female H-Y TCR transgenic mice, positive selection is enhanced and even converted to negative selection. Similarly, mature peripheral T cells are hyperresponsive towards TCR-mediated stimuli and produce larger amounts of T-helper 1 (TH1) cytokines, and SIT-deficient mice show an increased susceptibility to develop experimental autoimmune encephalomyelitis, a mouse model of multiple sclerosis. These results demonstrate that SIT is a critical negative regulator of TCR-mediated signaling and finely tunes the signals required for thymic selection and peripheral T-cell activation.

A requirement for sustained ERK signaling during thymocyte positive selection in vivo

It is unknown how the contrasting events of positive and negative selection can lead to the distinct biological outcomes of life or death. An increasing body of evidence suggests that the duration of extracellular signal-regulated kinase (ERK) signaling plays a role in thymocyte selection. However, it remains unclear what the kinetics of ERK activation are during positive selection in vivo. In this study, we examined the magnitude and duration of ERK signaling in intact murine thymic tissues cultured under conditions of negative or positive selection. We found that negative selection induced a rapid and robust ERK activation that is associated with death, whereas positive selection stimulated a lower intensity and brief ERK activation that quickly declined and then gradually increased and was sustained over several days. The expression pattern of Egr-1 (early growth response-1), a downstream ERK effector, correlates with the biphasic kinetics of ERK during positive selection. Id3 (inhibitor of differentiation/DNA binding 3) also exhibits biphasic kinetics but appeared to be independent of ERK signaling. Furthermore, inhibitors of T cell receptor ligation and ERK activation block maturation of CD8 single-positive thymocytes even when added after 24 h. These results demonstrate that the in vivo duration of ERK signaling must be sustained to support positive selection.

Development of autoreactive diabetogenic T cells in the thymus of NOD mice

Type 1 diabetes results from destruction of pancreatic beta cells by beta cell-specific autoreactive T cells in the nonobese diabetic (NOD) mouse. Defects in thymic negative selection are thought to result in failure to delete potential beta cell-reactive T cells, contributing to the development of autoimmune diabetes. We investigated this possibility by comparing the deletion profile of double-positive (DP) thymocytes in NOD mice with diabetes-resistant strains of mice after anti-CD3 Ab treatment to trigger the TCR-mediated signaling pathway. We found that immature NOD CD4+CD8+ DP thymocytes have a lower activation threshold than C57BL/6 and Balb/c thymocytes. This was confirmed by showing that NOD DP thymocytes have a higher level of ERK and JNK phosphorylation. The low activation threshold of immature thymocytes resulted in rapid deletion of strongly activated immature DP thymocytes by negative selection, whereas weakly activated immature thymocytes differentiated more efficiently into CD69+CD3high DP thymocytes by positive selection. SP thymocytes, particularly CD4-CD8+ T cells that were efficiently generated from activated DP thymocytes, could induce severe insulitis and diabetes in NOD.scid mice. We conclude that the development of autoreactive diabetogenic T cells results from inordinate positive selection due to the low activation threshold of DP thymocytes in NOD mice.

Role of CTLA-4 in the activation of single- and double-positive thymocytes

CTLA-4, a homologue of CD28, is a negative regulator of T cell activation in the periphery and is transiently expressed on the cell surface after T cell activation. However, the role of CTLA-4 in T cell activation in the thymus is not clear. This investigation was initiated to determine the role of CTLA-4 in the activation of CD4(+)CD8(+) double-positive (DP) and CD4(+)CD8(-) and CD4(-)CD8(+) single-positive (SP) thymocytes using fetal thymic organ cultures (FTOC) of MHC class II-restricted, OVA(323-339)-restricted TCR transgenic mice (DO11.10). We found that treatment of the FTOC with anti-CTLA-4-blocking Ab during activation with OVA(323-339) increased the proportion and number of DP thymocytes, but decreased the proportion and number of SP thymocytes compared with OVA(323-339)-stimulated FTOC without anti-CTLA-4 Ab treatment. In addition, anti-CTLA-4 Ab treatment inhibited OVA(323-339)-induced expression of the early activation marker, CD69, in DP thymocytes, but increased CD69 in SP thymocytes. Similarly, CTLA-4 blockage decreased phosphorylation of ERK in DP thymocytes by Ag-specific TCR engagement, but increased phosphorylation of ERK in SP thymocytes. CTLA-4 blockage inhibited deletion of DP thymocytes treated with a high dose of OVA(323-339), whereas CTLA-4 blockage did not inhibit deletion of DP thymocytes treated with a low dose of OVA(323-339). We conclude that CTLA-4 positively regulates the activation of DP thymocytes, resulting in their deletion, whereas blocking CTLA-4 suppresses the activation of DP thymocytes, leading to inhibition of DP thymocyte deletion. In contrast, CTLA-4 negatively regulates the activation of SP thymocytes.

B-Raf contributes to sustained extracellular signal-regulated kinase activation associated with interleukin-2 production stimulated through the T cell receptor

A T cell receptor (TCR) recognizes and responds to an antigenic peptide in the context of major histocompatibility complex-encoded molecules. This provokes T cells to produce interleukin-2 (IL-2) through extracellular signal-regulated kinase (ERK) activation. We investigated the roles of B-Raf in TCR-mediated IL-2 production coupled with ERK activation in the Jurkat human T cell line. We found that TCR cross-linking could induce up-regulation of both B-Raf and Raf-1 activities, but Raf-1 activity was decreased rapidly. On the other hand, TCR-stimulated kinase activity of B-Raf was sustained. Expression of a dominant-negative mutant of B-Raf abrogated sustained but not transient TCR-mediated MEK/ERK activation. The inhibition of sustained ERK activation by either expression of a dominant-negative B-Raf or treatment with a MEK inhibitor resulted in a decrease of the TCR-stimulated nuclear factor of activated T cells (NFAT) activity and IL-2 production. Collectively, our data provide the first direct evidence that B-Raf is a positive regulator of TCR-mediated sustained ERK activation, which is required for NFAT activation and the full production of IL-2.

Ternary complex factor SAP-1 is required for Erk-mediated thymocyte positive selection

Thymocyte selection and differentiation requires extracellular signal-regulated kinase (Erk) signaling, but transcription factor substrates of Erk in thymocytes are unknown. We have characterized the function of SAP-1 (Elk4), an Erk-regulated transcription factor, in thymocyte development. Early thymocyte development was normal, but single-positive thymocyte and peripheral T cell numbers were reduced, reflecting a T cell-autonomous defect. T cell receptor-induced activation of SAP-1 target genes such as Egr1 was substantially impaired in double-positive thymocytes, although Erk activation was normal. Analysis of T cell receptor transgenes showed that positive selection was reduced by 80-90% in SAP-1-deficient mice; heterozygous mice showed a moderate defect. Negative selection was unimpaired. SAP-1 thus directly links Erk signaling to the transcriptional events required for thymocyte positive selection.

Vav1 transduces T cell receptor signals to the activation of the Ras/ERK pathway via LAT, Sos, and RasGRP1

Vav1 is a signaling protein required for both positive and negative selection of CD4(+)CD8(+) double positive thymocytes. Activation of the ERK MAPK pathway is also required for positive selection. Previous work has shown that Vav1 transduces T cell receptor (TCR) signals leading to an intracellular calcium flux. We now show that in double positive thymocytes Vav1 is required for TCR-induced activation of the ERK1 and ERK2 kinases via a pathway involving the Ras GTPase, and B-Raf, MEK1, and MEK2 kinases. Furthermore, we show that Vav1 transduces TCR signals to Ras by controlling the membrane recruitment of two guanine nucleotide exchange factors. First, Vav1 transduces signals via phospholipase Cgamma1 leading to the membrane recruitment of RasGRP1. Second, Vav1 is required for recruitment of Sos1 and -2 to the transmembrane adapter protein LAT. Finally, we show that Vav1 is required for TCR-induced LAT phosphorylation, a key event for the activation of both phospholipase Cgamma1 and Sos1/2. We propose that reduced LAT phosphorylation is the key reason for defective TCR-induced calcium flux and ERK activation in Vav1-deficient cells.

Involvement of MKK6 in TCRalphabeta(int)CD69lo: a target population for apoptotic cell death in thymocytes

By analyzing real-time caspase activity and DNA fragmentation in live thymocytes, we found that apoptosis occurs predominantly in a TCRalphabeta(int)/hiCD69lo population. The number of caspase-active cells and DNA-fragmented cells in MKK6-deficient mice, which were originally generated in our laboratory by gene targeting, was decreased in the TCRalphabeta(int)CD69lo population but not in the TCRalphabetahiCD69lo population. The percentage of caspase-active cells in the H-Y-specific TCRint population was more clearly decreased in male MKK6-deficient H-Y TCR-transgenic mice. Furthermore, the absolute number of TCRhiCD4loCD8lo cells, which are developmentally next to TCRintCD4hiCD8hi cells, was increased in MKK6-deficient H-Y TCR-transgenic mice. Deletion of TCRalphabeta(int)CD4hiCD8hi cells by injecting antigenic lymphocytic chorio-meningitis virus (LCMV) peptide into LCMV-specific TCR-transgenic mice was incomplete in MKK6-deficient mice. Cellular death of TCRalphabeta(int) fetal thymocytes induced by adding an antigenic peptide into an in vitro fetal thymic organ culture system was also diminished in MKK6-deficient TCR-transgenic thymi. These results indicate that MKK6 plays a role in the developing thymocytes, especially in the population of TCRalphabeta(int)CD69lo cells, which possibly undergo negative selection.

Characterization of transcriptional regulation during negative selection in vivo

Negative selection is the process whereby immature thymocytes expressing TCRs with high affinity for self-peptide:MHC complexes are induced to undergo apoptosis. The transcriptional events that occur as a result of TCR signaling during negative selection are not well-characterized. Using oligonucleotide arrays, we have identified 33 genes that exhibit changes in RNA levels in CD4(+)CD8(+) thymocytes during negative selection in vivo. Of 18 genes that have been further characterized, 13 are regulated in response to stimulation with Ag or anti-CD3 and anti-CD28 Abs ex vivo, indicating that these genes are regulated independently of activation of the peripheral immune system. These data also support the idea that anti-CD3/CD28-mediated thymocyte apoptosis is a valid model for negative selection in vivo. A detailed examination of the regulation of many of the identified genes in response to treatment with dexamethasone or gamma-radiation or in response to anti-CD3/anti-CD28 stimulation in the presence of pharmacological inhibitors of mitogen-activated protein kinase kinase kinase 1, p38 mitogen-activated protein kinase, phosphatidylinositol 3-kinase, calcineurin, and cyclin-dependent kinase 2 has facilitated the elucidation of a map of the transcriptional events that occur downstream of the TCR. These studies support a model whereby similar signal transduction pathways are activated by stimuli that induce positive and negative selection and are consistent with the idea that the balance between opposing proapoptotic and antiapoptotic pathways determines cell fate. The data presented in this study also suggest that calcineurin functions to amplify TCR signals by promoting sustained increases in the levels of specific transcripts.

Mek2 is dispensable for mouse growth and development

MEK is a dual-specificity kinase that activates the extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase upon agonist binding to receptors. The ERK/MAP kinase cascade is involved in cell fate determination in many organisms. In mammals, this pathway is proposed to regulate cell growth and differentiation. Genetic studies have shown that although a single Mek gene is present in Caenorhabditis elegans, Drosophila melanogaster, and Xenopus laevis, two Mek homologs, Mek1 and Mek2, are present in the mammalian cascade. The inactivation of the Mek1 gene leads to embryonic lethality and has revealed the unique role played by Mek1 during embryogenesis. To investigate the biological function of the second homolog, we have generated mice deficient in Mek2 function. Mek2 mutant mice are viable and fertile, and they do not present flagrant morphological alteration. Although several components of the ERK/MAP kinase cascade have been implicated in thymocyte development, no such involvement was observed for MEK2, which appears to be nonessential for thymocyte differentiation and T-cell-receptor-induced proliferation and apoptosis. Altogether, our findings demonstrate that MEK2 is not necessary for the normal development of the embryo and T-cell lineages, suggesting that the loss of MEK2 can be compensated for by MEK1.

Regulation of thymocyte development: only the meek survive

T cell antigen receptor-induced signals are required for normal T cell development and function. Recent studies have investigated the mechanism(s) by which signals of different strengths are converted into distinct cellular fates during thymocyte development. These studies indicate the importance of the strength and duration of signals activated through PLC and PKC pathways in shaping the mature TCR repertoire.

The Ras/MAPK cascade and the control of positive selection

Immature double positive (DP) thymocytes bearing a T cell receptor (TCR) that interacts with self-major histocompatibility complex (MHC) molecules receive signals that induce either their differentiation (positive selection) or apoptosis (negative selection). Furthermore, those cells that are positively selected develop into two different lineages, CD4 or CD8, depending on whether their TCRs bind to MHC class II or I, respectively. Positive selection therefore involves rescue from the default fate (death), lineage commitment, and progression to the single positive (SP) stage. These are probably temporally distinct events that may require both unique and overlapping signals. Work in the past several years has started to unravel the signaling networks that control these processes. One of the first pathways identified as important for positive selection was Ras and its downstream effector, the Erk mitogen-activated protein kinase (MAPK) cascade. In this review we examine the factors that connect the TCR to the Ras/Erk cascade in DP thymocytes, as well as what we know about the downstream effectors of the Ras/Erk cascade important for positive selection. We also consider the possible role of this cascade in CD4/CD8 lineage development, and the possible interactions of the Ras/Erk cascade with Notch during these cell fate determination processes.

Transgenic expression of RasGRP1 induces the maturation of double-negative thymocytes and enhances the production of CD8 single-positive thymocytes

RasGRP1 is a guanine nucleotide exchange factor for Ras that is required for the efficient production of both CD4 and CD8 single-positive thymocytes. We found that RasGRP1 expression is rapidly up-regulated in double-negative thymocytes following pre-TCR ligation. Transgenic overexpression of RasGRP1 compensated for deficient pre-TCR signaling in vivo, enabling recombinase-activating gene 2(-/-) double-negative thymocytes to mature to the double-positive stage. RasGRP1 transgenic mice had a 4-fold increase in CD8 single-positive thymocytes, most of which had atypically low levels of CD3. The RasGRP1 transgene lowered the threshold of TCR signaling needed to initiate proliferation of single-positive thymocytes, with this effect being particularly evident among CD8 single-positive cells. In 3-day cultures, TCR stimulation via anti-CD3 caused a 10-fold increase in the ratio of CD8 to CD4 thymocytes among RasGRP1 transgenic vs nontransgenic thymocytes. These results demonstrate that in addition to driving the double-negative to double-positive transition, increased expression of RasGRP1 selectively increases CD8 single-positive thymocyte numbers and enhances their responsiveness to TCR signaling.

RasGRP1 transduces low-grade TCR signals which are critical for T cell development, homeostasis, and differentiation

Two important Ras-guanyl nucleotide exchange factors, Sos and RasGRP1, control Ras activation in thymocytes. However, the relative contribution of these two exchange factors to Ras/ERK activation and their resulting impact on positive and negative selection is unclear. We have produced two lines of RasGRP1(-/-) TCR transgenic mice to determine the effect of RasGRP1 in T cell development under conditions of defined TCR signaling. Our results demonstrate that RasGRP1 is crucial for thymocytes expressing weakly selecting TCRs whereas those that express stronger selecting TCRs are more effective at utilizing RasGRP1-independent mechanisms for ERK activation and positive selection. Analysis of RasGRP1(-/-) peripheral T cells also revealed hitherto unidentified functions of RasGRP1 in regulating T cell homeostasis and sustaining antigen-induced developmental programming.

Thymocyte development in early growth response gene 1-deficient mice

Early growth response gene 1 (Egr1) codes for a transcriptional regulator that contains a zinc-finger DNA binding domain. Egr1 expression is induced by a variety of extracellular stimuli including TCR-ligand interactions. Its pattern of expression in the thymus and dependence on ERK activation have led to speculation that it has a role in T cell development, but the exact nature of this role has been undefined. To more clearly define the role of Egr1 in thymocyte development, we have analyzed thymocytes from Egr1-deficient mice. We find that thymuses from Egr1-deficient mice contain twice as many cells as age-matched controls, and the increase in thymocyte number is apparent at the early CD4/CD8 double negative stage of development. Subsequent maturation to the CD4/CD8 double positive stage and survival of the double positive cells both appear normal in Egr1-deficient animals. We also find that Egr1 promotes positive selection of both CD4 and CD8 single positive cells without playing a major role in negative selection. Egr1 influences positive selection by enhancing expression of the helix-loop-helix inhibitor Id3 and the anti-apoptosis molecule bcl-2. Thus, Egr1 translates developmental signals into appropriate changes in gene expression at multiple stages of thymocyte development.

Grap negatively regulates T-cell receptor-elicited lymphocyte proliferation and interleukin-2 induction

Grb-2-related adaptor protein (Grap) is a Grb2-like SH3-SH2-SH3 adaptor protein with expression restricted to lymphoid tissues. Grap(-/-) lymphocytes isolated from targeted Grap-deficient mice exhibited enhanced proliferation, interleukin-2 production, and c-fos induction in response to mitogenic T-cell receptor (TCR) stimulation, compared to wild-type cells. Ectopic expression of Grap led to a suppression of Elk-1-directed transcription induced by the Ras/Erk pathway, without having effects on gene expression mediated by Jnk and p38 mitogen-activated protein kinases. Together, these data suggest that Grap, unlike Grb2, acts as a negative regulator of TCR-stimulated intracellular signaling by downregulating signal relay through the Ras/Erk pathway.

RelB reduces thymocyte apoptosis and regulates terminal thymocyte maturation

Thymocyte maturation is controlled by successive developmental checkpoints connected to the acquisition of a functional T cell receptor (TCR). During thymocyte selection, engagement of the TCR regulates the fine balance between death and survival signals. At the final stages of single-positive (SP) thymocyte maturation, the coupling of the TCR changes from death- to proliferation-inducing signals, a competence required for optimal effector functions in the periphery. We show here that in RelB mutant thymuses, thymocyte differentiation of CD24(-) SP cells is partially impaired. Competitive bone marrow reconstitution experiments show that this defect is constitutive to the lymphoid compartment. This is accompanied by an increased proportion of apoptotic thymocytes and a drastically reduced proliferation upon activation with anti-CD3 antibody/PMA stimulation. Thus, the RelB protein contributes to the quality of cell signaling in thymocytes by providing anti-apoptotic signals. These results suggest that in addition to its major role on the activation of antigen-presenting cell function, the RelB protein is intrinsically required for terminal thymocyte differentiation and activation.

Duration and strength of extracellular signal-regulated kinase signals are altered during positive versus negative thymocyte selection

During thymocyte development, high-affinity/avidity TCR engagement leads to the induction of negative selection and apoptosis, while lower TCR affinity-avidity interactions lead to positive selection and survival. To elucidate how these extracellular interactions are translated into intracellular signals that distinguish between positive and negative selection, we developed a culture system in which naive double-positive thymocytes were either induced to differentiate along the CD8(+) lineage pathway or were triggered for clonal deletion. Using this system, we show that sustained low level activation of extracellular signal-regulated kinases (ERKs) promotes positive selection, whereas strong but transient ERK activation is coupled with negatively selecting stimuli. Importantly, similar ERK activation profiles were demonstrated during positive selection for strong agonist ligands presented at low concentrations or weak agonist ligands. This is consistent with the affinity/avidity model and a role for strong or weak agonists during positive selection. Surprisingly, the addition of a pharmacological inhibitor which blocks ERK activation prevented the induction of negative selection. These data suggest that the duration and strength of the TCR signal is involved in discriminating between positive and negative selection.

Heterogeneity in the phosphorylation of human death receptors by p42(mapk/erk2)

Phosphorylation of murine CD120a by p42(mapk/erk2) has been shown to inhibit its ability to initiate apoptosis while preserving signaling events such as NF-kappaB activation. Therefore, we sought to determine if p42(mapk/erk2) was also capable of phosphorylating additional human death receptors within the TNF receptor superfamily. These studies showed that CD120a and DR3 are significantly phosphorylated by p42(mapk/erk2) but Fas, DR4 and DR5 are not. Additionally, we demonstrated that (i) the p42(mapk/erk2)-dependent phosphorylation of CD120a and DR3 occurred on Ser and Thr residues, (ii) p42(mapk/erk2) phosphorylated residues located in the membrane proximal regions but not the death domains of CD120a and DR3, (iii) Ser 253 is a preferred site of phosphorylation on CD120a, and (iv) the p42(mapk/erk2)-dependent phosphorylation of the DR3 cytoplasmic domain occurred exclusively at non-p42/44(mapk/erk2/1) consensus sites. These findings suggest that human death receptors segregate into two groups along lines of phylogeny with respect to Ser/Thr phosphorylation by p42(mapk/erk2).

MAP-kinase signaling pathways in T cells

The family of MAP-kinases include ERKs, p38 MAP-kinases and JNKs. Recently, the role of MAP-kinases in T lymphocytes has attracted particular interest. Genetically modified mouse models have brought insight into the specific function of each MAP-kinase pathway in T lymphocyte biology. Studies clearly show that these pathways are not redundant and that the role of each pathway depends on the T cell type and differentiation stage.

Molecular determinants of TCR expression and selection

The process of T cell development in the thymus is tightly regulated, being dependent on the integration of signals required for thymocyte maturation and survival. Rearrangements, expression and signaling of TCR genes play an indispensable role in this developmental program. Recent advances have provided insights into the molecular mechanisms that regulate TCR repertoire formation at the level of alphabeta versus gammadelta T cell fate and CD4(+) versus CD8(+) lineage determination.

Effects of a constitutively active form of calcineurin on T cell activation and thymic selection

Calcineurin is a calcium/calmodulin-dependent phosphatase whose activity is required for the induction of T cell lymphokine production and proliferation. Although its specific role in T cell development is less well defined, studies with the immunosuppressive drugs cyclosporin A and FK-506 suggest that it is involved in both positive and negative selection of immature thymocytes. To more completely characterize a role for calcineurin in T cell development in vivo, we have generated transgenic mice that express an activated form of this enzyme in thymocytes and peripheral T cells. We find that the transgene causes a block in early thymic development, resulting in a reduction in the steady-state number of CD4 and CD8 double positives, but not on the number of mature T cells. We also find that thymocytes and mature T cells expressing this transgene are more sensitive to signals through their TCR. In thymocytes this sensitivity difference is manifested as an increase in positive selection, although negative selection seems to remain unaffected. Therefore, these studies confirm and extend past reports that suggested a role for calcineurin in thymic development and selection.