Antigen receptor-mediated regulation of sustained polyphosphoinositide turnover in a human T cell line. Evidence for a receptor-regulated pathway for production of phosphatidylinositol 4,5-bisphosphate

The phosphatidylinositol-transfer protein Nir3 promotes PI(4,5)P2 replenishment in response to TCR signaling during T cell development and survival

Hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) by phospholipase C-γ (PLCγ1) represents a critical step in T cell antigen receptor (TCR) signaling and subsequent thymocyte and T cell responses. PIP₂ replenishment following its depletion in the plasma membrane (PM) is dependent on delivery of its precursor phosphatidylinositol (PI) from the endoplasmic reticulum (ER) to the PM. We show that a PI transfer protein (PITP), Nir3 (Pitpnm2), promotes PIP₂ replenishment following TCR stimulation and is important for T cell development. In Nir3^-/- T lineage cells, the PIP₂ replenishment following TCR stimulation is slower. Nir3 deficiency attenuates calcium mobilization in double-positive (DP) thymocytes in response to weak TCR stimulation. This impaired TCR signaling leads to attenuated thymocyte development at TCRβ selection and positive selection as well as diminished mature T cell fitness in Nir3^-/- mice. This study highlights the importance of PIP₂ replenishment mediated by PITPs at ER-PM junctions during TCR signaling.

EWI-2 association with α-actinin regulates T cell immune synapses and HIV viral infection

EWI motif-containing protein 2 (EWI-2) is a member of the Ig superfamily that links tetraspanin-enriched microdomains to the actin cytoskeleton. We found that EWI-2 colocalizes with CD3 and CD81 at the central supramolecular activation cluster of the T cell immune synapse. Silencing of the endogenous expression or overexpression of a cytoplasmic truncated mutant of EWI-2 in T cells increases IL-2 secretion upon Ag stimulation. Mass spectrometry experiments of pull-downs with the C-term intracellular domain of EWI-2 revealed the specific association of EWI-2 with the actin-binding protein α-actinin; this association was regulated by PIP2. α-Actinin regulates the immune synapse formation and is required for efficient T cell activation. We extended these observations to virological synapses induced by HIV and found that silencing of either EWI-2 or α-actinin-4 increased cell infectivity. Our data suggest that the EWI-2-α-actinin complex is involved in the regulation of the actin cytoskeleton at T cell immune and virological synapses, providing a link between membrane microdomains and the formation of polarized membrane structures involved in T cell recognition.

Phosphoinositide and inositol phosphate analysis in lymphocyte activation

Lymphocyte antigen receptor engagement profoundly changes the cellular content of phosphoinositide lipids and soluble inositol phosphates. Among these, the phosphoinositides phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 3,4,5-trisphosphate (PIP3) play key signaling roles by acting as pleckstrin homology (PH) domain ligands that recruit signaling proteins to the plasma membrane. Moreover, PIP2 acts as a precursor for the second messenger molecules diacylglycerol and soluble inositol 1,4,5-trisphosphate (IP3), essential mediators of PKC, Ras/Erk, and Ca2+ signaling in lymphocytes. IP3 phosphorylation by IP3 3-kinases generates inositol 1,3,4,5-tetrakisphosphate (IP4), an essential soluble regulator of PH domain binding to PIP3 in developing T cells. Besides PIP2, PIP3, IP3, and IP4, lymphocytes produce multiple other phosphoinositides and soluble inositol phosphates that could have important physiological functions. To aid their analysis, detailed protocols that allow one to simultaneously measure the levels of multiple different phosphoinositide or inositol phosphate isomers in lymphocytes are provided here. They are based on thin layer, conventional and high-performance liquid chromatographic separation methods followed by radiolabeling or non-radioactive metal-dye detection. Finally, less broadly applicable non-chromatographic methods for detection of specific phosphoinositide or inositol phosphate isomers are discussed. Support protocols describe how to obtain pure unstimulated CD4+CD8+ thymocyte populations for analyses of inositol phosphate turnover during positive and negative selection, key steps in T cell development.

The cytoplasmic tail of the T cell receptor CD3 epsilon subunit contains a phospholipid-binding motif that regulates T cell functions

The CD3 epsilon subunit of the TCR complex contains two defined signaling domains, a proline-rich sequence and an ITAM. We identified a third signaling sequence in CD3 epsilon, termed the basic-rich stretch (BRS). Herein, we show that the positively charged residues of the BRS enable this region of CD3 epsilon to complex a subset of acidic phospholipids, including PI(3)P, PI(4)P, PI(5)P, PI(3,4,5)P(3), and PI(4,5)P(2). Transgenic mice containing mutations of the BRS exhibited varying developmental defects, ranging from reduced thymic cellularity to a complete block in T cell development. Peripheral T cells from BRS-modified mice also exhibited several defects, including decreased TCR surface expression, reduced TCR-mediated signaling responses to agonist peptide-loaded APCs, and delayed CD3 epsilon localization to the immunological synapse. Overall, these findings demonstrate a functional role for the CD3 epsilon lipid-binding domain in T cell biology.

Analysis of inositol phospholipid turnover during lymphocyte activation

Receptor-mediated activation of phospholipase C (PLC) leads to the hydrolysis of membrane inositol phospholipids, generating diacylglycerol (DAG) and water-soluble inositol phosphates. This signaling mechanism is used by antigen receptors on T and B cells that have been implicated as mediators of receptor-induced influx of extracellular Ca(2+). This unit provides protocols that describe the resolution of InsP by Dowex anion-exchange chromatography. This technique provides a reliable means of separating inositol monophosphate, inositol bisphosphate, and inositol trisphosphate, but does not resolve isomers of these. An Alternate Protocol describes the separation of inositol phosphates by anion-exchange HPLC. A protocol for resolution of inositol phospholipids by thin-layer chromatography (TLC) is also provided.

Type II phosphatidylinositol 4-kinase β associates with TCR-CD3 ζ chain in Jurkat cells

Phosphatidylinositol lipid signaling cascades are integral part of TCR-CD3 signaling. The mechanisms by which phosphatidylinositol kinases are coupled to TCR-CD3 complex remain elusive. Here we report an association of type II PtdIns 4-kinase with TCR-CD3 zeta chain upon cross-linking. Mapping studies have revealed that the C-terminal ITAM is critical for docking of the enzyme on the zeta chain. The association is shown to be tyrosyl phosphorylation dependent as mutation of Y-151 and Y-142 on the C-terminal ITAM disrupts interaction of the two proteins. Identification of the associated type II PtdIns 4-kinase revealed that the beta isoform of the enzyme interacts with the zeta chain in vivo.

Changes in actin dynamics at the T-cell/APC interface: implications for T-cell anergy?

Over the past 20 years the role of the actin cytoskeleton in the formation of the immunological synapse and in T-cell activation has been the subject of intense scrutiny. T-cell receptor (TCR) signaling leads to tyrosine phosphorylation of numerous adapter proteins whose function is to relay signals to downstream components of the TCR signaling pathway and, in particular, to molecules implicated in remodeling the actin cytoskeleton. Here, we discuss how signals from the TCR converge on two key regulators of the actin cytoskeleton, Ena/vasodilator-stimulated phosphoproteins (VASPs) and the actin-related protein (ARP2/3) complex. We also discuss the implications of TCR signaling in the process of T-cell anergy with particular emphasis on the actin remodeling and molecules involved in the control of T-cell proliferation.

CD28 co-stimulates TCR/CD3-induced phosphoinositide turnover in human T lymphocytes

Upon engagement of TCR with peptide-MHC complexes displayed on the surface of antigen-presenting cells, T lymphocytes undergo a sustained elevation of intracellular Ca(2+) concentration([Ca(2+)](i)), which is required for cytokine production. In the present work, we investigate how inositol lipid metabolism can be activated for a prolonged time to ensure a sustained link between receptor triggering and downstream signaling effectors. Four lines of evidence indicate that an extensive phosphoinositide turnover induced by TCR and CD28 engagement allows this task to be accomplished: (i) continuous phosphoinositide breakdown is required for a sustained [Ca(2+)](i )increase in antigen-stimulated T cells; (ii) TCR triggering results in a continuous release of inositol phosphates from the cell membrane paralleled by a massive and sustained phosphoinositide re-synthesis due to free inositol re-incorporation; (iii) TCR-induced phosphoinositide turnover is strongly increased by CD28 ligation; and (iv) CD28 engagement augments and sustains the TCR-induced [Ca(2+)](i )increase. Our results show that the T cell pool of phosphoinositides is continuously re-formed during T cell-APC cognate interaction, thereby explaining how sustained receptor triggering can transduce an equally sustained [Ca(2+)](i) increase. Importantly, our data identify a novel step in the signaling cascade where co-stimulation converges with TCR-generated signals to sustain and amplify the activation process.

Concanavalin A modulates tyrosine phosphorylation and activation of a type II phosphatidylinositol 4-kinase in rat splenic lymphocytes

Activation of rat splenic lymphocytes by concanavalin A resulted in two-fold increase in Ptdlns 4-kinase activity and rapid tyrosine phosphorylation of the enzyme. The activation kinetics showed a strong correlation with tyrosine phosphorylation state of the enzyme. Characterization of the enzyme activity suggests that it is a type II PtdIns 4-kinase. Kinetic analysis of the enzyme reaction showed three-fold decrease in Km for PtdIns and two-fold increase in Vmax in Con A stimulated cells. These results suggest that a type II PtdIns 4-kinase is an integral component of the early signal transduction machinery during T-cell activation by concanavalin A and is actively regulated by protein tyrosine phosphorylation-dephosphorylation.

Dietary fish oil appears to prevent the activation of phospholipase C-gamma in lymphocytes

Rats were fed on a low fat diet or on high fat diets which included coconut oil, olive oil, safflower oil, evening primrose oil or fish oil as the principal fat source. The level of phosphatidylinositol-4, 5-bisphosphate in spleen lymphocytes was unaffected by diet. However, the fish oil diet significantly decreased the concentration of inositol-1,4,5-trisphosphate in stimulated lymphocytes; this concentration was also reduced following olive oil feeding. Diet did not significantly affect the level of phospholipase C-gamma1 in spleen lymphocytes but the tyrosine phosphorylation state of this enzyme in stimulated lymphocytes, as well as that of a range of other proteins, was decreased following feeding the fish oil and, to a lesser extent, the olive oil diets. It is concluded that fish oil feeding appears to result in inhibition of one or more tyrosine kinases.

Reconstitution of a T cell receptor-stimulated plasma membrane calcium transporter: lack of dependence on inositol phosphates

The activation of T lymphocytes, like many cells, requires a rapid rise in intracellular Ca2+ secondary to both an influx and a release from intracellular stores. The latter is activated by inositol-1,4,5-trisphosphate [Ins(1,4,5)P3]. It is controversial if inositol phosphates can also stimulate a plasma membrane Ca2+ channel in T cells. We have studied the human T cell line HPB-ALL which, upon stimulation of its antigen receptor, does not generate detectable levels of Ins(1,4,5)P3 or internal Ca2+ release, but does have a Ca2+ influx. We have reconstituted a receptor-activated Ca2+ transporter from plasma membranes from these cells which has properties similar to the transporter observed in vivo and does not require inositol phosphates for activation. These data show that mitogens may activate more than one type of ligand-gated Ca2+ transport mechanism in T lymphocytes.

Signal transduction in T cells

Rapid progress was made during the past year in the delineation of the nature of the initial biochemical events triggered by the T-cell antigen receptor. Antigen-mediated activation of phospholipase C was demonstrated to require protein tyrosine phosphorylation and, most surprising, activation of the Ras family of signal transduction molecules was shown to closely follow stimulation of the T-cell antigen receptor. Major controversy continues over which events are relevant to the various effector functions of T cells.