Antigen Receptor Engagement Research Articles

Deletion of the G2A receptor fails to attenuate experimental autoimmune encephalomyelitis

Lysophosphatidylcholine (LPC) is a chemotactic lysolipid produced during inflammation by the hydrolytic action of phospholipase A 2 enzymes. LPC stimulates chemotaxis of T cells in vitro through activation of the G protein-coupled receptor, G2A. This has led to the proposition that G2A contributes to the recruitment of T cells to sites of inflammation and thus promotes chronic inflammatory autoimmune diseases associated with the generation and subsequent tissue infiltration of auto-antigen-specific effector T cells. However, one study suggests that G2A may negatively regulate T cell proliferative responses to antigen receptor engagement and thereby attenuates autoimmunity by reducing the generation of autoreactive T cells. To address the relative contribution of these G2A-mediated effects to the pathophysiology of T cell-mediated autoimmune disease, we examined the impact of G2A inactivation on the onset and severity of murine experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis (MS). Wild type (G2A +/+) and G2A-deficient (G2A −/−) C57BL/6J mice exhibited a similar incidence and onset of disease following immunization with MOG 35–55 peptide. Disease severity was only moderately reduced in G2A −/− mice. Similar numbers of MOG 35–55 specific T cells were generated in secondary lymphoid organs of MOG 35–55-immunized G2A +/+ and G2A −/− mice. Comparable numbers of T cells were detected in spinal cords of G2A +/+ and G2A −/− mice. We conclude that the proposed anti-proliferative and chemotactic functions of G2A are not manifested in vivo and therefore therapeutic targeting of G2A is unlikely to be beneficial in the treatment of MS.

Calcium signaling in immune cells

Calcium acts as a second messenger in many cell types, including lymphocytes. Resting lymphocytes maintain a low concentration of Ca2+. However, engagement of antigen receptors induces calcium influx from the extracellular space by several routes. A chief mechanism of Ca2+ entry in lymphocytes is through store-operated calcium (SOC) channels. The identification of two important molecular components of SOC channels, CRACM1 (the pore-forming subunit) and STIM1 (the sensor of stored calcium), has allowed genetic and molecular manipulation of the SOC entry pathway. In this review, we highlight advances in the understanding of Ca2+ signaling in lymphocytes with special emphasis on SOC entry. We also discuss outstanding questions and probable future directions of the field.

Tec Kinase Inhibits the Expression of IL-2 Receptor Alpha (CD25) in Human T-Lymphocyte.

Activation and development of lymphocytes is regulated by the engagement of cell surface immune cell antigen receptors. Following receptor engagement, these receptors transmit signals by the activation of cytoplasmic protein tyrosine kinases (PTKs). The Tec PTK belongs to a group of structurally related nonreceptor PTKs that also includes Btk, Itk (Emt), Rlk (Txk), and Bmx (Etk). We have found the contribution of Tec in the antigen receptor signaling in B-lymphoid cells in the previous studies. Despite the presence of evidences suggesting the involvement of Tec in T-lymphocyte activation pathway via T-cell receptor (TCR) and CD28, the role of Tec in T-lymphocyte still remains unclear because of the lack of apparent defects in T-lymphocyte function in Tec-deficient mice. In this study, we investigated the role of Tec in T-lymphocyte using Jurkat human T-lymphoid cell line stably transfected with a cDNA encoding Tec. We found that the expression of wild type Tec but not kinase-deleted Tec inhibited the expression of IL-2 receptor alpha (CD25) induced by TCR cross-linking. The percentage of CD25 expressing cells after TCR cross-linking was 41.7% in mock-transfected cells, 18.3% in the wild type Tec expressing cells and 41.3% in the kinase-deleted Tec expressing cells. Second, the selective inhibitor of Tec family PTK, LFM-A13, rescued the suppression of TCR-induced CD25 expression observed in the wild type Tec-expressing Jurkat cells. We conclude that Tec PTK mediates signals that negatively regulate CD25 expression induced by TCR cross-linking, implying that this PTK plays a role in the attenuation of the IL-2 activity in human T-lymphocytes.

New insights into the activation mechanism of store-operated calcium channels: roles of STIM and Orai

The activation of Ca2+ entry through store-operated channels by agonists that deplete Ca2+ from the endoplasmic reticulum (ER) is a ubiquitous signaling mechanism, the molecular basis of which has remained elusive for the past two decades. Store-operated Ca2+-release-activated Ca2+ (CRAC) channels constitute the sole pathway for Ca2+ entry following antigen-receptor engagement. In a set of breakthrough studies over the past two years, stromal interaction molecule 1 (STIM1, the ER Ca2+ sensor) and Orai1 (a pore-forming subunit of the CRAC channel) have been identified. Here we review these recent studies and the insights they provide into the mechanism of store-operated Ca2+ channels (SOCCs).

Identification of a new transmembrane adaptor protein that constitutively binds Grb2 in B cells

Transmembrane adaptor proteins couple antigen receptor engagement to downstream signaling cascades in lymphocytes. One example of these proteins is the linker for activation of T cells (LAT), which plays an indispensable role in T cell activation and development. Here, we report identification of a new transmembrane adaptor molecule, namely growth factor receptor-bound protein 2 (Grb2)-binding adaptor protein, transmembrane (GAPT), which is expressed in B cells and myeloid cells. Similar to LAT, GAPT has an extracellular domain, a transmembrane domain, and a cytoplasmic tail with multiple Grb2-binding motifs. In contrast to other transmembrane adaptor proteins, GAPT is not phosphorylated upon BCR ligation but associates with Grb2 constitutively through its proline-rich region. Targeted disruption of the gapt gene in mice affects neither B cell development nor a nitrophenylacetyl-specific antibody response. However, in the absence of GAPT, B cell proliferation after BCR cross-linking is enhanced. In aged GAPT(-/-) mice, the number of marginal zone (MZ) B cells is increased, and other B cell subsets are normal. The serum concentrations of IgM, IgG2b, and IgG3 are also elevated in these mice. These data indicate that GAPT might play an important role in control of B cell activation and proper maintenance of MZ B cells.

The proapoptotic and antimitogenic protein p66SHC acts as a negative regulator of lymphocyte activation and autoimmunity

AbstractThe ShcA locus encodes 3 protein isoforms that differ in tissue specificity, subcellular localization, and function. Among these, p66Shc inhibits TCR coupling to the Ras/MAPK pathway and primes T cells to undergo apoptotic death. We have investigated the outcome of p66Shc deficiency on lymphocyte development and homeostasis. We show that p66Shc−/− mice develop an age-related lupus-like autoimmune disease characterized by spontaneous peripheral T- and B-cell activation and proliferation, autoantibody production, and immune complex deposition in kidney and skin, resulting in autoimmune glomerulonephritis and alopecia. p66Shc−/− lymphocytes display enhanced proliferation in response to antigen receptor engagement in vitro and more robust immune responses both to vaccination and to allergen sensitization in vivo. The data identify p66Shc as a negative regulator of lymphocyte activation and show that loss of this protein results in breaking of immunologic tolerance and development of systemic autoimmunity.

Micro-scale flow cytometry-based and biochemical analysis of lipid signaling in primary B cell subpopulations.

B cell subpopulations in the spleen have been extensively characterized phenotypically; however, biochemical properties of these cell populations following B cell antigen receptor engagement have not been fully determined due to technical difficulties and limiting cell numbers. We therefore employed mini-scale protocols to assess lipid signaling, particularly that of diacylglycerol and inositol trisphosphate, with as few as 0.5x106 purified early (T1) and late (T2) transitional B cells. Additionally, utilizing flow cytometric techniques, we determined levels of phosphatidylinositol bisphosphate and calcium mobilization in T1 and T2 cells, as well as mature follicular and marginal zone B cells using less than 1x106 primary B cells. Thus, these biochemical and flow cytometric methodologies can be used to analyse signal-induced changes in phosphatidylinositol bisphosphate levels, diacylglycerol and inositol triphosphate production and calcium in each B cell population.

3-Hydroxyanthranilic acid inhibits PDK1 activation and suppresses experimental asthma by inducing T cell apoptosis

3-Hydroxyanthranilic acid (HAA), a compound generated during tryptophan metabolism initiated by indoleamine 2,3-dioxygenase, is known to induce T cell death, but its molecular target is not known. Here we report that HAA inhibits NF-kappaB activation upon T cell antigen receptor engagement by specifically targeting PDK1. Inhibition of NF-kappaB by HAA leads to dysfunction and cell death of activated Th2 cells, which in turn suppresses experimental asthma. Inhibition of NF-kappaB and induction of apoptosis is specific to CD4 T cells because HAA does not inhibit NF-kappaB activation or induce cell death upon Toll-like receptor 4 stimulation in dendritic cells. Thus, HAA is a natural inhibitor that restrains T cell expansion and activation.

Prostaglandin E2 Activates HPK1 Kinase Activity via a PKA-dependent Pathway

Hematopoietic progenitor kinase 1 (HPK1) is a hematopoietic cell-restricted member of the Ste20 serine/threonine kinase super family. We recently reported that the immunosuppressive eicosanoid, prostaglandin E(2) (PGE(2)), is capable of activating HPK1 in T cells. In this report, we demonstrate that unlike the TCR-induced activation of HPK1 kinase activity, the induction of HPK1 catalytic activity by PGE(2) does not require the presence of phosphotyrosine-based signaling molecules such as Lck, ZAP-70, SLP-76, and Lat. Nor does the PGE(2)-induced HPK1 activation require the intermolecular interaction between its proline-rich regions and the SH3 domain-containing adaptor proteins, as required by the signaling from the TCR to HPK1. Instead, our study reveals that PGE(2) signal to HPK1 via a 3' -5 '-cyclic adenosine monophosphate-regulated, PKA-dependent pathway. Consistent with this observation, changing the serine 171 residue that forms the optimal PKA phosphorylation site within the "activation loop" of HPK1 to alanine completely prevents this mutant from responding to PGE(2)-generated stimulation signals. Moreover, the inability of HPK1 to respond to PGE(2) stimulation in PKA-deficient S49 cells further supports the importance of PKA in this signaling pathway. We speculate that this unique signaling pathway enables PGE(2) signals to engage a proven negative regulator of TCR signal transduction pathway and uses it to inhibit T cell activation.

A Novel Clonal Selection Algorithm and Its Application to Traveling Salesman Problem

The Clonal Selection Algorithm (CSA) is employed by the natural immune system to define the basic features of an immune response to an antigenic stimulus. In the immune response, according to Burnet's clonal selection principle, the antigen imposes a selective pressure on the antibody population by allowing only those cells which specifically recognize the antigen to be selected for proliferation and differentiation. However ongoing investigations indicate that receptor editing, which refers to the process whereby antigen receptor engagement leads to a secondary somatic gene rearrangement event and alteration of the receptor specificity, is occasionally found in affinity maturation process. In this paper, we extend the traditional CSA approach by incorporating the receptor editing method, named RECSA, and applying it to the Traveling Salesman Problem. Thus, both somatic hypermutation (HM) of clonal selection theory and receptor editing (RE) are utilized to improve antibody affinity. Simulation results and comparisons with other general algorithms show that the RECSA algorithm can effectively enhance the searching efficiency and greatly improve the searching quality within reasonable number of generations.

COP9 signalosome subunit 8 is essential for peripheral T cell homeostasis and antigen receptor–induced entry into the cell cycle from quiescence

Engagement of antigen receptors triggers the proliferation and functional activation of lymphocytes. Here we report that T cell homeostasis and antigen-induced responses require the COP9 signalosome (CSN), a regulator of the ubiquitin-proteasome system. Conditional deletion of the CSN subunit Csn8 in peripheral T lymphocytes disrupted formation of the CSN complex, reduced T cell survival and proliferation in vivo and impaired antigen-induced production of interleukin 2. Moreover, Csn8-deficient T cells showed defective entry into the cell cycle from the G0 quiescent state. This phenotype was associated with a lack of signal-induced expression of cell cycle-related genes, including G1 cyclins and cyclin-dependent kinases, and with excessive induction of p21(Cip1). Our data define a CSN-dependent pathway of transcriptional control that is essential for antigen-induced initiation of T cell proliferation.

The B7 Family and Cancer Therapy: Costimulation and Coinhibition

The activation and development of an adaptive immune response is initiated by the engagement of a T-cell antigen receptor by an antigenic peptide-MHC complex. The outcome of this engagement is determined by both positive and negative signals, costimulation and coinhibition, generated mainly by the interaction between the B7 family and their receptor CD28 family. The importance of costimulation and coinhibition of T cells in controlling immune responses is exploited by tumors as immune evasion pathways. Absence of the expression of costimulatory B7 molecules renders tumors invisible to the immune system, whereas enhanced expression of inhibitory B7 molecules protects them from effective T cell destruction. Therefore, the manipulation of these pathways is crucial for developing effective tumor immunotherapy. Translation of our basic knowledge of costimulation and coinhibition into early clinical trials has shown considerable promise.

The Calponin Homology Domain of Vav1 Associates with Calmodulin and Is Prerequisite to T Cell Antigen Receptor-induced Calcium Release in Jurkat T Lymphocytes

Vav1 is a guanine nucleotide exchange factor that is expressed specifically in hematopoietic cells and plays important roles in T cell development and activation. Vav1 consists of multiple structural domains so as to facilitate both its guanine nucleotide exchange activity and scaffold function following T cell antigen receptor (TCR) engagement. Previous studies demonstrated that the calponin homology (CH) domain of Vav1 is required for TCR-stimulated calcium mobilization and thus downstream activation of nuclear factor of activated T cells. However, it remained obscure how Vav1 functions in regulating calcium flux. In an effort to explore molecules interacting with Vav1, we found that calmodulin bound to Vav1 in a calcium-dependent and TCR activation-independent manner. The binding site was mapped to the CH domain of Vav1. Reconstitution of vav1-null Jurkat T cells (J.Vav1) with CH-deleted Vav1 exhibited a severe deficiency in calcium release to the same extent as that of Jurkat cells treated with the calmodulin inhibitor or J.Vav1 cells. The defect persisted even when phospholipase-Cgamma1 was fully activated, indicating a prerequisite role of Vav1 CH domain in calcium signaling. The results suggest that Vav1 and calmodulin function cooperatively to potentiate TCR-induced calcium release. This study unveiled a mechanism by which the Vav1 CH domain is involved in calcium signaling and provides insight into our understanding of the role of Vav1 in T cell activation.

Histone Deacetylase 7 Functions as a Key Regulator of Genes Involved in both Positive and Negative Selection of Thymocytes

Histone deacetylase 7 (HDAC7) is highly expressed in CD4(+)/CD8(+) thymocytes and functions as a signal-dependent repressor of gene transcription during T-cell development. In this study, we expressed HDAC7 mutant proteins in a T-cell line and use DNA microarrays to identify transcriptional targets of HDAC7 in T cells. The changes in gene expression levels were compared to differential gene expression profiles associated with positive and negative thymic selection. This analysis reveals that HDAC7 regulates an extensive set of genes that are differentially expressed during both positive and negative thymic selection. Many of these genes play important functional roles in thymic selection, primarily via modulating the coupling between antigen receptor engagement and downstream signaling events. Consistent with the model that HDAC7 may play an important role in both positive and negative thymic selection, the expression of distinct HDAC7 mutants or the abrogation of HDAC7 expression can either enhance or inhibit the signal-dependent differentiation of a CD4(+)/CD8(+) cell line.

The lymphocyte function-associated antigen-1 receptor costimulates plasma membrane Ras via phospholipase D2

Ras activation as a consequence of antigen receptor (T-cell receptor; TCR) engagement on T lymphocytes is required for T-cell development, selection and function. Lymphocyte function-associated antigen-1 (LFA-1) mediates lymphocyte adhesion, stabilization of the immune synapse and bidirectional signalling. Using a fluorescent biosensor we found that TCR activation with or without costimulation of CD28 led to activation of Ras only on the Golgi apparatus, whereas costimulation with LFA-1 induced Ras activation on both the Golgi and the plasma membrane. Ras activation on both compartments required RasGRP1, an exchange factor regulated by calcium and diacylglycerol (DAG), but phospholipase C (PLC) activity was required only for activation on the Golgi. Engagement of LFA-1 increased DAG levels at the plasma membrane by stimulating phospholipase D (PLD). PLD2 and phosphatidic acid phosphatase (PAP) were required for Ras activation on the plasma membrane. Thus, LFA-1 acts through PLD2 to reshape the pattern of Ras activation downstream of the TCR.

Interleukin 2 gene transcription is regulated by Ikaros-induced changes in histone acetylation in anergic T cells

AbstractIn T cells anergy may be evoked by an unbalanced stimulation of the T-cell receptor in the absence of costimulation. Anergic T cells are unresponsive to new antigen receptor engagement and do not produce interleukin 2. We present evidence that anergizing stimuli induce changes in histone acetylation, which mediates transcriptional repression of interleukin 2 expression. In response to calcium signaling, anergic T cells up-regulate the expression of Ikaros, a zinc finger transcription factor essential for lymphoid lineage determination. Ikaros binds to the interleukin 2 promoter where it induces histone deacetylation. Confirming the role of Ikaros in the induction of T-cell anergy, cells with reduced Ikaros activity show defective inactivation in response to an anergizing stimulus. We propose a model in which tolerizing stimuli induce epigenetic changes on the interleukin 2 locus that are responsible for the stable inhibition of the expression of this cytokine in anergic T cells.

Engagement of the B-cell antigen receptor activates STAT through Lyn in a Jak-independent pathway

Engagement of the B-cell antigen receptor (BCR) initiated by the Src kinase Lyn triggers rapid signaling cascades, leading to proliferation, differentiation or growth arrest of B cells. The Janus kinase (JAK)-STAT (signal transducer and activator of transcription) pathway, activated through cytokine receptors, mediates similar responses. Hypothesizing that Src and JAK pathways engage in crosstalk in B-cell signaling, we studied wild-type and Lyn-null B-cell lines, which express BCR. We found that activated BCR results in tyrosine phosphorylation of JAK-STAT, which required Lyn. To confirm that STAT activation is not due to JAK, we cloned the chicken homologs of JAK1 and JAK2 and made their antisense constructs. In cells expressing antisense JAK1 and JAK2, tyrosine phosphorylation of STAT was not inhibited following BCR stimulation. Using activation loop-specific phosphotyrosine antibodies, we did not detect phospho-JAK1 and phospho-JAK2 after BCR stimulation. The JAK inhibitor AG490 did not inhibit the tyrosine phosphorylation of Lyn or STAT after BCR simulation. An in vitro phosphorylation assay showed that Lyn directly phosphorylates STAT3. In an electrophoretic mobility shift assay, BCR stimulation led to enhanced DNA binding of the STAT3 in DT40, but not in the Lyn-null cells. We conclude that BCR engagement activates the STAT pathway via Lyn, independent of JAK.

T‐Cell Antigen Receptor‐Induced Signaling Complexes: Internalization Via a Cholesterol‐Dependent Endocytic Pathway

T-cell antigen receptor engagement causes the rapid assembly of signaling complexes. The adapter protein SLP-76, detected as SLP-yellow fluorescent protein, initially clustered with the TCR and other proteins, then translocated medially on microtubules. As shown by total internal reflection fluorescence microscopy and the inhibition of SLP-76 movement at 16 degrees C, this movement required endocytosis. Immunoelectron microscopy showed SLP-76 staining of smooth pits and tubules. Cholesterol depletion decreased the movement of SLP-76 clusters, as did coexpression of the ubiquitin-interacting motif domain from eps15. These data are consistent with the internalization of SLP-76 via a lipid raft-dependent pathway that requires interaction of the endocytic machinery with ubiquitinylated proteins. The endocytosed SLP-76 clusters contained phosphorylated SLP-76 and phosphorylated LAT. The raft-associated, transmembrane protein LAT likely targets SLP-76 to endocytic vesicles. The endocytosis of active SLP-76 and LAT complexes suggests a possible mechanism for downregulation of signaling complexes induced by TCR activation.

Recruitment and activation of PLCγ1 in T cells: a new insight into old domains

Engagement of the T-cell antigen receptor leads to recruitment of phospholipase Cgamma1 (PLCgamma1) to the LAT-nucleated signaling complex and to PLCgamma1 activation in a tyrosine phosphorylation-dependent manner. The mechanism of PLCgamma1 recruitment and the role of PLCgamma1 Src homology (SH) domains in this process remain incompletely understood. Using a combination of biochemical methods and real-time fluorescent imaging, we show here that the N-terminal SH2 domain of PLCgamma1 is necessary but not sufficient for its recruitment. Either the SH3 or C-terminal SH2 domain of PLCgamma1, with the participation of Vav1, c-Cbl and Slp76, are required to stabilize PLCgamma1 recruitment. All three PLCgamma1 SH domains are required for phosphorylation of PLCgamma1 Y783, which is critical for enzyme activation. These novel findings entailed revision of the currently accepted model of PLCgamma1 recruitment and activation in T lymphocytes.

Phosphorylation of the CARMA1 Linker Controls NF-κB Activation

PKC isoforms and CARMA1 play crucial roles in immunoreceptor-dependent NF-kappaB activation. We tested whether PKC-dependent phosphorylation of CARMA1 directly regulates this signaling cascade. B cell antigen receptor (BCR) engagement led to the progressive recruitment of CARMA1 into lipid rafts and to the association of CARMA1 with, and phosphorylation by, PKCbeta. Furthermore, PKCbeta interacted with the serine-rich CARMA1 linker, and both PKCbeta and PKCtheta phosphorylated identical serine residues (S564, S649, and S657) within this linker. Mutation of two of these sites ablated the functional activity of CARMA1. In contrast, deletion of the linker resulted in constitutive, receptor- and PKC-independent NF-kappaB activation. Together, our data support a model whereby CARMA1 phosphorylation controls NF-kappaB activation by triggering a shift from an inactive to an active CARMA1 conformer. This PKC-dependent switch regulates accessibility of the CARD and CC domains and controls assembly and full activation of the membrane-associated IkappaB kinase (IKK) signalosome.