The T-cell receptor regulates Akt (protein kinase B) via a pathway involving Rac1 and phosphatidylinositide 3-kinase

The serine/threonine kinase Akt (also known as protein kinase B) (Akt/PKB) is activated upon T-cell antigen receptor (TCR) engagement or upon expression of an active form of phosphatidylinositide (PI) 3-kinase in T lymphocytes. Here we report that the small GTPase Rac1 is implicated in this pathway, connecting the receptor with the lipid kinase. We show that in Jurkat cells, activated forms of Rac1 or Cdc42, but not Rho, stimulate an increase in Akt/PKB activity. TCR-induced Akt/PKB activation is inhibited either by PI 3-kinase inhibitors (LY294002 and wortmannin) or by overexpression of a dominant negative mutant of Rac1 but not Cdc42. Accordingly, triggering of the TCR rapidly stimulates a transient increase in GTP-Rac content in these cells. Similar to TCR stimulation, L61Rac-induced Akt/PKB kinase activity is also LY294002 and wortmannin sensitive. However, induction of Akt/PKB activity by constitutive active PI 3-kinase is unaffected when dominant negative Rac1 is coexpressed, placing Rac1 upstream of PI 3-kinase in the signaling pathway. When analyzing the signaling hierarchy in the pathway leading to cytoskeleton rearrangements, we found that Rac1 acts downstream of PI 3-kinase, a finding that is in accordance with numerous studies in fibroblasts. Our results reveal a previously unrecognized role of the GTPase Rac1, acting upstream of PI 3-kinase in linking the TCR to Akt/PKB. This is the first report of a membrane receptor employing Rac1 as a downstream transducer for Akt/PKB activation.     

STAM2, a new member of the STAM family, binding to the Janus kinases

We here cloned a cDNA encoding STAM2, a new member of the STAM family, which contains an SH3 domain and ITAM. STAM2 like STAM1 is associated with Jak2 and Jak3, and involved in the signaling for DNA synthesis and c-myc induction mediated by IL-2 and GM-CSF. Co-expression of the SH3 deletion mutants of STAM1 and STAM2 induces an additive effect on suppressing DNA synthesis upon stimulation with IL-2 and GM-CSF, suggesting that STAM1 and STAM2 exhibit compensatory effects on the signaling pathways downstream of Jak2 and Jak3 upon stimulation with GM-SCF and IL-2, respectively.     

Negative regulation of interleukin-2 and p38 mitogen-activated protein kinase during T-cell activation by the adaptor ALX

Activation of na?ve T cells requires synergistic signals produced by the T-cell receptor (TCR) and by CD28. We previously identified the novel adaptor ALX, which, upon overexpression in Jurkat T cells, inhibited activation of the interleukin-2 (IL-2) promoter by TCR/CD28, suggesting that it is a negative regulator of T-cell activation. To further understand the physiological role of ALX, ALX-deficient mice were generated. Purified T cells from ALX-deficient mice demonstrated increased IL-2 production, CD25 expression, and proliferation in response to TCR/CD28 stimulation. Enhanced IL-2 production and proliferation were also observed when ALX-deficient mice were primed in vivo with ovalbumin-complete Freund's adjuvant and then restimulated ex vivo. Consistent with our initial overexpression studies, these data demonstrate that ALX is a negative regulator of T-cell activation. While TCR/CD28-mediated activations of phosphotyrosine induction, extracellular signal-regulated kinase 1/2, Jun N-terminal protein kinase, IkappaB kinase alpha/beta, and Akt were unaltered, constitutive activation of p38 mitogen-activated protein kinase and its upstream regulators MKK3/6 were observed for ALX-deficient splenocytes. The phenotype of ALX-deficient mice resembled the phenotype of those deficient in the transmembrane adaptor LAX, and an association between ALX and LAX proteins was demonstrated. These results suggest that ALX, in association with LAX, negatively regulates T-cell activation through inhibition of p38.     

Loss of hippocampal CA3 pyramidal neurons in mice lacking STAM1

STAM1, a member of the STAM (signal transducing adapter molecule) family, has a unique structure containing a Src homology 3 domain and ITAM (immunoreceptor tyrosine-based activation motif). STAM1 was previously shown to be associated with the Jak2 and Jak3 tyrosine kinases and to be involved in the regulation of intracellular signal transduction mediated by interleukin-2 (IL-2) and granulocyte-macrophage colony-stimulating factor (GM-CSF) in vitro. Here we generated mice lacking STAM1 by using homologous recombination with embryonic stem cells. STAM1(-/-) mice were morphologically indistinguishable from their littermates at birth. However, growth retardation in the third week after birth was observed for the STAM1(-/-) mice. Unexpectedly, despite the absence of STAM1, hematopoietic cells, including T- and B-lymphocyte and other hematopoietic cell populations, developed normally and responded well to several cytokines, including IL-2 and GM-CSF. However, histological analyses revealed the disappearance of hippocampal CA3 pyramidal neurons in STAM1(-/-) mice. Furthermore, we observed that primary hippocampal neurons derived from STAM1(-/-) mice are vulnerable to cell death induced by excitotoxic amino acids or an NO donor. These data suggest that STAM1 is dispensable for cytokine-mediated signaling in lymphocytes but may be involved in the survival of hippocampal CA3 pyramidal neurons.     

Constitutively active protein kinase B enhances Lck and Erk activities and influences thymocyte selection and activation

Protein kinase B (PKB), a serine threonine kinase is critically involved in cellular proliferation and survival. To characterize its role in T cell development in vivo, we have analyzed transgenic mice that express a membrane-targeted constitutively active version of PKB (myr PKB) in thymocytes and peripheral T cells. We report that myr PKB renders proliferative responses of thymocytes more sensitive to TCR signals by increased and sustained activation of Src kinase Lck and the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway. In addition, the proliferative response of myr PKB T cells is relatively independent of calcium mobilization and calcineurin activity. We also find that myr PKB enhances phosphorylation of glycogen synthase kinase 3, a negative regulator of NFAT and T cell activation, and the recruitment of the adapter protein Cbl-c. Interestingly, we demonstrate that upon TCR/CD3 stimulation of wild-type T cells PKB is translocated into lipid rafts, adding a new role for PKB in TCR-initiated signalosome formation in T cell activation. Localization of transgenic PKB in lipid rafts could contribute to the higher TCR sensitivity of myr PKB thymocytes which is reflected in an increase in positive selection toward the CD4 lineage and variable effects on negative selection depending on the model system analyzed. Thus, our observations clearly indicate a cross-talk between PKB and important signaling molecules downstream of TCR that modulate the thresholds of thymocyte selection and T cell activation.     

Loss of c-Cbl RING finger function results in high-intensity TCR signaling and thymic deletion

Signaling from the T-cell receptor (TCR) in thymocytes is negatively regulated by the RING finger-type ubiquitin ligase c-Cbl. To further investigate this regulation, we generated mice with a loss-of-function mutation in the c-Cbl RING finger domain. These mice exhibit complete thymic deletion by young adulthood, which is not caused by a developmental block, lack of progenitors or peripheral T-cell activation. Rather, this phenotype correlates with greatly increased expression of the CD5 and CD69 activation markers and increased sensitivity to anti-CD3-induced cell death. Thymic loss contrasts the normal fate of the c-Cbl-/- thymus, even though thymocytes from both mutant mice show equivalent enhancement in proximal TCR signaling, Erk activation and calcium mobilization. Remarkably, only the RING finger mutant thymocytes show prominent TCR-directed activation of Akt. We show that the mutant c-Cbl protein itself is essential for activating this pathway by recruiting the p85 regulatory subunit of PI 3-kinase. This study provides a unique model for analyzing high-intensity TCR signals that cause thymocyte deletion and highlights multiple roles of c-Cbl in regulating this process.     

Disruption of Mekk2 in mice reveals an unexpected role for MEKK2 in modulating T-cell receptor signal transduction

MEKK2 is a member of the mitogen-activated protein kinase (MAPK) kinase kinase gene family involved in regulating multiple MAPK signaling pathways. To elucidate the in vivo function of MEKK2, we generated mice carrying a targeted mutation in the Mekk2 locus. Mekk2(-/-) mice are viable and fertile. Major subsets of thymic and spleen T cells in Mekk2-deficient mice were indistinguishable from those in wild-type mice. B-cell development appeared to proceed similarly in the bone marrow of Mekk2-deficient and wild-type mice. However, Mekk2(-/-) T-cell proliferation was augmented in response to anti-CD3 monoclonal antibody (MAb) stimulation, and these T cells produced more interleukin 2 and gamma interferon than did the wild-type T cells, suggesting that MEKK2 may be involved in controlling the strength of T-cell receptor (TCR) signaling. Consistently, Mekk2(-/-) thymocytes were more susceptible than wild-type thymocytes to anti-CD3 MAb-induced cell death. Furthermore, TCR-mediated c-Jun N-terminal kinase activation was not blocked but moderately enhanced in Mekk2(-/-) T cells. Neither extracellular signal-regulated kinase nor p38 MAPK activation was affected in Mekk2(-/-) T cells. In conclusion, we found that MEKK2 may be required for controlling the strength of TCR/CD3 signaling.     

PTP1B Targets the Endosomal Sorting Machinery: DEPHOSPHORYLATION OF REGULATORY SITES ON THE ENDOSOMAL SORTING COMPLEX REQUIRED FOR TRANSPORT COMPONENT STAM2*

Dephosphorylation and endocytic down-regulation are distinct processes that together control the signaling output of a variety of receptor tyrosine kinases (RTKs). PTP1B can directly dephosphorylate several RTKs, but it can also promote activation of downstream pathways through largely unknown mechanisms. These positive signaling functions likely contribute to the tumor-promoting effect of PTP1B in mouse cancer models. Here, we have identified STAM2, an endosomal protein involved in sorting activated RTKs for lysosomal degradation, as a substrate of PTP1B. PTP1B interacts with STAM2 at defined phosphotyrosine sites, and knockdown of PTP1B expression augments STAM2 phosphorylation. Intriguingly, manipulating the expression and phosphorylation state of STAM2 did not have a general effect on epidermal growth factor (EGF)-induced EGF receptor trafficking, degradation, or signaling. Instead, phosphorylated STAM2 specifically suppressed Akt activation, and a phosphorylation-deficient STAM2 mutant displayed prolonged localization on endosomes following EGF stimulation. These results reveal a novel link between the dephosphorylation and endocytic machinery and suggest that PTP1B can affect RTK signaling in a previously unrecognized manner.     

Rapid activation of protein kinase B/Akt has a key role in antiapoptotic signaling during liver regeneration

Liver regeneration is controlled by multiple signaling pathways induced by a variety of growth factors, hormones, and cytokines. Here we report that protein kinase B (PKB)/Akt, part of a key cell survival signaling pathway, is markedly activated after partial hepatectomy (PHX). The antiapoptotic protein Bad, a downstream target of PKB/Akt, is also phosphorylated. This cascade can be activated by various factors in primary hepatocytes, with the strongest activation by insulin and the alpha1-adrenergic agonist phenylephrine (PE), followed by IL-6, epidermal growth factor (EGF), and hepatocyte growth factor (HGF). Pretreatment of cells with the specific PI3 kinase inhibitor LY294002 abolished insulin- or PE-activation of PKB/Akt, suggesting that activation of PKB/Akt is mediated by a PI3 kinase-dependent mechanism. In vivo administration of PE, insulin, IL-6, HGF, or EGF to mice markedly stimulated PKB/Akt in the liver, with the strongest stimulation induced by insulin and PE. Moreover, HGF and insulin were able to attenuate transforming growth factor beta-induced apoptosis in hepatic cells, and these effects were antagonized by LY294002. Taken together, these findings suggest that rapid activation of PKB/Akt is a key antiapoptotic signaling pathway involved in liver regeneration.     

Identification of a novel immunoreceptor tyrosine-based activation motif-containing molecule, STAM2, by mass spectrometry and its involvement in growth factor and cytokine receptor signaling pathways

In an effort to clone novel tyrosine-phosphorylated substrates of the epidermal growth factor receptor, we have initiated an approach coupling affinity purification using anti-phosphotyrosine antibodies to mass spectrometry-based identification. Here, we report the identification of a signaling molecule containing a Src homology 3 domain as well as an immunoreceptor tyrosine-based activation motif (ITAM). This molecule is 55% identical to a previously isolated molecule designated signal transducing adaptor molecule (STAM) that was identified as an interleukin (IL)-2-induced phosphoprotein and is therefore designated STAM2. Tyrosine phosphorylation of STAM2 is induced by growth factors such as epidermal growth factor and platelet-derived growth factor as well as by cytokines like IL-3. Several of the deletion mutants tested except the one containing only the amino-terminal region underwent tyrosine phosphorylation upon growth factor stimulation, implying that STAM2 is phosphorylated on several tyrosine residues. STAM2 is downstream of the Jak family of kinases since coexpression of STAM2 with Jak1 or Jak2 but not an unrelated Tec family kinase, Etk, resulted in its tyrosine phosphorylation. In contrast to epidermal growth factor receptor-induced phosphorylation, this required the ITAM domain since mutants lacking this region did not undergo tyrosine phosphorylation. Finally, overexpression of wild type STAM2 led to an increase in IL-2-mediated induction of c-Myc promoter activation indicating that it potentiates cytokine receptor signaling.     

pp59fyn mutant mice display differential signaling in thymocytes and peripheral T cells.

We have generated mutant mice that do not express pp59fyn, a nonreceptor protein tyrosine kinase related to pp60src, by homologous recombination in embryonic stem cells. fyn- mice did not display an overt phenotype. Because fyn is associated with the T cell receptor (TCR), thymocyte and T cell signaling was analyzed in the mutant background. Cross-linking of TCR-CD3 in thymocytes led to markedly reduced calcium fluxes and abrogated proliferation, whereas mature splenic T cells retained largely normal proliferation despite depressed calcium movements and IL-2 production. Similarly, proliferation induced by Thy-1 cross-linking was reduced in thymocytes but not in splenic T cells. fyn- thymocytes were impaired at a late stage of maturation and showed limited clonal deletion to the Mls-1a self-super-antigen but not to staphylococcal enterotoxin A. These results implicate fyn as a critical component in TCR signaling in thymocytes and, potentially, in the process that determines T cell repertoire in the adult mouse.     

JNK2 is required for efficient T-cell activation and apoptosis but not for normal lymphocyte development

BACKGROUND: The Jun N-terminal kinase (JNK) signaling pathway has been implicated in cell proliferation and apoptosis, but its function seems to depend on the cell type and inducing signal. In T cells, JNK has been implicated in both antigen-induced activation and apoptosis. RESULTS: We generated mice lacking the JNK2 isozymes. The mutant mice were healthy and fertile but defective in peripheral T-cell activation induced by antibody to the CD3 component of the T-cell receptor (TCR) complex - proliferation and production of interleukin-2 (IL-2), IL-4 and interferon-gamma (IFN-gamma) were reduced. The proliferation defect was restored by exogenous IL-2. B-cell activation was normal in the absence of JNK2. Activation-induced peripheral T-cell apoptosis was comparable between mutant and wild-type mice, but immature (CD4(+) CD8(+)) thymocytes lacking JNK2 were resistant to apoptosis induced by administration of anti-CD3 antibody in vivo. The lack of JNK2 also resulted in partial resistance of thymocytes to anti-CD3 antibody in vitro, but had little or no effect on apoptosis induced by anti-Fas antibody, dexamethasone or ultraviolet-C (UVC) radiation. CONCLUSIONS: JNK2 is essential for efficient activation of peripheral T cells but not B cells. Peripheral T-cell activation is probably required indirectly for induction of thymocyte apoptosis resulting from administration of anti-CD3 antibody in vivo. JNK2 functions in a cell-type-specific and stimulus-dependent manner, being required for apoptosis of immature thymocytes induced by anti-CD3 antibody but not for apoptosis induced by anti-Fas antibody, UVC or dexamethasone. JNK2 is not required for activation-induced cell death of mature T cells.     

CH-ILKBP regulates cell survival by facilitating the membrane translocation of protein kinase B/Akt

Cell survival depends on proper propagation of protective signals through intracellular signaling intermediates. We report here that calponin homology domain-containing integrin-linked kinase (ILK)-binding protein (CH-ILKBP), a widely expressed adaptor protein localized at plasma membrane-actin junctions, is essential for transmission of survival signals. Cells that are depleted of CH-ILKBP undergo extensive apoptosis despite the presence of cell-extracellular matrix contacts and soluble growth factors. The activating phosphorylation of protein kinase B (PKB/Akt), a key regulator of apoptosis, is impaired in the absence of CH-ILKBP. Importantly, loss of CH-ILKBP prevents the membrane translocation of PKB/Akt. Furthermore, forced membrane targeting of PKB/Akt bypasses the requirement of CH-ILKBP for the activating phosphorylation of PKB/Akt, suggesting that CH-ILKBP is required for the membrane translocation but not the subsequent phosphorylation of PKB/Akt. Finally, we show that loss of CH-ILKBP is also required for the full activation of extracellular signal-regulated kinase (ERK)1/2. However, restoration of the PKB/Akt activation is sufficient for protection of cells from apoptosis induced by the depletion of CH-ILKBP despite the persistent suppression of the ERK1/2 activation. Thus, CH-ILKBP is an important component of the prosurvival signaling pathway functioning primarily by facilitating the membrane translocation of PKB/Akt and consequently the activation of PKB/Akt in response to extracellular survival signals.     

Reactive oxygen species mediate the activation of Akt/protein kinase B by angiotensin II in vascular smooth muscle cells.

Angiotensin II, a hypertrophic/anti-apoptotic hormone, utilizes reactive oxygen species (ROS) as growth-related signaling molecules in vascular smooth muscle cells (VSMCs). Recently, the cell survival protein kinase Akt/protein kinase B (PKB) was proposed to be involved in protein synthesis. Here we show that angiotensin II causes rapid phosphorylation of Akt/PKB (6- +/- 0.4-fold increase). Exogenous H(2)O(2) (50-200 microM) also stimulates Akt/PKB phosphorylation (maximal 8- +/- 0.2-fold increase), suggesting that Akt/PKB activation is redox-sensitive. Both angiotensin II and H(2)O(2) stimulation of Akt/PKB are abrogated by the phosphatidylinositol 3-kinase (PI3-K) inhibitors wortmannin and LY294002 (2(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one), suggesting that PI3-K is an upstream mediator of Akt/PKB activation in VSMCs. Furthermore, diphenylene iodonium, an inhibitor of flavin-containing oxidases, or overexpression of catalase to block angiotensin II-induced intracellular H(2)O(2) production significantly inhibits angiotensin II-induced Akt/PKB phosphorylation, indicating a role for ROS in agonist-induced Akt/PKB activation. In VSMCs infected with dominant-negative Akt/PKB, angiotensin II-stimulated [(3)H]leucine incorporation is attenuated. Thus, our studies indicate that Akt/PKB is part of the remarkable spectrum of angiotensin II signaling pathways and provide insight into the highly organized signaling mechanisms coordinated by ROS, which mediate the hypertrophic response to angiotensin II in VSMCs.     

A regulator of G protein signaling-containing kinase is important for chemotaxis and multicellular development in dictyostelium

We have identified a gene encoding RGS domain-containing protein kinase (RCK1), a novel regulator of G protein signaling domain-containing protein kinase. RCK1 mutant strains exhibit strong aggregation and chemotaxis defects. rck1 null cells chemotax approximately 50% faster than wild-type cells, suggesting RCK1 plays a negative regulatory role in chemotaxis. Consistent with this finding, overexpression of wild-type RCK1 reduces chemotaxis speed by approximately 40%. On cAMP stimulation, RCK1 transiently translocates to the membrane/cortex region with membrane localization peaking at approximately 10 s, similar to the kinetics of membrane localization of the pleckstrin homology domain-containing proteins CRAC, Akt/PKB, and PhdA. RCK1 kinase activity also increases dramatically. The RCK1 kinase activity does not rapidly adapt, but decreases after the cAMP stimulus is removed. This is particularly novel considering that most other chemoattractant-activated kinases (e.g., Akt/PKB, ERK1, ERK2, and PAKa) rapidly adapt after activation. Using site-directed mutagenesis, we further show that both the RGS and kinase domains are required for RCK1 function and that RCK1 kinase activity is required for the delocalization of RCK1 from the plasma membrane. Genetic evidence suggests RCK1 function lies downstream from Galpha2, the heterotrimeric G protein that couples to the cAMP chemoattractant receptors. We suggest that RCK1 might be part of an adaptation pathway that regulates aspects of chemotaxis in Dictyostelium.     

RasGRP1 transmits prodifferentiation TCR signaling that is crucial for CD4 T cell development

TCR signaling plays a governing role in both the survival and differentiation of bipotent double-positive thymocytes into the CD4(+) and CD8(+) single-positive T cell lineages. A central mediator of this developmental program is the small GTPase Ras, emitting cytoplasmic signals through downstream MAPK pathways and eventually affecting gene expression. TCR signal transduction orchestrates the activation of Ras by integrating at least two Ras-guanyl nucleotide exchange factors, RasGRP1 and Sos. In this study, we have characterized the relationship between RasGRP1 function and its potential roles in promoting ERK activity, cell survival, maturation, and lineage commitment. Investigations on RasGRP1(-/-) mice expressing a transgenic (Tg) MHC class II-restricted TCR revealed that the development of CD4 T cells expressing this Tg TCR is completely dependent on RasGRP1. Unexpectedly, a small number of functional CD8 single-positive thymocytes expressing the Tg MHC class II-restricted TCR exists in mutant mice. In addition, RasGRP1(-/-) double-positive thymocytes exhibit marked deficits in TCR-stimulated up-regulation of the positive selection marker CD69 and the antiapoptotic protein Bcl-2, whereas CD5 induction is unaffected. To evaluate the role of RasGRP1 in providing cellular survival signaling, we enforced Bcl-2 expression in RasGRP1(-/-) thymocytes. These studies demonstrate that RasGRP1 function cannot be fully complemented by Tg Bcl-2 expression. Therefore, we propose that RasGRP1 transmits differentiation signaling critically required for CD4 T cell development.     

IL-10 synergistically enhances GM-CSF-induced CCR1 expression in myelomonocytic cells

CC chemokine receptor 1 (CCR1) has been implicated in inflammation. The present study examined the signaling mechanisms that mediate GM-CSF/IL-10-induced synergistic CCR1 protein expression in monocytic U937 cells. GM-CSF alone markedly increased both the mRNA and protein expression of CCR1. IL-10 augmented GM-CSF-induced CCR1 protein expression with no effect on mRNA expression. PD098059 and U0126 (two MEK inhibitors), and LY294002 (a PI3K inhibitor) inhibited GM-CSF/IL-10-induced CCR1 gene and protein expression. PD098059, U0126, and LY294002 also attenuated chemotaxis of GM-CSF/IL-10-primed U937 cells in response to MIP-1alpha. Immunoblotting studies show that GM-CSF alone induced ERK2 phosphorylation; whereas, IL-10 alone induced p70(S6k) phosphorylation in U937 cells. Neither cytokine when used alone induced PKB/Akt phosphorylation. Combined GM-CSF/IL-10 treatment of U937 cells induced phosphorylation of ERK2, p70(S6k), and PKB/Akt. PD098059 and U0126 completely abrogated ERK2 phosphorylation; whereas, LY294002 completely blocked PKB/Akt and p70(S6k) phosphorylation. Our findings indicate that IL-10 may potentiate GM-CSF-induced CCR1 protein expression in U937 cells via activation of PKB/Akt and p70(S6k).     

PI3-kinase and MAP-kinase signaling cascades in AILIM/ICOS- and CD28-costimulated T-cells have distinct functions between cell proliferation and IL-10 production

Both AILIM/ICOS and CD28 provide positive costimulatory signals for T-cell activation, resulting in proliferation and cytokine production. In this study, we attempted to clarify the key signaling molecules in T-cell proliferation, and also IL-2 and IL-10 production, during T-cell activation by CD3 induced by costimulation with either AILIM/ICOS or CD28. We examined the role of both the PI3-kinase/Akt pathway and MAP kinase family members such as ERK1/2, JNK, and p38 kinase in this process. PI3-kinase and Erk1/2 were shown to potentially regulate primary T-cell activation and subsequent proliferation via both AILIM/ICOS- or CD28-mediated costimulation and the Erk signaling cascade was essential for this proliferation induction and also for IL-2 production. The JAK inhibitor, AG490, inhibited this induction. Our studies indicate that IL-2 is necessary for induction of T-cell proliferation and that the quantities of IL-2 produced by AILIM/ICOS ligation are also sufficient for T-cells to proliferate. In contrast, inhibition of Akt and p38, that are phosphorylated by both AILIM/ICOS and CD28-ligation, could downregulate IL-10 production but not T-cell proliferation. These data raise the interesting possibility that the signaling cascades between T-cell proliferation and IL-10 production are regulated by different molecules in AILIM/ICOS- and CD28-costimulated T-cells.     

TCR signaling thresholds regulating T cell development and activation are dependent upon SHP-1

An examination of thymocytes and peripheral T cells from SHP-1-deficient motheaten mice possessing a transgenic MHC class I-restricted TCR has implicated SHP-1 in regulating TCR signaling thresholds at three checkpoints in T cell development and activation. First, in the population of CD4-CD8- double negative thymocytes, SHP-1 appears capable of regulating signals from TCR complexes that control the maturation and proliferation of double negative thymocytes. Second, the loss of SHP-1 increased the number of CD4+CD8+ double positive thymocytes capable of maturing as TCRhigh single positive thymocytes. Third, the loss of SHP-1 altered the basal level of activation of naive lymph node T cells. Accordingly, SHP-1-deficient lymph node T cells bearing the transgenic TCR demonstrated a hyperresponsiveness to stimulation with cognate peptide. However, the loss of SHP-1 did not alter the cytolytic ability of mature effector cytotoxic T lymphocytes. Together these results suggest that SHP-1 contributes to establishing thresholds for TCR signaling in thymocytes and naive peripheral T cells.