TCR-mediated Signaling Research Articles

Mitochondria-derived Hydrogen Peroxide Selectively Enhances T Cell Receptor-initiated Signal Transduction

T cell receptor (TCR)-initiated signal transduction is reported to increase production of intracellular reactive oxygen species, such as superoxide (O2˙(-)) and hydrogen peroxide (H2O2), as second messengers. Although H2O2 can modulate signal transduction by inactivating protein phosphatases, the mechanism and the subcellular localization of intracellular H2O2 as a second messenger of the TCR are not known. The antioxidant enzyme superoxide dismutase (SOD) catalyzes the dismutation of highly reactive O2˙(-) into H2O2 and thus acts as an intracellular generator of H2O2. As charged O2˙(-) is unable to diffuse through intracellular membranes, cells express distinct SOD isoforms in the cytosol (Cu,Zn-SOD) and mitochondria (Mn-SOD), where they locally scavenge O2˙(-) leading to production of H2O2. A 2-fold organelle-specific overexpression of either SOD in Jurkat T cell lines increases intracellular production of H2O2 but does not alter the levels of intracellular H2O2 scavenging enzymes such as catalase, membrane-bound peroxiredoxin1 (Prx1), and cytosolic Prx2. We report that overexpression of Mn-SOD enhances tyrosine phosphorylation of TCR-associated membrane proximal signal transduction molecules Lck, LAT, ZAP70, PLCγ1, and SLP76 within 1 min of TCR cross-linking. This increase in mitochondrial H2O2 specifically modulates MAPK signaling through the JNK/cJun pathway, whereas overexpressing Cu,Zn-SOD had no effect on any of these TCR-mediated signaling molecules. As mitochondria translocate to the immunological synapse during TCR activation, we hypothesize this translocation provides the effective concentration of H2O2 required to selectively modulate downstream signal transduction pathways.

Modulation of CD8+ T-cell activation events by monocytic and granulocytic myeloid-derived suppressor cells

Myeloid-derived suppressor cells are immature myeloid cells, consisting of a monocytic and a granulocytic fraction, that are known to suppress anti-tumor immune responses. Important targets of the immunosuppressive capacity of MDSC are CD8+ T cells, which are crucial cytotoxic effector cells in immunotherapeutic settings. CD8+ T-cell activation and differentiation comprises a well-orchestrated series of events, starting from early TCR-mediated signaling and leading to cytokine secretion, the expression of activation markers, proliferation and the differentiation into several subsets of effector and memory cells. In this review, we summarize the available data on how the production of reactive oxygen species, nitric oxide, the arginase-mediated depletion of l-arginine and Cystine depletion by MDSCs interfere with the signaling molecules necessary for normal CTL differentiation and activation.

Different Modulation of Ptpn22 in Effector and Regulatory T Cells Leads to Attenuation of Autoimmune Diabetes in Transgenic Nonobese Diabetic Mice

Ptpn22 encodes PEST domain-enriched tyrosine phosphatase (Pep), which negatively regulates TCR proximal signaling and is strongly associated with a variety of autoimmune diseases in humans. The net effect of Pep on the balance of immunity and tolerance is uncertain because of the simultaneous inhibition of TCR-mediated signaling of effector and regulatory T cells (T(regs)). In this study, we generated transgenic NOD mice that overexpressed Pep in T cells. The transgenic mice had a significantly lower incidence of spontaneous autoimmune diabetes, which was accompanied by fewer IFN-γ-producing T cells, and an increased ratio of CD4(+)Foxp3(+) T(regs)to CD4(+)IFN-γ(+) or to CD8(+)IFN-γ(+) T cells, respectively, in pancreatic islets. Transgenic T cells showed markedly decreased TCR-mediated effector cell responses such as proliferation and Th1 differentiation. By contrast, the inhibitory effect of transgenic Pep on TCR signaling did not affect the differentiation of T(regs) or their suppressive activity. Adoptive transfer experiments showed that transgenic splenocytes exhibited attenuated diabetogenic ability. To examine further the pathogenic features of transgenic T cells, we generated Ptpn22/BDC2.5 doubly transgenic mice and found reduced proliferation and Th1 differentiation in CD4(+) T lymphocytes with additional Pep in pancreatic lymph nodes but not in inguinal lymph nodes of NOD/SCID recipients. This finding indicates that transgenic Pep attenuates T cell functions in an islet Ag-driven manner. Taken together, our results demonstrate that Pep overexpression in T cells attenuates autoimmune diabetes in NOD mice by preferentially modulating TCR signaling-mediated functions in diabetogenic T cells but not in T(regs).

NKT Cell-TCR Expression Activates Conventional T Cells in Vivo, but Is Largely Dispensable for Mature NKT Cell Biology

Natural killer T (NKT) cell development depends on recognition of self-glycolipids via their semi-invariant Vα14i-TCR. However, to what extent TCR-mediated signals determine identity and function of mature NKT cells remains incompletely understood. To address this issue, we developed a mouse strain allowing conditional Vα14i-TCR expression from within the endogenous Tcrα locus. We demonstrate that naïve T cells are activated upon replacement of their endogenous TCR repertoire with Vα14i-restricted TCRs, but they do not differentiate into NKT cells. On the other hand, induced TCR ablation on mature NKT cells did not affect their lineage identity, homeostasis, or innate rapid cytokine secretion abilities. We therefore propose that peripheral NKT cells become unresponsive to and thus are independent of their autoreactive TCR.

GB Virus C Particles Inhibit T Cell Activation via Envelope E2 Protein-Mediated Inhibition of TCR Signaling

Viruses enter into complex interactions within human hosts, leading to facilitation or suppression of each other's replication. Upon coinfection, GB virus C (GBV-C) suppresses HIV-1 replication in vivo and in vitro, and GBV-C coinfection is associated with prolonged survival in HIV-infected people. GBV-C is a lymphotropic virus capable of persistent infection. GBV-C infection is associated with reduced T cell activation in HIV-infected humans, and immune activation is a critical component of HIV disease pathogenesis. We demonstrate that serum GBV-C particles inhibited activation of primary human T cells. T cell activation inhibition was mediated by the envelope glycoprotein E2, because expression of E2 inhibited TCR-mediated activation of Lck. The region on the E2 protein was characterized and revealed a highly conserved peptide motif sufficient to inhibit TCR-mediated signaling. The E2 region contained a predicted Lck substrate site, and substitution of an alanine or histidine for the tyrosine reversed TCR-signaling inhibition. GBV-C E2 protein and a synthetic peptide representing the inhibitory amino acid sequence were phosphorylated by Lck in vitro. The synthetic peptide also inhibited TCR-mediated activation of primary human CD4(+) and CD8(+) T cells. Extracellular microvesicles from GBV-C E2-expressing cells contained E2 protein and inhibited TCR signaling in bystander T cells not expressing E2. Thus, GBV-C reduced global T cell activation via competition between its envelope protein E2 and Lck following TCR engagement. This novel inhibitory mechanism of T cell activation may provide new approaches for HIV and immunoactivation therapy.

OX40L/Jagged1 Cosignaling by GM-CSF–Induced Bone Marrow-Derived Dendritic Cells Is Required for the Expansion of Functional Regulatory T Cells

Abstract Earlier, we had demonstrated that treatment with low dose of GM-CSF can prevent the development of experimental autoimmune thyroiditis (EAT), experimental autoimmune myasthenia gravis, and type 1 diabetes, and could also reverse ongoing EAT and experimental autoimmune myasthenia gravis. The protective effect was mediated through the induction of tolerogenic CD11C+CD8α− dendritic cells (DCs) and consequent expansion of Foxp3+ regulatory T cells (Tregs). Subsequently, we showed that GM-CSF acted specifically on bone marrow precursors and facilitated their differentiation into tolerogenic dendritic cells (DCs; GM-CSF–induced bone marrow-derived DCs [GM-BMDCs]), which directed Treg expansion in a contact-dependent manner. This novel mechanism of Treg expansion was independent of TCR-mediated signaling but required exogenous IL-2 and cosignaling from DC-bound OX40L. In this study, we observed that OX40L-mediated signaling by GM-BMDCs, although necessary, was not sufficient for Treg expansion and required signaling by Jagged1. Concurrent signaling induced by OX40L and Jagged1 via OX40 and Notch3 receptors expressed on Tregs was essential for the Treg expansion with sustained FoxP3 expression. Adoptive transfer of only OX40L+Jagged1+ BMDCs led to Treg expansion, increased production of IL-4 and IL-10, and suppression of EAT in the recipient mice. These results showed a critical role for OX40L- and Jagged1-induced cosignaling in GM-BMDC–induced Treg expansion.

Enhanced PI3K/Akt-mediated survival of memory Th1 cells in type-1 diabetics and healthy controls carrying the autoimmunity-associated 1858T allelic variant of PTPN22 (P4097)

Abstract A C-to-T polymorphism (1858T) in the PTPN22 gene alters the function of the lymphoid phosphatase Lyp and confers increased risk for autoimmunity. The PTPN22 1858T allele is associated with altered TCR signaling and expanded memory T cell pool. To determine its impact on lineage commitment and survival, naïve CD4+ T cells from healthy subjects carrying risk or non-risk alleles were differentiated in vitro. Cultures of T cells bearing PTPN22 1858T displayed similar proliferative capacity but reduced activation-induced cell death, yielding more IFNγ+ Th1 cells than non-risk controls. In vivo, PTPN22 1858T subjects mounted enhanced IFNγ responses to the pneumococcal vaccine Prevnar, but not IL-2 production or T cell proliferation. Examination of TCR-mediated signaling revealed that elevated pAkt activity and BCL2 expression were associated with the PTPN22 1858T allele and correlated with enhanced T cell survival during in vitro stimulation. This phenotype was mirrored in T cell cultures from type-1 diabetic subjects, regardless of their PTPN22 genotype, and was reversed by a PI3Kδ-specific inhibitor. Our studies indicate that the autoimmunity-associated PTPN22 1858T allele protects T cells from apoptosis via elevated PI3K/Akt signaling, leading to the accumulation of pro-inflammatory memory Th1 cells. This T-cell intrinsic defect is a general feature of T1D and inhibition of PI3Kδ offers a potential avenue for pharmacological intervention.

The Requirement of Linker for Activation of T Cells in the Primary and Memory Responses of CD8 T Cells

Linker for activation of T cells (LAT) is a transmembrane adaptor protein that links TCR engagement to downstream signaling events. Although it is clear that LAT is essential in thymocyte development and initiation of T cell activation, its function during T cell expansion, contraction, and memory formation remains unknown. To study the role of TCR-mediated signaling in CD8 T cells during the course of pathogen infection, we used an inducible mouse model to delete LAT in Ag-specific CD8 T cells at different stages of Listeria infection and analyzed the effect of deletion on T cell responses. Our data showed that LAT is important for maintaining CD8 T cell expansion during the priming phase; however, it is not required for CD8 T cell contraction and memory maintenance. Moreover, LAT deficiency accelerates memory differentiation during the effector-to-memory transition, leading to a higher frequency of KLRG1(low)IL-7R(high)CD62L(high) memory T cells. Nonetheless, these LAT-deficient memory T cells were unable to proliferate or produce cytokines upon secondary infection. Our data demonstrated that, although TCR-mediated signaling is dispensable for contraction and memory maintenance, it regulates CD8 T cell memory differentiation and is essential for the memory response against pathogens.

Early signaling events that underlie fate decisions of naive CD4+ T cells toward distinct T‐helper cell subsets

CD4(+) T-helper (Th) cells are a major cell population that play an important role in governing acquired immune responses to a variety of foreign antigens as well as inducing some types of autoimmune diseases. There are at least four distinct Th cell subsets (Th1, Th2, Th17, and inducible T-regulatory cells), each of which has specialized functions to control immune responses. Each of these cell types emerge from naive CD4(+) T cells after encounter with foreign antigens presented by dendritic cells (DCs). Each Th cell subset expresses a unique set of transcription factors and produces hallmark cytokines. Both T-cell receptor (TCR)-mediated stimulation and the cytokine environment created by activated CD4(+) T cells themselves, by 'partner' DCs, and/or other cell types during the course of differentiation, play an important role in the fate decisions toward distinct Th subsets. Here, we review how TCR-mediated signals in collaboration with the cytokine environment influence the fate decisions of naive CD4(+) T cells toward distinct Th subsets at the early stages of activation. We also discuss the roles of TCR-proximal signaling intermediates and of the Notch pathway in regulating the differentiation to distinct Th phenotypes.

Kurarinone regulates immune responses through regulation of the JAK/STAT and TCR-mediated signaling pathways

Sophora flavescens is a medicinal herb that contains flavonoids and quinolizidine alkaloids and has a wide range of biological activities due to its anti-inflammatory, anti-bacterial and anti-cancer properties. We isolated a series of flavonoids from the roots of Sophora flavescens and examined their ability to inhibit immune responses. Among the flavonoids, kurarinone exhibited the strongest inhibitory effect on immune responses. Kurarinone suppressed the differentiation of CD4+ T cells by inhibiting the expression and production of T-cell lineage-specific master regulators and cytokines. Our results also demonstrated that kurarinone directly suppressed the cytokine-induced Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling and T-cell receptor (TCR) pathways. In two established animal models of chronic inflammatory skin disease, one in which psoriasis-like skin disease was induced by an interleukin 23 (IL-23) injection into mouse ears and another in which 2,4,6-trinitrochlorobenzene (TNCB) application on the abdomens of mice was used to induce contact dermatitis, kurarinone repressed disease development by inhibiting the expression of pro-inflammatory mediators, including cytokines, chemokines and enzyme in murine ear skin. This study provides new evidence that kurarinone may ameliorate chronic inflammatory skin diseases through the suppression of pathogenic CD4+ T-cell differentiation and the overall immune response.

TCR activation kinetics and feedback regulation in primary human T cells

BackgroundSignaling through the TCR is crucial for the generation of different cellular responses including proliferation, differentiation, and apoptosis. A growing body of evidence indicates that differences in the magnitude and the duration of the signal are critical determinants in eliciting cellular responses.ResultsHere, we have analyzed signaling dynamics correlating with either unresponsiveness or proliferation induced upon TCR/CD28 ligation in primary human T cells. We used two widely employed methods to stimulate T cells in vitro, antibodies either cross-linked in solution (sAbs) or immobilized on microbeads (iAbs). A comparative analysis of the signaling properties of iAbs and sAbs revealed that, under proliferation-inducing conditions, feedback regulation is markedly different from that leading to an unresponsive state. In fact, upon iAbs stimulation TCR-mediated signaling is prolonged by a positive feedback loop involving Erk, whereas sAbs strongly activate inhibitory molecules that likely terminate signaling. We additionally found that, by enhancing the phosphorylation of Src family kinases under proliferation-inducing conditions, signaling and T-cell activation are terminated.ConclusionsIn summary, our analysis documents TCR signaling kinetics and feedback regulation under conditions of stimulation inducing either unresponsiveness or proliferation.

Inducible Costimulator (ICOS) facilitates T-dependent B cell differentiation by augmenting IL-4 translation

Event Abstract Back to Event Inducible Costimulator (ICOS) facilitates T-dependent B cell differentiation by augmenting IL-4 translation Mathieu Gigoux1, 2*, Amanda Lovato1, 2, Julien Leconte1, 3, Joanne Leung1, 2, Nahum Sonenberg4, 5 and Woong-Kyung Suh1, 2, 3 1 Institut de Recherches Cliniques de Montréal, Canada 2 McGill University, Microbiology and Immunology, Canada 3 Université de Montréal, Médecine, Canada 4 McGill University, Biochemistry, Canada 5 The Rosalind and Morris Goodman Cancer Research Centre, Canada The Inducible Costimulator (ICOS) is a member of the CD28 family of costimulatory receptor that is highly expressed in follicular helper T cells (Tfh), a subset of CD4 T cells that migrate to the B cell zone and facilitate germinal center reactions. Although ICOS is known to play a critical role in shaping the Tfh cell population during immune reactions, its role in the effector function of Tfh cells has been unclear. Here we demonstrate that ICOS assists TCR-mediated signal transduction by potentiating PI3K-Akt-mTOR signaling cascade that leads to hyperphosphorylation of the key translational regulators p70S6K and 4E-BP1. Consistently, ICOS costimulation promotes the formation of polysomes on IL-4 mRNA in a PI3K-signaling dependent manner. Here we provide evidence that IL-4 production is diminished during T-B cell co-culture when ICOS-PI3K signaling axis is disrupted in T cells. We also show that the 5’ untranslated region of IL-4 mRNA is able to repress the translation of firefly luciferase. Thus, ICOS-PI3K signaling facilitates translation of IL-4 mRNA and this may augment IL-4 production during T-B interaction in germinal centers. Keywords: ICOS, IL-4, PI3K/AKT/mTOR, translation, costimulation Conference: 15th International Congress of Immunology (ICI), Milan, Italy, 22 Aug - 27 Aug, 2013. Presentation Type: Abstract Topic: Immune receptors and signaling Citation: Gigoux M, Lovato A, Leconte J, Leung J, Sonenberg N and Suh W (2013). Inducible Costimulator (ICOS) facilitates T-dependent B cell differentiation by augmenting IL-4 translation. Front. Immunol. Conference Abstract: 15th International Congress of Immunology (ICI). doi: 10.3389/conf.fimmu.2013.02.00538 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 10 Jun 2013; Published Online: 22 Aug 2013. * Correspondence: Mr. Mathieu Gigoux, Institut de Recherches Cliniques de Montréal, Montréal, Canada, Mathieu.Gigoux@ircm.qc.ca Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Mathieu Gigoux Amanda Lovato Julien Leconte Joanne Leung Nahum Sonenberg Woong-Kyung Suh Google Mathieu Gigoux Amanda Lovato Julien Leconte Joanne Leung Nahum Sonenberg Woong-Kyung Suh Google Scholar Mathieu Gigoux Amanda Lovato Julien Leconte Joanne Leung Nahum Sonenberg Woong-Kyung Suh PubMed Mathieu Gigoux Amanda Lovato Julien Leconte Joanne Leung Nahum Sonenberg Woong-Kyung Suh Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Distinct signaling pathways regulate TLR2 co-stimulatory function in human T cells

Toll-like receptor 2 (TLR2) serves as a co-stimulatory receptor for human T cells by enhancing T cell receptor (TCR)-induced cytokine production and proliferation. However, it is unknown where signals from the TCR and TLR2 converge to enhance T cell activation. To address this gap, we examined changes in TCR-induced signaling following concurrent TLR2 activation in human T cells. Both proximal TCR-mediated signaling and early NFκB activation were not enhanced by TCR andTLR2 co-activation, potentially due to the association of TLR2 with TLR10. Instead, TLR2 co-induction did augment Akt and Erk1/Erk2 activation in human T cells. These findings demonstrate that TLR2 activates distinct signaling pathways in human T cells and suggest that alterations in expression of TLR2 co-receptors may contribute to aberrant T cell responses.

Membrane microdomain organization, calcium signal, and NFAT activation as an important axis in polarized Th cell function

T helper lymphocytes become polarized upon antigen and cytokine stimuli received after their maturation in the thymus. Since the balance of Th1 and Th2 responses is critical in healthy and pathological immune responses, understanding the molecular base of T cell polarization still remained an important question. Using our Th0/Th1/Th2 hybridoma model system, we performed a comparative study on polarized Th1 and Th2 cells in terms of their membrane raft expression/composition, their TCR mediated activation signaling, and sensitivity to activation-induced cell death (AICD) using flow and image cytometric methods. We show here that the TCR stimulation induced more intense and sustained Ca(2+) -response in Th1 cells compared to Th2 ones correlates well with a shorter nuclear residence time of the Ca(2+) -dependent NFAT transcription factor in Th2 cells. In addition, NFAT translocation directly depended on lipid raft integrity/membrane cholesterol level. Expression pattern of raftophilic accessory proteins (CD4, CD59, and CD48) and lipids (GM1, cholesterol) were also different in the Th1 and Th2 hybridomas, similarly to differentiated spleen Th cells. The activation-induced, remarkably clustered and polarized membrane distribution of TCR/CD3 complex in Th1, but not in Th2 cells, together with an increased raft localization of Kv1.3 ion channels regulating the Ca(2+) -response, are consistent with the above properties of NFAT. Finally, the polarized Th cells, especially Th1, were more sensitive to AICD than their unpolarized Th0 precursor. These results suggest that the membrane microdomain organization-Ca(2+) -signaling-NFAT activation axis is an important determinant of polarized Th cell effector function and fate.

Analysis of TCR activation kinetics in primary human T cells upon focal or soluble stimulation

Signaling through the TCR is crucial for the generation of different cellular responses including proliferation, differentiation, and apoptosis. A growing body of evidence indicates that differences in the magnitude and the duration of the signal are critical determinants in eliciting cellular responses. Here, we have analyzed signaling dynamics induced upon TCR ligation in primary human T cells. We used CD3 antibodies either cross-linked in solution (sAbs) or immobilized on microbeads (iAbs), two widely employed methods to stimulate T cells in vitro. We show that classical sAbs stimulation induces a transient and abortive response, whereas iAbs induce sustained TCR-mediated signaling, resulting in productive T-cell responses previously observed only in antigen-specific murine systems. In summary, our analysis documents TCR signaling kinetics and suggests that iAbs are better suited for studying TCR-mediated signaling as they mimic antigen specific systems.

Phosphorylation of Tyrosine 340 in the Pleckstrin Homology Domain of RIAM Is Required for Inside-Out Activation of LFA-1 and LFA-1: ICAM-1-Mediated Adhesion

Abstract 837Adhesion of lymphocytes to antigen presenting cells (APCs) is a critical event linking innate and adaptive immunity and a mandatory step for the initiation of T cell immune responses. Control of lymphocyte adhesion to APC is accomplished through the regulation of the principle adhesion molecule on the lymphocyte surface, the β2 integrin designated lymphocyte functional antigen 1 (LFA-1), which binds to intercellular adhesion molecule 1 (ICAM-1) on the surface of APCs. In order to mediate its adhesive function LFA-1 must be activated via a process referred to as inside-out signaling, which results in conformation changes of the receptor that extends the ectodomains of the α and β chains leading to a high affinity state. Because LFA-1-mediated adhesion is a central link between innate and adaptive immunity, understanding the mechanisms implicated in inside-out-mediated LFA-1 activation is a subject of intense investigation. Currently, it is poorly understood how signals originating from the TCR are linked to specific mechanisms that regulate inside-out activation of LFA-1. Among the few signaling molecules that have been implicated in this process in hematopoietic cells are the small GTPase Rap1 and its downstream effector RIAM. RIAM is a multidomain protein that includes a talin binding region, two coiled-coiled regions, a small N-terminus proline-rich region, sequential Ras association (RA) and pleckstrin homology (PH) domains, and a large C-terminus proline-rich region, via which interacts with Ena/VASP family proteins and profilin and is recruited to the sites of actin turnover. Through its C-terminus RIAM also interacts with the SH3-domain of PLC-γ1. The RA and PH domains of RIAM function as an integral unit and as a proximity detector since both are required for translocation of RIAM to the plasma membrane. The RA domain of RIAM has specificity for Rap1-GTP whereas the PH domain binds to the PLC-γ1 substrate PI(4,5)P2. We determined that the integral RA-PH structural unit of RIAM has also an integral functional role because both the RA and PH regions are required for LFA-1-mediated adhesion. Here we sought to determine how TCR proximal signaling events regulate RIAM function leading to inside-out activation of LFA-1 and LFA-1:ICAM-1 mediated adhesion. Using primary human T lymphocytes and Jurkat T cells we determined that upon TCR/CD3-mediated stimulation, activated Src family kinases Fyn and Lck associate with and induce tyrosine phosphorylation of RIAM. To identify the precise domain(s) of RIAM, which undergo phosphorylation by these kinases, we co-expressed individual truncation constructs of RIAM N-terminus, RA-PH, and C-terminus regions along with the active or inactive form of Fyn or Lck in COS cells. Immunoprecipitations and immunoblot assays revealed that active Fyn and Lck mediated robust and selective tyrosine phosphorylation of the RA-PH structural unit of RIAM. Detailed analysis by mass spectrometry identified that tyrosine 340 (Y340) within the PH domain of RIAM was the specific target. The PH domain of RIAM has specificity for PI(4,5)P2 and is an integral component of the RA-PH proximity detector. We hypothesized that phosphorylation of tyrosine 340 in the PH domain might have a regulatory role in the function of the RA-PH structural unit in mediating LFA-1 activation. To address this issue we used site directed mutagenesis and we introduced a tyrosine to phenylalanine mutation in site 340 (Y34F) of RIAM, rendering this residue resistant to phosphorylation mediated by Fyn and Lck. Expression of RIAM Y340F in Jurkat T cells abrogated LFA-1 activation and LFA-1-mediated adhesion in response to TCR/CD3 stimulation. Under these conditions, activation of LFA-1 in response to PMA, which bypasses TCR-mediated signaling, remained intact. Thus, besides recruitment of RIAM to the plasma membrane, TCR-mediated activation of Src family kinases and phosphorylation of RIAM PH domain is a mandatory requirement for the function of the RA-PH proximity detector as a regulator of LFA-1 activation. Our results provide a biochemical, molecular and mechanistic link between TCR-mediated signaling and inside-out activation of LFA-1 thereby initiating LFA-1: ICAM-1-mediated adhesion and cross-talk between T cells and APC. Disclosures:No relevant conflicts of interest to declare.

Reciprocal Control of G1-Phase Progression Is Required for Th-POK/Runx3–Mediated CD4/8 Thymocyte Cell Fate Decision

After receiving a TCR-mediated differentiation signal, CD4 and CD8 double-positive thymocytes diverge into CD4 or CD8 single-positive T cells, for which Th-POK and Runx3 have been identified as pivotal transcription factors, respectively. The cross-antagonistic regulation of Th-POK and Runx3 seems to be essential for CD4/8 thymocyte lineage commitment. However, the process for determining which pivotal factor acts dominantly has not been established. To explore the determining process, we used an in vitro culture system in which CD4 or CD8 single-positive cells are selectively induced from CD4/8 double-positive cells. Surprisingly, we found that control of G(1) cell cycle phase progression is critical for the determination. In the CD4 pathway, sustained TCR signal, as well as Th-POK, induces G(1)-phase extension and represses CD8 expression in a G(1) extension-dependent manner. In the CD8 pathway, after receiving a transient TCR signal, the IL-7R signal, as well as Runx3, antagonizes TCR signal-mediated G(1) extension and CD8 repression. Importantly, forced G(1) extension cancels the functions of Runx3 to repress Th-POK and CD4 and to reactivate CD8. In contrast, it is suggested that forced G(1) progression inhibits Th-POK function to repress CD8. Collectively, Th-POK and Runx3 are reciprocally involved in the control of G(1)-phase progression, on which they exert their functions dependently. These findings may provide novel insight into how CD4/CD8 cell lineages are determined by Th-POK and Runx3.

The Importance of the Erk Pathway in the Development of Linker for Activation of T Cells-Mediated Autoimmunity

The ability of the transmembrane adaptor protein linker for activation of T cells (LAT) to regulate T cell development, activation, survival, and homeostasis depends upon phosphorylation of its multiple tyrosine residues. The mutation of tyrosine 136 on LAT abrogates its interaction with phospholipase C-γ1, causing severe ramifications on TCR-mediated signaling. Mice harboring this mutation, LATY136F mice, have significantly impaired thymocyte development; however, they rapidly develop a fatal lymphoproliferative disease marked by the uncontrolled expansion of Th2-skewed CD4(+) T cells, high levels of IgE and IgG1, and autoantibody production. In this study, we assessed the contribution of multiple signaling pathways in LATY136F disease development. The deletion of the critical signaling proteins Gads and RasGRP1 caused a further block in thymocyte development, but, over time, could not prevent CD4(+) T cell hyperproliferation. Also, restoring signaling through the NF-κB and NFAT pathways was unable to halt the development of disease. However, expression of a constitutively active Raf transgene enhanced lymphoproliferation, indicating a role for the Ras-MAPK pathway in LAT-mediated disease.

Nitric oxide and glucocorticoids synergize in inducing apoptosis of CD4+8+ thymocytes: implications for ‘Death by Neglect’ and T-cell function

Thymic epithelial cells (TECs) play a central role in T-cell development by presenting self-antigens on MHC proteins. Double-positive (DP) thymocytes that fail to interact with TEC via their TCR die by 'Death by Neglect'. We demonstrated a role for TEC-derived glucocorticoids (GCs) in this process. In a previous study, we used an in vitro system recapitulating Death by Neglect, to demonstrate the involvement of nitric oxide (NO) and inducible NO synthase (iNOS) in this process. In this study, we show that NO synergizes with GCs to induce apoptosis of DP thymocytes in a fetal thymic organ culture. Also, DP thymocytes from iNOS⁻/⁻ mice are less sensitive to GC-induced apoptosis. Furthermore, the number of DP thymocytes in iNOS⁻/⁻ mice is higher than in wild-type mice, suggesting a role for NO in Death by Neglect. This phenomenon effects T-cell function profoundly: iNOS⁻/⁻ T cells do not respond to TCR-mediated activation signals, measured by up-regulation of CD69, IL-2R and IFNγ secretion. This failure to activate is a result of TCR incompetence because iNO⁻/⁻ T cells respond to TCR-independent stimuli (phorbol myristate acetate and calcium ionophore). This study suggests that NO and GCs synergize to execute TEC-induced death of DP thymocytes.

Phospholipase Cγ2 Plays a Role in TCR Signal Transduction and T Cell Selection

One of the important signaling events following TCR engagement is activation of phospholipase Cγ (PLCγ). PLCγ has two isoforms, PLCγ1 and PLCγ2. It is known that PLCγ1 is important for TCR signaling and TCR-mediated T cell selection and functions, whereas PLCγ2 is critical for BCR signal transduction and BCR-mediated B cell maturation and functions. In this study, we report that PLCγ2 was expressed in primary T cells, and became associated with linker for activated T cells and Src homology 2-domain containing leukocyte protein of 76 kDa and activated upon TCR stimulation. PLCγ1/PLCγ2 double-deficient T cells displayed further block from CD4 and CD8 double-positive to single-positive transition compared with PLCγ1 single-deficient T cells. TCR-mediated proliferation was further impaired in PLCγ1/PLCγ2 double-deficient T cells compared with PLCγ1 single-deficient T cells. TCR-mediated signal transduction, including Ca²⁺ mobilization and Erk activation, was further impaired in PLCγ1/PLCγ2 double-deficient relative to PLCγ1 single-deficient T cells. In addition, in HY TCR transgenic mouse model, thymic positive and negative selections were reduced in PLCγ1 heterozygous- and PLCγ2 homozygous-deficient (PLCγ1⁺/⁻PLCγ2⁻/⁻) relative to wild-type, PLCγ2 single-deficient (PLCγ2⁻/⁻), or PLCγ1 heterozygous-deficient (PLCγ1⁺/⁻) mice. Taken together, these data demonstrate that PLCγ2 participates in TCR signal transduction and plays a role in T cell selection.