Direct regulation of ZAP-70 by SHP-1 in T cell antigen receptor signaling

Negative immune checkpoints on T lymphocytes and their relevance to cancer immunotherapy

The term 'inhibitory checkpoint' refers to the broad spectrum of co-receptors expressed by T cells that negatively regulate T cell activation thus playing a crucial role in maintaining peripheral self-tolerance. Co-inhibitory receptor ligands are highly expressed by a variety of malignancies allowing evasion of anti-tumour immunity. Recent studies demonstrate that manipulation of these co-inhibitory pathways can remove the immunological brakes that impede endogenous immune responses against tumours. Antibodies that block the interactions between co-inhibitory receptors and their ligands have delivered very promising clinical responses, as has been shown by recent successful trials targeting the CTLA-4 and PD-1 pathways. In this review, we discuss the mechanisms of action and expression pattern of co-inhibitory receptors on different T cells subsets, emphasising differences between CD4(+) and CD8(+) T cells. We also summarise recent clinical findings utilising immune checkpoint blockade.

Protein Tyrosine Phosphatase SHP-1 Modulates T Cell Responses by Controlling Cbl-b Degradation

Previously, we demonstrated that CD28 and CTLA-4 signaling control Casitas-B-lineage lymphoma (Cbl)-b protein expression, which is critical for T cell activation and tolerance induction. However, the molecular mechanism(s) of this regulation remains to be elucidated. In this study, we found that Cbl-b fails to undergo tyrosine phosphorylation upon CD3 stimulation because SHP-1 is recruited to and dephosphorylates Cbl-b, whereas CD28 costimulation abrogates this interaction. In support of this finding, T cells lacking SHP-1 display heightened tyrosine phosphorylation and ubiquitination of Cbl-b upon TCR stimulation, which correlates with decreased levels of Cbl-b protein. The aberrant Th2 phenotype observed in T cell-specific Shp1(-/-) mice is reminiscent of heightened Th2 response in Cblb(-/-) mice. Indeed, overexpressing Cbl-b in T cell-specific Shp1(-/-) T cells not only inhibits heightened Th2 differentiation in vitro, but also Th2 responses and allergic airway inflammation in vivo. Therefore, SHP-1 regulates Cbl-b-mediated T cell responses by controlling its tyrosine phosphorylation and ubiquitination.

Commentary: IL-4 and IL-13 receptors and signaling

Interleukin (IL)-4 and IL-13 were discovered approximately 30years ago and were immediately linked to allergy and atopic diseases. Since then, new roles for IL-4 and IL-13 and their receptors in normal gestation, fetal development and neurological function and in the pathogenesis of cancer and fibrosis have been appreciated. Studying IL-4/-13 and their receptors has revealed important clues about cytokine biology and led to the development of numerous experimental therapeutics. Here we aim to highlight new discoveries and consolidate concepts in the field of IL-4 and IL-13 structure, receptor regulation, signaling and experimental therapeutics.

A THEMIS:SHP1 complex promotes T-cell survival

THEMIS is critical for conventional T-cell development, but its precise molecular function remains elusive. Here, we show that THEMIS constitutively associates with the phosphatases SHP1 and SHP2. This complex requires the adapter GRB2, which bridges SHP to THEMIS in a Tyr-phosphorylation-independent fashion. Rather, SHP1 and THEMIS engage with the N-SH3 and C-SH3 domains of GRB2, respectively, a configuration that allows GRB2-SH2 to recruit the complex onto LAT. Consistent with THEMIS-mediated recruitment of SHP to the TCR signalosome, THEMIS knock-down increased TCR-induced CD3-ζ phosphorylation, Erk activation and CD69 expression, but not LCK phosphorylation. This generalized TCR signalling increase led to augmented apoptosis, a phenotype mirrored by SHP1 knock-down. Remarkably, a KI mutation of LCK Ser59, previously suggested to be key in ERK-mediated resistance towards SHP1 negative feedback, did not affect TCR signalling nor ligand discrimination in vivo. Thus, the THEMIS:SHP complex dampens early TCR signalling by a previously unknown molecular mechanism that favours T-cell survival. We discuss possible implications of this mechanism in modulating TCR output signals towards conventional T-cell development and differentiation.

T cell receptor-associated protein tyrosine kinases: the dynamics of tolerance regulation by phosphorylation and its role in systemic lupus erythematosus

There are different abnormalities that lead to the autoreactive phenotype in T cells from systemic lupus erythematosus (SLE) patients. Proximal signaling, involving the T-cell receptor (TCR) and its associated protein tyrosine kinases (PTKs), is significantly affected in SLE. This ultimately leads to aberrant responses, which include enhanced tyrosine phosphorylation and calcium release, as well as decreased IL-2 secretion. Lck, ZAP70 and Syk, which are PTKs with a major role in proximal signaling, all present abnormal functioning that contributes to an altered T cell response in these patients. A number of other molecules, especially regulatory proteins, are also involved. This review will focus on the PTKs that participate in proximal signaling, with specific emphasis on their relevance in maintaining peripheral tolerance, their abnormalities in SLE and how these contribute to an altered T cell response.

Fine-tuning T cell receptor signaling to control T cell development

T cell development from immature CD4(+)CD8(+) double-positive (DP) thymocytes to the mature CD4 or CD8 single-positive (SP) stage requires proper T cell receptor (TCR) signaling. The current working model of thymocyte development is that the strength of the TCR-mediated signal - from little-or-none, through intermediate, to strong - received by the immature cells determines whether they will undergo death by neglect, positive selection, or negative selection, respectively. In recent years, several developmentally regulated, stage-specifically expressed proteins and miRNAs have been found that act like fine-tuners for signal transduction and propagation downstream of the TCR. This allows them to govern thymocyte positive selection. Here, we summarize recent findings on these molecules and suggest new concepts of TCR positive-selection signaling.

Dynamic functional modulation of CD4+ T cell recall responses is dependent on the inflammatory environment of the secondary stimulus

The parameters that modulate the functional capacity of secondary Th1 effector cells are poorly understood. In this study, we employ a serial adoptive transfer model system to show that the functional differentiation and secondary memory potential of secondary CD4+ effector T cells are dependent on the inflammatory environment of the secondary challenge. Adoptive transfer of TCR transgenic lymphocytic choriomeningitis virus (LCMV) Glycoprotein-specific SMARTA memory cells into LCMV-immune hosts, followed by secondary challenge with Listeria monocytogenes recombinantly expressing a portion of the LCMV Glycoprotein (Lm-gp61), resulted in the rapid emergence of SMARTA secondary effector cells with heightened functional avidity (as measured by their ability to make IFNγ in response to ex vivo restimulation with decreasing concentrations of peptide), limited contraction after pathogen clearance and stable maintenance secondary memory T cell populations. In contrast, transfer of SMARTA memory cells into naïve hosts prior to secondary Lm-gp61 challenge, which resulted in a more extended infectious period, resulted in poor functional avidity, increased death during the contraction phase and poor maintenance of secondary memory T cell populations. The modulation of functional avidity during the secondary Th1 response was independent of differences in antigen load or persistence. Instead, the inflammatory environment strongly influenced the function of the secondary Th1 response, as inhibition of IL-12 or IFN-I activity respectively reduced or increased the functional avidity of secondary SMARTA effector cells following rechallenge in a naïve secondary hosts. Our findings demonstrate that secondary effector T cells exhibit inflammation-dependent differences in functional avidity and memory potential, and have direct bearing on the design of strategies aimed at boosting memory T cell responses.

A sorafenib derivative and novel SHP-1 agonist, SC-59, acts synergistically with radiotherapy in hepatocellular carcinoma cells through inhibition of STAT3

Radiotherapy shows limited benefit as treatment for hepatocellular carcinoma (HCC). In this study, we aimed to overcome the radioresistance of HCC by using a novel sorafenib derivative, SC-59 that targets SHP-1-related signaling. HCC cell lines (SK-Hep1, Hep3B, and Huh7) were treated with sorafenib, SC-59, radiation, sorafenib plus radiation, or SC-59 plus radiation, and then apoptosis, colony formation, signal transduction and the phosphatase activity were analyzed. The synergistic effect of radiotherapy and SC-59 was analyzed using a combination index (CI) approach. In vivo efficacy was determined in a Huh7-bearing subcutaneous model. Mice were treated with radiation (5 Gy, one fraction per day) for 4 days, SC-59 (10mg/kg/day) for 24 days, or a combination. Tumor samples were further analyzed for p-STAT3 and SHP-1 activity. SC-59 displayed a better synergistic effect when used in combination with radiotherapy than sorafenib in HCC cell lines. SC-59 downregulated p-STAT3 and its downstream targets and increased SHP-1 phosphatase activity. Both ectopic STAT3 and inhibition of SHP-1 abolished SC-59-induced radiosensitization. Moreover, SC-59 significantly synergized radiotherapy in a Huh7 xenograft model by targeting SHP-1/STAT3 signaling. The novel sorafenib derivative, SC-59, acting as a SHP-1 agonist, displays a better synergistic effect when used in combination with radiotherapy than sorafenib for the treatment of HCC. Further clinical investigation is warranted.

Insights into the suppressor of T-cell receptor (TCR) signaling-1 (Sts-1)-mediated regulation of TCR signaling through the use of novel substrate-trapping Sts-1 phosphatase variants

High affinity substrate-trapping protein tyrosine phosphatases have been widely used both to investigate the endogenous targets of many phosphatases and to address questions of substrate specificity. Herein, we extend the concept of a substrate-trapping phosphatase to include an enzyme of the histidine phosphatase superfamily. This is the first description of substrate-trapping technology applied to a member of the histidine phosphatase family. The phosphatase suppressor of T-cell receptor signaling (Sts)-1 has recently been reported to negatively regulate signaling downstream of the T-cell receptor. We generated high-affinity substrate-trapping variants of Sts-1 by mutagenesis of key active site residues within the phosphatase catalytic domain. Mutation of both the nucleophilic His380 and the general acid Glu490 yielded Sts-1 enzymes that were catalytically inactive but showed high affinity for an important tyrosine kinase in T cells that Sts-1 is known to regulate, Zap-70. Sts-1 substrate-trapping mutants isolated tyrosine-phosphorylated Zap-70 from lysates of activated T cells, validating Zap-70 as a possible substrate for Sts-1 and highlighting the efficacy of the mutants as substrate-trapping agents. Inhibition of the Zap-70 interaction by vanadate suggests that the substrate-trapping effect occurred via the Sts-1 phosphatase active site. Finally, overexpression of Sts-1 substrate-trapping mutants in T cells blocked T-cell receptor signaling, confirming the inhibitory effect of Sts-1 on Zap-70.

Shp1 regulates T cell homeostasis by limiting IL-4 signals

Absence of the phosphatase Shp1 in T cells does not affect the TCR signaling threshold but results in IL-4 sensitivity and memory phenotype cells.

The protein-tyrosine phosphatase Shp1 is expressed ubiquitously in hematopoietic cells and is generally viewed as a negative regulatory molecule. Mutations in Ptpn6, which encodes Shp1, result in widespread inflammation and premature death, known as the motheaten (me) phenotype. Previous studies identified Shp1 as a negative regulator of TCR signaling, but the severe systemic inflammation in me mice may have confounded our understanding of Shp1 function in T cell biology. To define the T cell–intrinsic role of Shp1, we characterized mice with a T cell–specific Shp1 deletion (Shp1^fl/fl CD4-cre). Surprisingly, thymocyte selection and peripheral TCR sensitivity were unaltered in the absence of Shp1. Instead, Shp1^fl/fl CD4-cre mice had increased frequencies of memory phenotype T cells that expressed elevated levels of CD44. Activation of Shp1-deficient CD4⁺ T cells also resulted in skewing to the Th2 lineage and increased IL-4 production. After IL-4 stimulation of Shp1-deficient T cells, Stat 6 activation was sustained, leading to enhanced Th2 skewing. Accordingly, we observed elevated serum IgE in the steady state. Blocking or genetic deletion of IL-4 in the absence of Shp1 resulted in a marked reduction of the CD44^hi population. Therefore, Shp1 is an essential negative regulator of IL-4 signaling in T lymphocytes.

In vivo modulation of avidity in highly sensitive CD8(+) effector T cells following viral infection

Numerous studies have demonstrated a critical role for T cell avidity in predicting in vivo efficacy. Even though the measurement of avidity is now a routine assessment for the analysis of effector and memory T cell populations, our understanding of how this property is controlled in vivo at both the population and individual cell levels is limited. Our previous studies have identified high avidity as a property of the initial effector population generated in mice following respiratory virus infection. As the response progresses, lower avidity cells appear in the effector pool. The studies described here investigate the mechanistic basis of this in vivo regulation of avidity. We present data supporting in vivo avidity modulation within the early high avidity responders that results in a population of lower avidity effector cells. Changes in avidity were correlated with decreased lck expression and increased sensitivity to lck inhibitors in effector cells present at late versus early times postinfection. The possibility of tuning within select individual effectors is a previously unappreciated mechanism for the control of avidity in vivo.

Thrombospondin-1 modulates VEGF signaling via CD36 by recruiting SHP-1 to VEGFR2 complex in microvascular endothelial cells

Thrombospondin-1 (TSP-1) inhibits growth factor signaling at the receptor level in microvascular endothelial cells (MVEC), and CD36 has been suggested to be involved in this inhibition, but the mechanisms are not known. We hypothesized that CD36-TSP-1 interaction recruits Src homology 2 domain-containing protein tyrosine phosphatase (SHP)-1 to the vascular endothelial growth factor receptor 2 (VEGFR2) signaling complex and attenuates vascular endothelial growth factor (VEGF) signaling. Western blots of anti-CD36 and anti-VEGFR2 immunoprecipitates from VEGF-treated MVEC showed that exposure of the cells to a recombinant protein containing the CD36 binding domain of thrombospondin-1 (known as the TSR domain) induced association of SHP-1 with the VEGFR2/CD36 signaling complex and thereby suppressed VEGFR2 phosphorylation. SHP-1 phosphatase activity was increased in immunoprecipitated VEGFR2 complexes from TSR-treated cells. Silencing CD36 expression in MVEC by small interfering RNA (siRNA) or genetic deletion of cd36 in mice showed that TSR-induced SHP-1/VEGFR2 complex formation required CD36 in vitro and in vivo. Silencing SHP-1 expression in MVEC by siRNA abrogated TSR-mediated inhibition of VEGFR2 phosphorylation as well as TSR-mediated inhibition of VEGF-induced endothelial cell migration and tube formation. These studies reveal a SHP-1-mediated antiangiogenic pathway induced by CD36-TSP-1 interaction that inhibits VEGFR2 signaling and they provide a novel target to modulate angiogenesis therapeutically.

Molecular insights for optimizing T cell receptor specificity against cancer

Cytotoxic CD8 T cells mediate immunity to pathogens and they are able to eliminate malignant cells. Immunity to viruses and bacteria primarily involves CD8 T cells bearing high affinity T cell receptors (TCRs), which are specific to pathogen-derived (non-self) antigens. Given the thorough elimination of high affinity self/tumor-antigen reactive T cells by central and peripheral tolerance mechanisms, anti-cancer immunity mostly depends on TCRs with intermediate-to-low affinity for self-antigens. Because of this, a promising novel therapeutic approach to increase the efficacy of tumor-reactive T cells is to engineer their TCRs, with the aim to enhance their binding kinetics to pMHC complexes, or to directly manipulate the TCR-signaling cascades. Such manipulations require a detailed knowledge on how pMHC-TCR and co-receptors binding kinetics impact the T cell response. In this review, we present the current knowledge in this field. We discuss future challenges in identifying and targeting the molecular mechanisms to enhance the function of natural or TCR-affinity optimized T cells, and we provide perspectives for the development of protective anti-tumor T cell responses.

Phosphatase regulation of immunoreceptor signaling in T cells, B cells and mast cells

Recent progress has begun to reveal the often complex and changing roles of phosphotyrosine and phosphoinositide phosphatases in regulation of immunoreceptor signaling. The resultant confusion has been further increased by discoveries of new players. Here we provide a review of recent progress in defining the roles of these enzymes in immunoreceptor-dependent mast cell, T cell and B cell activation.

A new look at T cell receptor signaling to nuclear factor-κB

Antigen stimulation of T cell receptor (TCR) signaling to nuclear factor (NF)-κB is required for T cell proliferation and differentiation of effector cells. The TCR-to-NF-κB pathway is generally viewed as a linear sequence of events in which TCR engagement triggers a cytoplasmic cascade of protein-protein interactions and post-translational modifications, ultimately culminating in the nuclear translocation of NF-κB. However, recent findings suggest a more complex picture in which distinct signalosomes, previously unrecognized proteins, and newly identified regulatory mechanisms play key roles in signal transmission. In this review, we evaluate recent data and suggest areas of future emphasis in the study of this important pathway.

Regulation of TCR signalling by tyrosine phosphatases: from immune homeostasis to autoimmunity

More than half of the known protein tyrosine phosphatases (PTPs) in the human genome are expressed in T cells, and significant progress has been made in elucidating the biology of these enzymes in T-cell development and function. Here we provide a systematic review of the current understanding of the roles of PTPs in T-cell activation, providing insight into their mechanisms of action and regulation in T-cell receptor signalling, the phenotypes of their genetically modified mice, and their possible involvement in T-cell-mediated autoimmune disease. Our projection is that the interest in PTPs as mediators of T-cell homeostasis will continue to rise with further functional analysis of these proteins, and PTPs will be increasingly considered as targets of immunomodulatory therapies.

Quantitative phosphoproteomics reveals SLP-76 dependent regulation of PAG and Src family kinases in T cells

The SH2-domain-containing leukocyte protein of 76 kDa (SLP-76) plays a critical scaffolding role in T cell receptor (TCR) signaling. As an adaptor protein that contains multiple protein-binding domains, SLP-76 interacts with many signaling molecules and links proximal receptor stimulation to downstream effectors. The function of SLP-76 in TCR signaling has been widely studied using the Jurkat human leukaemic T cell line through protein disruption or site-directed mutagenesis. However, a wide-scale characterization of SLP-76-dependant phosphorylation events is still lacking. Quantitative profiling of over a hundred tyrosine phosphorylation sites revealed new modes of regulation of phosphorylation of PAG, PI3K, and WASP while reconfirming previously established regulation of Itk, PLCγ, and Erk phosphorylation by SLP-76. The absence of SLP-76 also perturbed the phosphorylation of Src family kinases (SFKs) Lck and Fyn, and subsequently a large number of SFK-regulated signaling molecules. Altogether our data suggests unique modes of regulation of positive and negative feedback pathways in T cells by SLP-76, reconfirming its central role in the pathway.

Protein clusters on the T cell surface may suppress spurious early signaling events

T cells play an important role in the adaptive immune system, quickly activating effector functions in response to small numbers of antigenic peptides but rarely activating in response to constant interaction with most endogenous peptides. Emerging experimental evidence suggests that key membrane-bound signaling proteins such as the T cell receptor and the adaptor protein Lat are spatially organized into small clusters on the T cell membrane. We use spatially resolved, stochastic computer simulations to study how the inhomogeneous distribution of molecules affects the portion of the T cell signaling network in which the kinase ZAP-70, originating in T cell receptor clusters, phosphorylates Lat. To gain insight into the effects of protein clustering, we compare the signaling response from membranes with clustered proteins to the signaling response from membranes with homogeneously distributed proteins. Given a fixed amount of ZAP-70 (a proxy for degree of TCR stimulation) that must diffuse into contact with Lat molecules, the spatially homogeneous system responds faster and results in higher levels of phosphorylated Lat. Analysis of the spatial distribution of proteins demonstrates that, in the homogeneous system, nearest ZAP-70 and Lat proteins are closer on average and fewer Lat molecules share the same closest ZAP-70 molecule, leading to the faster response time. The results presented here suggest that spatial clustering of proteins on the T cell membrane may suppress the propagation of signal from ZAP-70 to Lat, thus providing a regulatory mechanism by which T cells suppress transient, spurious signals induced by stimulation of T cell receptors by endogenous peptides. Because this suppression of spurious signals may occur at a cost to sensitivity, we discuss recent experimental results suggesting other potential mechanisms by which ZAP-70 and Lat may interact to initiate T cell activation.

Abrogation of SRC homology region 2 domain-containing phosphatase 1 in tumor-specific T cells improves efficacy of adoptive immunotherapy by enhancing the effector function and accumulation of short-lived effector T cells in vivo

T cell expression of inhibitory proteins can be a critical component for the regulation of immunopathology owing to self-reactivity or potentially exuberant responses to pathogens, but it may also limit T cell responses to some malignancies, particularly if the tumor Ag being targeted is a self-protein. We found that the abrogation of Src homology region 2 domain-containing phosphatase-1 (SHP-1) in tumor-reactive CD8(+) T cells improves the therapeutic outcome of adoptive immunotherapy in a mouse model of disseminated leukemia, with benefit observed in therapy employing transfer of CD8(+) T cells alone or in the context of also providing supplemental IL-2. SHP-1(-/-) and SHP-1(+/+) effector T cells were expanded in vitro for immunotherapy. Following transfer in vivo, the SHP-1(-/-) effector T cells exhibited enhanced short-term accumulation, followed by greater contraction, and they ultimately formed similar numbers of long-lived, functional memory cells. The increased therapeutic effectiveness of SHP-1(-/-) effector cells was also observed in recipients that expressed the tumor Ag as a self-antigen in the liver, without evidence of inducing autoimmune toxicity. SHP-1(-/-) effector CD8(+) T cells expressed higher levels of eomesodermin, which correlated with enhanced lysis of tumor cells. Furthermore, reduction of SHP-1 expression in tumor-reactive effector T cells by retroviral transduction with vectors that express SHP-1-specific small interfering RNA, a translatable strategy, also exhibited enhanced antitumor activity in vivo. These studies suggest that abrogating SHP-1 in effector T cells may improve the efficacy of tumor elimination by T cell therapy without affecting the ability of the effector cells to persist and provide a long-term response.

Phosphatase inhibitor, sodium stibogluconate, in combination with interferon (IFN) alpha 2b: phase I trials to identify pharmacodynamic and clinical effects

Since sodium stibogluconate (SSG) inhibited phosphatases including SHP-1 and augmented anti-tumor actions of IFN-α2b in vitro and in mice, two Phase I trials of SSG/IFN-α2b combination were undertaken to evaluate safety and target inhibition. Escalating doses of SSG (200-1200 mg/m2) and fixed doses of IFN-α2b (3x106 units/m2) with or without chemotherapy (dacarbazine, vinblastine, cisplatin) were evaluated for side effects and impact on SHP-1 phospho-substrates and IFNα-stimulated-genes (ISGs) in peripheral blood in 40 patients with metastatic melanoma, soft tissue sarcomas, gastrointestinal stromal tumors, and breast or colorectal carcinomas who did not have other established treatment options. Common adverse events were bone marrow suppression, fatigue, gastrointestinal upset, and asymptomatic lipase elevation (n=13); the latter was dose related and mostly after 10d of SSG/IFN-α2b in combination. Levels of SHP-1 substrates (pSTAT1, pSTAT3, pLck and pSlp76) were increased (up to 3x) in peripheral blood cells following SSG with no potentiation by combination with IFN-α2b. Representative ISGs in peripheral blood were induced after IFN-α2b at 4 and 24 hrs with selective modulations by combination. The median time on trials was 2.3 months (10-281d) with no objective regression of disease. Alive at 1y were 17/40 (43%) patients and after 2y were 8/40 (20%) following treatment initiation. These data demonstrate that SSG impacted signal molecules consistent with PTP inhibition and was tolerated in combination with IFN-α2b. Phase II investigations of SSG could safely utilize doses of up to 1200 mg/m2 of SSG for up to 10d alone or in combination with IFN-α2b with or without chemotherapy.

T cell protein tyrosine phosphatase attenuates T cell signaling to maintain tolerance in mice

Many autoimmune diseases exhibit familial aggregation, indicating that they have genetic determinants. Single nucleotide polymorphisms in PTPN2, which encodes T cell protein tyrosine phosphatase (TCPTP), have been linked with the development of several autoimmune diseases, including type 1 diabetes and Crohn's disease. In this study, we have identified TCPTP as a key negative regulator of TCR signaling, which might explain the association of PTPN2 SNPs with autoimmune disease. We found that TCPTP dephosphorylates and inactivates Src family kinases to regulate T cell responses. Using T cell-specific TCPTP-deficient mice, we established that TCPTP attenuates T cell activation and proliferation in vitro and blunts antigen-induced responses in vivo. TCPTP deficiency lowered the in vivo threshold for TCR-dependent CD8(+) T cell proliferation. Consistent with this, T cell-specific TCPTP-deficient mice developed widespread inflammation and autoimmunity that was transferable to wild-type recipient mice by CD8(+) T cells alone. This autoimmunity was associated with increased serum levels of proinflammatory cytokines and anti-nuclear antibodies, T cell infiltrates in non-lymphoid tissues, and liver disease. These data indicate that TCPTP is a critical negative regulator of TCR signaling that sets the threshold for TCR-induced naive T cell responses to prevent autoimmune and inflammatory disorders arising.

Phosphatase inhibitor, sodium stibogluconate, in combination with interferon (IFN) alpha 2b: phase I trials to identify pharmacodynamic and clinical effects

Since sodium stibogluconate (SSG) inhibited phosphatases including SHP-1 and augmented anti-tumor actions of IFN-α2b in vitro and in mice, two Phase I trials of SSG/IFN-α2b combination were undertaken to evaluate safety and target inhibition. Escalating doses of SSG (200-1200 mg/m2) and fixed doses of IFN-α2b (3x106 units/m2) with or without chemotherapy (dacarbazine, vinblastine, cisplatin) were evaluated for side effects and impact on SHP-1 phospho-substrates and IFNα-stimulated-genes (ISGs) in peripheral blood in 40 patients with metastatic melanoma, soft tissue sarcomas, gastrointestinal stromal tumors, and breast or colorectal carcinomas who did not have other established treatment options. Common adverse events were bone marrow suppression, fatigue, gastrointestinal upset, and asymptomatic lipase elevation (n=13); the latter was dose related and mostly after 10d of SSG/IFN-α2b in combination. Levels of SHP-1 substrates (pSTAT1, pSTAT3, pLck and pSlp76) were increased (up to 3x) in peripheral blood cells following SSG with no potentiation by combination with IFN-α2b. Representative ISGs in peripheral blood were induced after IFN-α2b at 4 and 24 hrs with selective modulations by combination. The median time on trials was 2.3 months (10-281d) with no objective regression of disease. Alive at 1y were 17/40 (43%) patients and after 2y were 8/40 (20%) following treatment initiation. These data demonstrate that SSG impacted signal molecules consistent with PTP inhibition and was tolerated in combination with IFN-α2b. Phase II investigations of SSG could safely utilize doses of up to 1200 mg/m2 of SSG for up to 10d alone or in combination with IFN-α2b with or without chemotherapy.

A new pathway of CD5 glycoprotein-mediated T cell inhibition dependent on inhibitory phosphorylation of Fyn kinase

Triggering of the T cell receptor initiates a signaling cascade resulting in the activation of the T cell. These signals are integrated alongside those resulting from the triggering of other receptors whose function is to modulate the overall response. CD5 is an immunotyrosine-based inhibition motif-bearing receptor that antagonizes the overt T cell receptor activation response by recruiting inhibitory intracellular mediators such as SHP-1, RasGAP, or Cbl. We now propose that the inhibitory effects of CD5 are also mediated by a parallel pathway that functions at the level of inhibition of Fyn, a kinase generally associated with T cell receptor-mediated activation. After CD5 ligation, phosphorylation of the negative regulatory tyrosine (Tyr(531)) of Fyn increases, and this correlates with a substantial reduction in the kinase activity of Fyn and a profound inhibition of ZAP-70 activation. The effect requires the last 23 amino acids of the cytoplasmic domain of the receptor, strongly implying the involvement of a new CD5-interacting signaling or adaptor protein. Furthermore, we show that upon CD5 ligation there is a profound shift in its distribution from the bulk fluid phase to the lipid raft environment, where it associates with Fyn, Lck, and PAG. We suggest that the relocation of CD5, which we also show is capable of forming homodimers, to the proximity of raft-resident molecules enables CD5 to inhibit membrane proximal signaling by controlling the phosphorylation and activity of Fyn, possibly by interfering with the disassembly of C-terminal Src kinase (Csk)-PAG-Fyn complexes during T cell activation.

T cell recognition of weak ligands: roles of signaling, receptor number, and affinity

T cell recognition of antigen is a crucial aspect of the adaptive immune response. One of the most common means of pathogen immune evasion is mutation of T cell epitopes. T cell recognition of such ligands can result in a variety of outcomes including activation, apoptosis and anergy. The ability of a given T cell to respond to a specific peptide-MHC ligand is regulated by a number of factors, including the affinity, on- and off-rates and half-life of the TCR-peptide-MHC interaction. Interaction of T cells with low-potency ligands results in unique signaling patterns and requires engagement with a larger number of T cell receptors than agonist ligands. This review will address these aspects of T cell interaction with weak ligands and the ways in which these ligands have been utilized therapeutically.

Increased sensitivity to antigen in high avidity CD8(+) T cells results from augmented membrane proximal T-cell receptor signal transduction

The functional avidity of a cytotoxic T lymphocyte (CTL) is known to be a critical determinant of the efficacy with which it clears pathogens. High avidity cells, which are by definition highly sensitive to peptide antigen, are superior for elimination of viruses and tumours. Our studies have established the ability of T cells to undergo avidity modulation as a result of antigen encounter. High and low avidity cells established in this manner exhibit significant differences in the amount of peptide required to elicit effector function. However, how signalling is regulated in these cells as it relates to the control of peptide sensitivity remains to be defined. To address this question, we compared T-cell receptor (TCR) signal transduction events in high and low avidity CTL generated from OT-I(rag2-) TCR transgenic mice. Our data suggest that divergent signalling is initiated at the TCR-associated CD3ζ, with low avidity CTL requiring higher amounts of pMHC to achieve threshold levels of phosphorylated CD3ζ compared with high avidity CTL. Further, this difference is transduced further downstream to mitogen-activated protein kinase and Ca(2+) signalling pathways. These results suggest that regulated control of the initiation of TCR signalling in high versus low avidity cells determines the amount of peptide required for T-cell activation.

A discrete affinity-driven elevation of ZAP-70 kinase activity initiates negative selection

CONTEXT

Although ZAP-70 is required for T-cell development, it's unclear how this kinase controls both positive and negative selection.

OBJECTIVE AND METHODS

Using OT-I pre-selection thymocytes and a panel of peptide major histocompatibility complex (pMHC) ligands of defined affinity, the recruitment, phosphorylation and activity of ZAP-70 was determined at the interface with antigen-presenting cells (APCs).

RESULTS

pMHC ligands promoting negative selection induce a discrete elevation of ZAP-70 recruitment, phosphorylation and enzymatic activity in the thymocyte:APCs interface.

DISCUSSION

The quantity of ZAP-70 kinase activity per cell is a key parameter controlling the fate of a developing thymocyte since partial inhibition of ZAP-70 kinase activity converted negative into positive selection. Surprisingly, the amount of ZAP-70 enzymatic activity observed during negative selection is not controlled by differential phosphorylation of the ZAP-70 protein but rather by the total amount of T-cell receptor and co-associated ZAP-70 recruited to the thymocyte:APC interface.

CONCLUSIONS

These data provide evidence that a burst of ZAP-70 activity initiates the signaling pathways for negative selection.

SHP-1 in T cells limits the production of CD8 effector cells without impacting the formation of long-lived central memory cells

During responses against viruses and malignancies, naive CD8 T lymphocytes expand to form both short-lived effector cells and a population containing cells with the potential to be long-lived and participate in memory responses (memory precursor effector cells). The strength of antigenic, costimulatory, and cytokine signals during responses impacts the magnitude and type of CD8 populations formed. In vitro studies have revealed that the tyrosine phosphatase Src homology region 2 domain-containing phosphatase-1 (SHP-1) regulates signal transduction from receptors on T cells including the TCR, helping set the activation threshold, and therefore may shape responses of mature CD8 T cells in vivo. Analysis of CD8 T cells from motheaten mice, which are globally deficient in SHP-1, proved problematic due to cell-extrinsic effects of SHP-1 deficiency in non-T cells on CD8 T cells. Therefore, a conditional knockout of SHP-1 in mature single-positive T cells was developed to analyze cell-intrinsic consequences of complete and partial SHP-1 deficiency on CD8 T cell responses to acute viral infection. The results demonstrated that SHP-1 has disparate effects on subpopulations of responding cells, limiting the magnitude and quality of primary and secondary responses by reducing the number of short-lived effector cells generated without affecting the size of the memory precursor effector cell pool that leads to formation of long-term memory.

Zap70 functions to maintain stemness of mouse embryonic stem cells by negatively regulating Jak1/Stat3/c-Myc signaling

Zeta-chain-associated protein kinase-70 (Zap70), a Syk family tyrosine kinase, has been reported to be present exclusively in normal T-cells, natural killer cells, and B cells, serving as a pivotal regulator of antigen-mediated receptor signaling and development. In this study, we report that Zap70 is expressed in undifferentiated mouse embryonic stem cells (mESCs) and may critically regulate self-renewal and pluripotency in mESCs. We found that Zap70 knocked-down mESCs (Zap70KD) show sustained self-renewal and defective differentiation. In addition, we present evidence that the sustained self-renewal in Zap70KD is associated with enhanced Jak/Stat3 signaling and c-Myc induction. These altered signaling appears to result from upregulated leukemia inhibitory factor receptor and downregulated src homology region 2 domain containing phosphatase 1 (SHP-1) phosphatase activity. On the basis of these results, we propose that in undifferentiated mESCs, Zap70 plays important roles in modulating the balance between self-renewal capacity and pluripotent differentiation ability as a key regulator of the Jak/Stat3/c-Myc signaling pathway.

The nonphagocytic NADPH oxidase Duox1 mediates a positive feedback loop during T cell receptor signaling

Production of reactive oxygen species, often by NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidases, plays a role in the signaling responses of cells to many receptor stimuli. Here, we describe the function of the calcium-dependent, nonphagocytic NADPH oxidase Duox1 in primary human CD4(+) T cells and cultured T cell lines. Duox1 bound to inositol 1,4,5-trisphosphate receptor 1 and was required for early T cell receptor (TCR)-stimulated production of hydrogen peroxide (H(2)O(2)) through a pathway that was dependent on TCR-proximal kinases. Transient or stable knockdown of Duox1 inhibited TCR signaling, especially phosphorylation of tyrosine-319 of zeta chain-associated protein kinase of 70 kilodaltons (ZAP-70), store-operated entry of calcium ions (Ca(2+)), and activation of extracellular signal-regulated kinase. The production of cytokines was also inhibited by knockdown of Duox1. Duox1-mediated inactivation of Src homology 2 domain-containing protein tyrosine phosphatase 2 promoted the phosphorylation of ZAP-70 and its association with the Src family tyrosine kinase Lck and the CD3zeta chain of the TCR complex. Thus, we suggest that activation of Duox1, downstream of proximal TCR signals, generates H(2)O(2) that acts in a positive feedback loop to enhance and sustain further TCR signaling.

Perspectives for computer modeling in the study of T cell activation

The T cell receptor (TCR) is responsible for discriminating between self- and foreign-derived peptides, translating minute differences in amino-acid sequence into large differences in response. Because of the great variability in the TCR and its ligands, activation of T cells by foreign peptides is a quantitative process, dependent on a mix of upstream signals and downstream integration. Accordingly, quantitative data and computational models have shed light on many important aspects of this process: molecular noise in ligand recognition, spatial dynamics in T cell-APC (antigen presenting cell) interactions, graded versus all-or-none decision making by the TCR apparatus, mechanisms of peptide antagonism and synergism, and the tunability and robustness of activation thresholds. Though diverse in their formalism, these studies together paint a picture of how modeling has shaped and will continue to shape understanding of T cell immunobiology.

Novel SHP-1 inhibitors tyrosine phosphatase inhibitor-1 and analogs with preclinical anti-tumor activities as tolerated oral agents

Src homology region 2 domain-containing phosphatase 1 (SHP-1) has been implicated as a potential cancer therapeutic target by its negative regulation of immune cell activation and the activity of the SHP-1 inhibitor sodium stibogluconate that induced IFN-gamma(+) cells for anti-tumor action. To develop more potent SHP-1-targeted anti-cancer agents, inhibitory leads were identified from a library of 34,000 drug-like compounds. Among the leads and active at low nM for recombinant SHP-1, tyrosine phosphatase inhibitor-1 (TPI-1) selectively increased SHP-1 phospho-substrates (pLck-pY394, pZap70, and pSlp76) in Jurkat T cells but had little effects on pERK1/2 or pLck-pY505 regulated by phosphatases SHP-2 or CD45, respectively. TPI-1 induced mouse splenic-IFN-gamma(+) cells in vitro, approximately 58-fold more effective than sodium stibogluconate, and increased mouse splenic-pLck-pY394 and -IFN-gamma(+) cells in vivo. TPI-1 also induced IFN-gamma(+) cells in human peripheral blood in vitro. Significantly, TPI-1 inhibited ( approximately 83%, p < 0.002) the growth of B16 melanoma tumors in mice at a tolerated oral dose in a T cell-dependent manner but had little effects on B16 cell growth in culture. TPI-1 also inhibited B16 tumor growth and prolonged tumor mice survival as a tolerated s.c. agent. TPI-1 analogs were identified with improved activities in IFN-gamma(+) cell induction and in anti-tumor actions. In particular, analog TPI-1a4 as a tolerated oral agent completely inhibited the growth of K1735 melanoma tumors and was more effective than the parental lead against MC-26 colon cancer tumors in mice. These results designate TPI-1 and the analogs as novel SHP-1 inhibitors with anti-tumor activity likely via an immune mechanism, supporting SHP-1 as a novel target for cancer treatment.

In vivo regulation of the allergic response by the IL-4 receptor alpha chain immunoreceptor tyrosine-based inhibitory motif

BACKGROUND

Signaling by IL-4 and IL-13 through the IL-4 receptor alpha chain (IL-4Ralpha) plays a critical role in the pathology of allergic diseases. The IL-4Ralpha is endowed with an immunoreceptor tyrosine-based inhibitory motif (ITIM) centered on tyrosine 709 (Y709) in the cytoplasmic domain that binds a number of regulatory phosphatases. The function of the ITIM in the in vivo regulation of IL-4 receptor signaling remains unknown.

OBJECTIVE

We sought to determine the in vivo function of the IL-4Ralpha ITIM by using mice in which the ITIM was inactivated by mutagenesis of the tyrosine Y709 residue into phenylalanine (F709).

METHODS

F709 ITIM mutant mice were derived by means of knock-in mutagenesis. Activation of intracellular signaling cascades by IL-4 and IL-13 was assessed by means of intracellular staining of phosphorylated signaling intermediates and gene expression analysis. In vivo responses to allergic sensitization were assessed by using models of allergic airway inflammation.

RESULTS

The F709 mutation increased signal transducer and activator of transcription 6 phosphorylation by IL-4 and, disproportionately, by IL-13. This was associated with exaggerated T(H)2 polarization, enhanced alternative macrophage activation by IL-13, augmented basal and antigen-induced IgE responses, and intensified allergen-induced eosinophilic airway inflammation and hyperreactivity.

CONCLUSIONS

These results point to a physiologic negative regulatory role for the Y709 ITIM in signaling through IL-4Ralpha, especially by IL-13.

Gene expression profiling of the short-term adaptive response to acute caloric restriction in liver and adipose tissues of pigs differing in feed efficiency

Residual feed intake (RFI) is a measure of feed efficiency, in which low RFI denotes improved feed efficiency. Caloric restriction (CR) is associated with feed efficiency in livestock species and to human health benefits, such as longevity and cancer prevention. We have developed pig lines that differ in RFI, and we are interested in identifying the genes and pathways that underlie feed efficiency. Prepubertal Yorkshire gilts with low RFI (n = 10) or high RFI (n = 10) were fed ad libitum or fed at restricted intake of 80% of maintenance energy requirements for 8 days. We measured serum metabolites and hormones and generated transcriptional profiles of liver and subcutaneous adipose tissue on these animals. Overall, 6,114 genes in fat and 305 genes in liver were differentially expressed (DE) in response to CR, and 311 genes in fat and 147 genes in liver were DE due to RFI differences. Pathway analyses of CR-induced DE genes indicated a dramatic switch to a conservation mode of energy usage by down-regulating lipogenesis and steroidogenesis in both liver and fat. Interestingly, CR altered expression of genes in immune and cell cycle/apoptotic pathways in fat, which may explain part of the CR-driven lifespan enhancement. In silico analysis of transcription factors revealed ESR1 as a putative regulator of the adaptive response to CR, as several targets of ESR1 in our DE fat genes were annotated as cell cycle/apoptosis genes. The lipid metabolic pathway was overrepresented by down-regulated genes due to both CR and low RFI. We propose a common energy conservation mechanism, which may be controlled by PPARA, PPARG, and/or CREB in both CR and feed-efficient pigs.

The Sts proteins target tyrosine phosphorylated, ubiquitinated proteins within TCR signaling pathways

The T cell receptor (TCR) detects the presence of infectious pathogens and activates numerous intracellular signaling pathways. Protein tyrosine phosphorylation and ubiquitination serve as key regulatory mechanisms downstream of the TCR. Negative regulation of TCR signaling pathways is important in controlling the immune response, and the Suppressor of TCR Signaling proteins (Sts-1 and Sts-2) have been shown to function as critical negative regulators of TCR signaling. Although their mechanism of action has yet to be fully uncovered, it is known that the Sts proteins possess intrinsic phosphatase activity. Here, we demonstrate that Sts-1 and Sts-2 are instrumental in down-modulating proteins that are dually modified by both protein tyrosine phosphorylation and ubiquitination. Specifically, both naïve and activated T cells derived from genetically engineered mice that lack the Sts proteins display strikingly elevated levels of tyrosine phosphorylated, ubiquitinated proteins following TCR stimulation. The accumulation of the dually modified proteins is transient, and in activated T cells but not naïve T cells is significantly enhanced by co-receptor engagement. Our observations hint at a novel regulatory mechanism downstream of the T cell receptor.

A new approach for quantitative phosphoproteomic dissection of signaling pathways applied to T cell receptor activation

Reversible protein phosphorylation plays a pivotal role in the regulation of cellular signaling pathways. Current approaches in phosphoproteomics focus on analysis of the global phosphoproteome in a single cellular state or of receptor stimulation time course experiments, often with a restricted number of time points. Although these studies have provided some insights into newly discovered phosphorylation sites that may be involved in pathways, they alone do not provide enough information to make precise predictions of the placement of individual phosphorylation events within a signaling pathway. Protein disruption and site-directed mutagenesis are essential to clearly define the precise biological roles of the hundreds of newly discovered phosphorylation sites uncovered in modern proteomics experiments. We have combined genetic analysis with quantitative proteomic methods and recently developed visual analysis tools to dissect the tyrosine phosphoproteome of isogenic Zap-70 tyrosine kinase null and reconstituted Jurkat T cells. In our approach, label-free quantitation using normalization to copurified phosphopeptide standards is applied to assemble high density temporal data within a single cell type, either Zap-70 null or reconstituted cells, providing a list of candidate phosphorylation sites that change in abundance after T cell stimulation. Stable isotopic labeling of amino acids in cell culture (SILAC) ratios are then used to compare Zap-70 null and reconstituted cells across a time course of receptor stimulation, providing direct information about the placement of newly observed phosphorylation sites relative to Zap-70. These methods are adaptable to any cell culture signaling system in which isogenic wild type and mutant cells have been or can be derived using any available phosphopeptide enrichment strategy.

PD-1 signaling in primary T cells

SUMMARY

Programmed death-1 (PD-1) is a cell surface molecule that regulates the adaptive immune response. Engagement of PD-1 by its ligands PD-L1 or PD-L2 transduces a signal that inhibits T-cell proliferation, cytokine production, and cytolytic function. While a great deal is known concerning the biologic roles PD-1 plays in regulating the primary immune response and in T-cell exhaustion, comparatively little is known regarding how PD-1 ligation alters signaling pathways. PD-1 ligation is known to inhibit membrane-proximal T-cell signaling events, while ligation of the related inhibitory molecule cytotoxic T-lymphocyte antigen-4 appears to target more downstream signaling pathways. A major obstacle to an in-depth understanding of PD-1 signaling is the lack of physiologic models in which to study signal transduction. This review focuses on: (i) signaling pathways altered by PD-1 ligation, (ii) factors recruited upon PD-1 phosphorylation, and (iii) exploring the hypothesis that PD-1 ligation induces distinct signals during various stages of immune-cell differentiation. Lastly, we describe models to dissect the function of the PD-1 cytoplasmic tail using primary cells in the absence of agonist antibodies.

SHP-1 and SHP-2 in T cells: two phosphatases functioning at many levels

Tyrosine phosphorylation and dephosphorylation of proteins play a critical role for many T-cell functions. The opposing actions of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) determine the level of tyrosine phosphorylation at any time. It is well accepted that PTKs are essential during T-cell signaling; however, the role and importance of PTPs are much less known and appreciated. Both transmembrane and cytoplasmic tyrosine phosphatases have been identified in T cells and shown to regulate T-cell responses. This review focuses on the roles of the two cytoplasmic PTPs, the Src-homology 2 domain (SH2)-containing SHP-1 and SHP-2, in T-cell signaling, development, differentiation, and function.

Signaling defects in anti-tumor T cells

The immune response to cancer has been long recognized, including both innate and adaptive responses, showing that the immune system can recognize protein products of genetic and epigenetic changes in transformed cells. The accumulation of antigen-specific T cells within the tumor, the draining lymph node, and the circulation, either in newly diagnosed patients or resultant from experimental immunotherapy, proves that tumors produce antigens and that priming occurs. Unfortunately, just as obviously, tumors grow, implying that anti-tumor immune responses are either not sufficiently vigorous to eliminate the cancer or that anti-tumor immunity is suppressed. Both possibilities are supported by current data. In experimental animal models of cancer and also in patients, systemic immunity is usually not dramatically suppressed, because tumor-bearing animals and patients develop T-cell-dependent immune responses to microbes and to either model antigens or experimental cancer vaccines. However, inhibition of specific anti-tumor immunity is common, and several possible explanations of tolerance to tumor antigens or tumor-induced immunesuppression have been proposed. Inhibition of effective anti-tumor immunity results from the tumor or the host response to tumor growth, inhibiting the activation, differentiation, or function of anti-tumor immune cells. As a consequence, anti-tumor T cells cannot respond productively to developmental, targeting, or activation cues. While able to enhance the number and phenotype of anti-tumor T cells, the modest success of immunotherapy has shown the necessity to attempt to reverse tolerance in anti-tumor T cells, and the vanguard of experimental therapy now focuses on vaccination in combination with blockade of immunosuppressive mechanisms. This review discusses several potential mechanisms by which anti-tumor T cells may be inhibited in function.

Tailoring T-cell receptor signals by proximal negative feedback mechanisms

The T-cell receptor (TCR) signalling machinery is central in determining the response of a T cell (establishing immunity or tolerance) following exposure to antigen. This process is made difficult by the narrow margin of self and non-self discrimination, and by the complexity of the genetic programmes that are induced for each outcome. Recent studies have identified novel negative feedback mechanisms that are rapidly induced by TCR engagement and that have key roles in the regulation of signal triggering and propagation. In vitro and in vivo data suggest that they are important in determining ligand discrimination by the TCR and in regulating signal output in response to antigen.

Survival of monocytes and macrophages and their role in health and disease

Macrophages are versatile cells involved in health and disease. These cells act as scavengers to rid the body of apoptotic and senescent cells and debris through their phagocytic function. Although this is a primary function of these cells, macrophages play vital roles in inflammation and repair of damaged tissue. Macrophages secrete a large number of cytokines, chemokines and growth factors that recruit and activate a variety of cell types to inflamed tissue compartments. These cells are also critical in cell-mediated immunity and in the resolution of inflammation. Since macrophages, and their precursors, blood monocytes, are important in regulating and resolving inflammation, prolonged cellular survival in tissue compartments could be detrimental. Thus, factors that regulate the fate of monocyte and macrophage survival are important in cellular homeostasis. In this article, we will explore stimuli and the intracellular pathways important in regulating macrophage survival and implication in human disease.

Activation of Cdk2 stimulates proteasome-dependent truncation of tyrosine phosphatase SHP-1 in human proliferating intestinal epithelial cells

SHP-1 is expressed in the nuclei of intestinal epithelial cells (IECs). Increased SHP-1 expression and phosphatase activity coincide with cell cycle arrest and differentiation in these cells. Suspecting the tumor-suppressive properties of SHP-1, a yeast two-hybrid screen of an IEC cDNA library was conducted using the full-length SHP-1 as bait. Characterization of many positive clones revealed sequences identical to a segment of the Cdk2 cDNA sequence. Interaction between SHP-1 and Cdk2 was confirmed by co-immunoprecipitations whereby co-precipitated Cdk2 phosphorylated SHP-1 protein. Inhibition of Cdk2 (roscovitine) or proteasome (MG132) was associated with an enhanced nuclear punctuate distribution of SHP-1. Double labeling localization studies with signature proteins of subnuclear domains revealed a co-localization between the splicing factor SC35 and SHP-1 in bright nucleoplasmic foci. Using Western blot analyses with the anti-SHP-1 antibody recognizing the C terminus, a lower molecular mass species of 45 kDa was observed in addition to the full-length 64-65-kDa SHP-1 protein. Treatment with MG132 led to an increase in expression of the full-length SHP-1 protein while concomitantly leading to a decrease in the levels of the lower mass 45-kDa molecular species. Further Western blots revealed that the 45-kDa protein corresponds to the C-terminal portion of SHP-1 generated from proteasome activity. Mutational analysis of Tyr(208) and Ser(591) (a Cdk2 phosphorylation site) residues on SHP-1 abolished the expression of the amino-truncated 45-kDa SHP-1 protein. In conclusion, our results indicate that Cdk2-associated complexes, by targeting SHP-1 for proteolysis, counteract the ability of SHP-1 to block cell cycle progression of IECs.

Carcinoembryonic antigen-related cell adhesion molecule 1 inhibits proximal TCR signaling by targeting ZAP-70

The long cytoplasmic tail (CT) isoforms of carcinoembryonic Ag-related cell adhesion molecule 1 (CEACAM1) are expressed on activated human T cells and possess two ITIM motifs in the CT. These isoforms of CEACAM1 are inhibitory for T cell responses initiated by the TCR/CD3 complex with the inhibition dependent upon the ITIMs of CEACAM1 and Src homology 2 domain-containing phosphatase 1 (SHP-1). However, the mechanism by which this inhibition occurs in T cells is unknown. We demonstrate here that the Src family kinase, Lck, and the ability of CEACAM1 to bind homophilically are required for the ITIM phosphorylation of CEACAM1 that is a prerequisite for CEACAM1 association with SHP-1. We further show that CEACAM1 associates with and recruits SHP-1 to the TCR/CD3 complex leading to decreased phosphorylation of CD3-zeta and ZAP-70 and consequently decreased activation of the elements downstream of ZAP-70. This is physiologically relevant because extinction of SHP-1 expression or blockade of homophilic binding by CEACAM1 using a Fab that specifically recognizes the homophilic binding region of human CEACAM1 increases the cytolytic function initiated by the TCR/CD3 complex. These studies show that long CT isoforms of CEACAM1 orchestrate an inhibitory program that abrogates extremely proximal events downstream of the TCR/CD3 complex by focusing on the activation of ZAP-70.

Involvement of CD45 in DNA fragmentation in apoptosis induced by mitochondrial perturbing agents

CD45 is a type I transmembrane molecule with phosphatase activity which comprises up to 10% of the cell surface area in nucleated haematopoietic cells. We have previously demonstrated the absence of nuclear apoptosis in CD45-negative T cells after chemical-induced apoptosis. The aim of this study was to characterize the role of CD45 in nuclear apoptosis. In contrast to wild type CD45-positive T cells, the CD45-deficient T cell lines are resistant to the induction of DNA fragmentation and chromatin condensation following tributyltin (TBT) or H2O2 exposure, but not to cycloheximide-induced apoptosis. CD45 transfection in deficient cell lines led to the restoration of chromatin condensation and DNA fragmentation following TBT exposure. In both CD45-positive and negative T cell lines, TBT exposure mediates intracellular calcium mobilization, caspase-3 activation and DFF45 cleavage. Moreover, DNA fragmentation was also induced by TBT in cells deficient in expression of p56lck, ZAP-70 and SHP-1. Subcellular partitioning showed a decrease in nuclear localisation of caspase-3 and DFF40. Together, these results demonstrate for the first time, that CD45 expression plays a key role in internucleosomal DNA fragmentation and chromatin condensation processes during apoptosis. CD45 activity or its substrates' activity, appears to be located downstream of caspase-3 activation and plays a role in retention of DFF40 in the nucleus.

BCR ligation induced by IgM stimulation results in gene expression and functional changes only in IgV H unmutated chronic lymphocytic leukemia (CLL) cells

Chronic lymphocytic leukemia (CLL) patients exhibit a variable clinical course. To investigate the association between clinicobiologic features and responsiveness of CLL cells to anti-IgM stimulation, we evaluated gene expression changes and modifications in cell-cycle distribution, proliferation, and apoptosis of IgV(H) mutated (M) and unmutated (UM) samples upon BCR cross-linking. Unsupervised analysis highlighted a different response profile to BCR stimulation between UM and M samples. Supervised analysis identified several genes modulated exclusively in the UM cases upon BCR cross-linking. Functional gene groups, including signal transduction, transcription, cell-cycle regulation, and cytoskeleton organization, were up-regulated upon stimulation in UM cases. Cell-cycle and proliferation analyses confirmed that IgM cross-linking induced a significant progression into the G(1) phase and a moderate increase of proliferative activity exclusively in UM patients. Moreover, we observed only a small reduction in the percentage of subG(0/1) cells, without changes in apoptosis, in UM cases; contrariwise, a significant increase of apoptotic levels was observed in stimulated cells from M cases. These results document that a differential genotypic and functional response to BCR ligation between IgV(H) M and UM cases is operational in CLL, indicating that response to antigenic stimulation plays a pivotal role in disease progression.