Microvillar Cartography: A Super-Resolution Single-Molecule Imaging Method to Map the Positions of Membrane Proteins with Respect to Cellular Surface Topography

We describe microvillar cartography (MC), a method to map proteins on cellular surfaces with respect to the membrane topography. The surfaces of many cells are not smooth, but are rather covered with various protrusions such as microvilli. These protrusions may play key roles in multiple cellular functions, due to their ability to control the distribution of specific protein assemblies on the cell surface. Thus, for example, we have shown that the T-cell receptor and several of its proximal signaling proteins reside on microvilli, while others are excluded from these projections. These results have indicated that microvilli can function as key signaling hubs for the initiation of the immune response. MC has facilitated our observations of particular surface proteins and their specialized distribution on microvillar and non-microvillar compartments. MC combines membrane topography imaging, using variable-angle total internal microscopy, with stochastic localization nanoscopy, which generates deep sub-diffraction maps of protein distribution. Since the method is based on light microscopy, it avoids some of the pitfalls inherent to electron-microscopy-based techniques, such as dehydration, the need for carbon coating, and immunogold clustering, and is amenable to future developments involving, for example, live-cell imaging. This protocol details the procedures we developed for MC, which can be readily adopted to study a broad range of cell-surface molecules and dissect their distribution within distinct surface assemblies under multiple cell activation states.

ERM-Dependent Assembly of T Cell Receptor Signaling and Co-stimulatory Molecules on Microvilli prior to Activation

T cell surfaces are covered with microvilli, actin-rich and flexible protrusions. We use super-resolution microscopy to show that ≥90% of T cell receptor (TCR) complex molecules TCRαβ and TCRζ, as well as the co-receptor CD4 (cluster of differentiation 4) and the co-stimulatory molecule CD2, reside on microvilli of resting human T cells. Furthermore, TCR proximal signaling molecules involved in the initial stages of the immune response, including the protein tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase) and the key adaptor LAT (linker for activation of T cells), are also enriched on microvilli. Notably, phosphorylated proteins of the ERM (ezrin, radixin, and moesin) family colocalize with TCRαβ as well as with actin filaments, implying a role for one or more ERMs in linking the TCR complex to the actin cytoskeleton within microvilli. Our results establish microvilli as key signaling hubs, in which the TCR complex and its proximal signaling molecules and adaptors are preassembled prior to activation in an ERM-dependent manner, facilitating initial antigen sensing.

Serine Phosphorylation of SLP76 Is Dispensable for T Cell Development but Modulates Helper T Cell Function

The adapter protein SLP76 is a key orchestrator of T cell receptor (TCR) signal transduction. We previously identified a negative feedback loop that modulates T cell activation, involving phosphorylation of Ser376 of SLP76 by the hematopoietic progenitor kinase 1 (HPK1). However, the physiological relevance of this regulatory mechanism was still unknown. To address this question, we generated a SLP76-S376A-expressing knock-in mouse strain and investigated the effects of Ser376 mutation on T cell development and function. We report here that SLP76-S376A-expressing mice exhibit normal thymocyte development and no detectable phenotypic alterations in mature T cell subsets or other lymphoid and myeloid cell lineages. Biochemical analyses revealed that mutant T cells were hypersensitive to TCR stimulation. Indeed, phosphorylation of several signaling proteins, including SLP76 itself, phospholipase Cγ1 and the protein kinases AKT and ERK1/2, was increased. These modifications correlated with increased Th1-type and decreased Th2-type cytokine production by SLP76-S376A T cells, but did not result in significant changes of proliferative capacity nor activation-induced cell death susceptibility. Hence, our results reveal that SLP76-Ser376 phosphorylation does not mediate all HPK1-dependent regulatory effects in T cells but it fine-tunes helper T cell responses.

Ezrin/radixin/moesin are required for the purinergic P2X7 receptor (P2X7R)-dependent processing of the amyloid precursor protein

The amyloid precursor protein (APP) can be cleaved by α-secretases in neural cells to produce the soluble APP ectodomain (sAPPα), which is neuroprotective. We have shown previously that activation of the purinergic P2X7 receptor (P2X7R) triggers sAPPα shedding from neural cells. Here, we demonstrate that the activation of ezrin, radixin, and moesin (ERM) proteins is required for the P2X7R-dependent proteolytic processing of APP leading to sAPPα release. Indeed, the down-regulation of ERM by siRNA blocked the P2X7R-dependent shedding of sAPPα. We also show that P2X7R stimulation triggered the phosphorylation of ERM. Thus, ezrin translocates to the plasma membrane to interact with P2X7R. Using specific pharmacological inhibitors, we established the order in which several enzymes trigger the P2X7R-dependent release of sAPPα. Thus, a Rho kinase and the MAPK modules ERK1/2 and JNK act upstream of ERM, whereas a PI3K activity is triggered downstream. For the first time, this work identifies ERM as major partners in the regulated non-amyloidogenic processing of APP.

Modulation of the immunological synapse: a key to HIV-1 pathogenesis?

AIDS is the result of a constant struggle between the lentivirus HIV and the immune system. Infection with HIV interferes directly with the function of CD4(+) T cells and manipulates the host immune response to the virus. Recent studies indicate that the viral protein Nef, a central player in HIV pathogenesis, impairs the ability of infected lymphocytes to form immunological synapses with antigen-presenting cells and affects T-cell-receptor-mediated stimulation. An integrative picture of the abnormal behaviour of HIV-infected lymphocytes is therefore emerging. We propose that modulating lymphocyte signalling, apoptosis and intracellular trafficking ensures efficient spread of the virus in the hostile environment of the immune system.

ZAP-70 kinase regulates HIV cell-to-cell spread and virological synapse formation

HIV efficiently spreads in lymphocytes, likely through virological synapses (VSs). These cell-cell junctions share some characteristics with immunological synapses, but cellular proteins required for their constitution remain poorly characterized. We have examined here the role of ZAP-70, a key kinase regulating T-cell activation and immunological synapse formation, in HIV replication. In lymphocytes deficient for ZAP-70, or expressing a kinase-dead mutant of the protein, HIV replication was strikingly delayed. We have characterized further this replication defect. ZAP-70 was dispensable for the early steps of viral cycle, from entry to expression of viral proteins. However, in the absence of ZAP-70, intracellular Gag localization was impaired. ZAP-70 was required in infected donor cells for efficient cell-to-cell HIV transmission to recipients and for formation of VSs. These results bring novel insights into the links that exist between T-cell activation and HIV spread, and suggest that HIV usurps components of the immunological synapse machinery to ensure its own spread through cell-to-cell contacts.

Role of ERM (ezrin-radixin-moesin) proteins in T lymphocyte polarization, immune synapse formation and in T cell receptor-mediated signaling

Following antigen recognition, T lymphocytes undergo strong actin cytoskeletal rearrangements. These play a crucial role in the molecular reorganization at the contact site between the T lymphocyte and the antigen presenting cell, termed the immunological synapse. Moreover, they are necessary for T cell activation that leads to cytokine secretion, T cell proliferation and effector function. Little is known on how membrane and signaling molecules interact with the actin cytoskeleton during these processes. Here we review the function of the ERM family of membrane-microfilament linkers, making emphasis on the role of these proteins in T lymphocyte physiology. We discuss how ERM proteins are involved in membrane reorganization during T lymphocyte polarization and immune synapse formation, and how these proteins may contribute to T cell receptor-mediated intracellular signaling that leads to T cell activation.

Vav proteins, masters of the world of cytoskeleton organization

Vav proteins are evolutionarily conserved from nematodes to mammals and play a pivotal role in many aspects of cellular signaling, coupling cell surface receptors to various effectors functions. In mammals, there are three family members; Vav1 is specifically expressed in the hematopoietic system, whereas Vav2 and Vav3 are more ubiquitously expressed. Vav proteins contain multiple domains that enable their function in various fashions. The participation of the Vav proteins in several processes that require cytoskeletal reorganization, such as the formation of the immunological synapse (IS), phagocytosis, platelet aggregation, spreading, and transformation will be discussed in this review. We will also cover how the Vav proteins succeed in controlling these processes by their function as guanine nucleotide exchange factors (GEFs) for the Rho/Rac family of GTPases. The contribution of the Vav proteins in a GEF-independent manner to the organization of the cytoskeleton will also be deliberated. The scope of this review is to highlight the numerous roles of the Vav signal transducer proteins in actin organization.

Over-expression of Rififylin, a new RING finger and FYVE-like domain-containing protein, inhibits recycling from the endocytic recycling compartment

Endocytosed membrane components are recycled to the cell surface either directly from early/sorting endosomes or after going through the endocytic recycling compartment (ERC). Studying recycling mechanisms is difficult, in part due to the fact that specific tools to inhibit this process are scarce. In this study, we have characterized a novel widely expressed protein, named Rififylin (Rffl) for RING Finger and FYVE-like domain-containing protein, that, when overexpressed in HeLa cells, induced the condensation of transferrin receptor-, Rab5-, and Rab11-positive recycling tubulovesicular membranes in the perinuclear region. Internalized transferrin was able to access these condensed endosomes but its exit from this compartment was delayed. Using deletion mutants, we show that the carboxy-terminal RING finger of Rffl is dispensable for its action. In contrast, the amino-terminal domain of Rffl, which shows similarities with the phosphatidylinositol-3-phosphate-binding FYVE finger, is critical for the recruitment of Rffl to recycling endocytic membranes and for the inhibition of recycling, albeit in a manner that is independent of PtdIns(3)-kinase activity. Rffl overexpression represents a novel means to inhibit recycling that will help to understand the mechanisms involved in recycling from the ERC to the plasma membrane.

Characterization of a gene encoding two isoforms of a mitochondrial protein up-regulated by cyclosporin A in activated T cells

Cyclosporin A (CSA) is an immunosuppressor used in organ transplantation. A recent proteomic analysis has revealed that activation of T cells in the presence of CSA induces the synthesis of hundreds of new proteins. Here we used representational difference analysis to characterize some of the corresponding induced genes. After cDNA bank screening we focused on one of these genes, which we named CSA-conditional, T cell activation-dependent (CSTAD) gene. This gene produces two mRNAs resulting from alternative splicing events. They encode two proteins of 104 and 141 amino acids, CSTADp-S and CSTADp-L, for the short and long forms, respectively. FK506 had the same effect as CSA, whereas rapamycin did not affect the level of CSTAD gene expression, demonstrating that inhibition of the calcineurin activation pathway is involved in CSTAD gene up-regulation. CSA also led to overexpression of CSTAD in mice immunized in the presence of CSA, confirming the in vitro analysis. Microscopic and cytofluorimetric analysis of cells expressing green fluorescent protein-tagged CSTADp-L and CSTADp-S showed that both proteins colocalize with mitochondrial markers and depolarize the mitochondrial transmembrane potential without causing release of cytochrome c, apoptosis, or necrosis. Both CSTADp isoforms are sensitive to proteinase K, implying that they are located in the mitochondrial outer membrane. These data reveal a new mechanism of action for CSA, which involves up-regulation of a gene whose products are sorted to mitochondria and depolarize the mitochondrial membrane.

Vav1 and Ly-GDI two regulators of Rho GTPases, function cooperatively as signal transducers in T cell antigen receptor-induced pathways

The Rho family GTPases are pivotal for T cell signaling; however, the regulation of these proteins is not fully known. One well studied regulator of Rho GTPases is Vav1; a hematopoietic cell-specific guanine nucleotide exchange factor critical for signaling in T cells, including stimulation of the nuclear factor of activated T cells (NFAT). Surprisingly, Vav1 associates with Ly-GDI, a hematopoietic cell-specific guanine nucleotide dissociation inhibitor of Rac. Here, we studied the functional significance of the interaction between Vav1 and Ly-GDI in T cells. Upon organization of the immunological synapse, both Ly-GDI and Vav1 relocalize to T cell extensions in contact with the antigen-presenting cell. Ly-GDI is phosphorylated on tyrosine residues following T cell receptor stimulation, and it associates with the Src homology 2 region of an adapter protein, Shc. In addition, the interaction between Ly-GDI and Vav1 requires tyrosine phosphorylation. Overexpression of Ly-GDI alone is inhibitory to NFAT stimulation and calcium mobilization. However, when co-expressed with Vav1, Ly-GDI enhances Vav1 induction of NFAT activation, phospholipase Cgamma phosphorylation, and calcium mobilization. Moreover, Ly-GDI does not alter the regulation of these phenomena when coexpressed with oncogenic Vav1. Since oncogenic Vav1 does not bind Ly-GDI, this suggests that the functional cooperativity of Ly-GDI and Vav1 is dependent upon their association. Thus, our data suggest that the interaction of Vav1 and Ly-GDI creates a fine tuning mechanism for the regulation of intracellular signaling pathways leading to NFAT stimulation.

Defective assembly of the B-cell receptor chains accounts for its low expression in B-chronic lymphocytic leukaemia

B-cell chronic lymphocytic leukaemia (B-CLL) characteristically displays low amounts of B-cell receptor (BCR), which mainly consists of the heterodimer CD79a/CD79b bound non-covalently with the surface immunoglobulin (SIg). This heterodimer is required for SIg expression and BCR signalling. To better define the mechanisms related to low BCR expression, we have investigated transcription, protein synthesis, assembly and transport of the BCR in B-CLL cells. Our results demonstrated that: (1) there was no major defect in transcriptional expression of the B29 (CD79b) gene; (2) the BCR components were intracellularly detected, thus adequately synthesized, in almost all patients; (3) neither a genetic defect in the transmembrane region of SIg, which associated with CD79a/CD79b, nor a genetic abnormality in the chaperone protein calnexin that is involved in folding and assembly of the BCR were found; (4) a constant defect in the assembly of IgM and CD79b chains occurred leading to abnormal accumulation of both chains in different intracellular compartments; (5) in a majority of CLL patients all of the nascent IgM failed to be processed into mature chains and remained unsuitable for transport. These findings demonstrated that a post-transcriptional defect located at the BCR intracellular assembly and/or trafficking levels could be involved in its low surface expression in B-CLL.

The membrane-microfilament linker ezrin is involved in the formation of the immunological synapse and in T cell activation

Dynamic interactions between membrane and cytoskeleton components are crucial for T cell antigen recognition and subsequent cellular activation. We report here that the membrane-microfilament linker ezrin plays an important role in these processes. First, ezrin relocalizes to the contact area between T cells and stimulatory antigen-presenting cells (APCs), accumulating in F-actin-rich membrane protrusions at the periphery of the immunological synapse. Second, T cell receptor (TCR)-mediated intracellular signals are sufficient to induce ezrin relocalization, indicating that this protein is an effector of TCR signaling. Third, overexpression of the membrane binding domain of ezrin perturbs T cell receptor clustering in the T cell-APC contact area and inhibits the activation of nuclear factor for activated T cells (NF-AT).

Internalization and intracellular fate of TCR-CD3 complexes

The number of surface TCR-CD3 complexes is maintained by an equilibrium between the synthesis and secretion of new polypeptides, their internalization, recycling, and degradation. The different subunits of the TCR-CD3 complex do not display the same intracellular trafficking dynamics. Thus, in the absence of stimuli, TCR and zeta chains may be degraded at a higher rate than CD3 subunits, which are mostly recycled. T-cell activation by antigen, anti-TCR-CD3 antibodies, or pharmacological activators of protein kinase C, results in increased TCR-CD3 internalization, followed by the downmodulation of TCR-CD3 surface levels. Once internalized, TCR-CD3 complexes may either enter a recycling pathway or be sorted to lysosomes and degraded. Protein serine kinases and protein tyrosine kinases may influence the internalization and intracellular sorting of TCR-CD3 complexes. In line with these results TCR-CD3 ligands stimulate both TCR-CD3 internalization and degradation, whereas protein kinase C activators stimulate internalization only. Depending on the stimulus applied, internalization motifs from one or several TCR-CD3 subunits mediate endocytic routing of the complex. The involvement of signaling molecules in the intracellular fate of TCR-CD3, the nature and location of sequences for internalization and intracellular sorting, and the role of downregulation in T-cell activation are still the main open questions.

Rho regulates T cell receptor ITAM-induced lymphocyte spreading in an integrin-independent manner

T cell receptor (TCR) engagement increases integrin-mediated adhesion to APC, resulting in the stabilization of the T cell : APC interaction and the close apposition of the two cell membranes. Here we show that engagement of either the TCR or CD3 chimeras with immobilized antibodies causes the rapid spreading of T cells in an integrin-independent fashion. This effect concurs with the polymerization of the actin cytoskeleton and is dependent on the integrity of the immunoreceptor tyrosine-based activation motifs of the CD3 subunits. Expression of a dominant negative mutant of RhoA, as well as the Rho-specific inhibitor C3 toxin, abolished TCR-induced spreading. In contrast, constitutively active or dominant negative forms of Rac and Cdc42 did not affect cell spreading. We conclude that signals emanating from the TCR can directly induce T cell spreading, independently of integrins, and via a Rho-dependent reorganization of the actin cytoskeleton.

CD28 utilizes Vav-1 to enhance TCR-proximal signaling and NF-AT activation

The mechanism through which CD28 costimulation potentiates TCR-driven gene expression is still not clearly defined. Vav-1, an exchange factor for Rho GTPases thought to regulate, mainly through Rac-1, various signaling components leading to cytokine gene expression, is tyrosine phosphorylated upon CD28 engagement. Here, we provide evidence for a key role of Vav-1 in CD28-mediated signaling. Overexpression of Vav-1 in Jurkat cells in combination with CD28 ligation strongly reduced the concentration of staphylococcus enterotoxin E/MHC required for TCR-induced NF-AT activation. Surprisingly, upon Vav-1 overexpression CD28 ligation sufficed to activate NF-AT in the absence of TCR engagement. This effect was not mediated by overexpression of ZAP-70 nor of SLP-76 but necessitated the intracellular tail of CD28, the intactness of the TCR-proximal signaling cascade, the Src-homology domain 2 (SH2) domain of Vav-1, and SLP-76 phosphorylation, an event which was favored by Vav-1 itself. Cells overexpressing Vav-1 formed lamellipodia and microspikes reminiscent of Rac-1 and Cdc42 activation, respectively, for which the SH2 domain of Vav-1 was dispensable. Together, these data suggest that CD28 engagement activates Vav-1 to boost TCR signals through a synergistic cooperation between Vav-1 and SLP-76 and probably via cortical actin changes to facilitate the organization of a signaling zone.

Dissection of the role of CD3gamma chains in profound but reversible T-cell receptor down-regulation

T-lymphocyte activity in the immune system is regulated by the quantity of surface membrane T-cell antigen receptors (TCR). The amount of surface-bound TCR is dependent on the rate of [1] biosynthesis, assembly and intracellular transport of the individual chains composing the TCR/CD3 complex and [2] the internalization and recycling of the receptors. The TCR-ligand interaction augments receptor internalization. In the present paper, we have studied short- and long-term down-regulation of TCR/CD3 complexes with monoclonal anti-TCR/CD3 antibodies, and attempted to determine which component(s) of the TCR/CD3 complex are responsible for these two phenomena. Our data indicate that short- and long-term down-regulation is mediated by different mechanisms, and that the extracellular and/or transmembrane regions of CD3gamma molecules appear to play an important role in chronic TCR/CD3 down-regulation and subsequent deficient re-expression. These results may have important implications for the understanding of induction of T-cell tolerance or anergy.

Convulxin binding to platelet receptor GPVI: competition with collagen related peptides

Convulxin (CVX), a potent platelet aggregating protein from the venom of the snake Crotalus durissus terrificus, is known to bind to the platelet collagen receptor, glycoprotein VI (GPVI). CVX binding to human platelets was investigated by flow cytometry, using fluorescein labeled convulxin (FITC-CVX). Scatchard analysis indicated high and low affinity binding sites with Kd values of 0.6 and 4 nM and Bmax values of 1200 and 2000 binding sites per platelet. FITC-CVX binding was inhibited by collagen related peptides (CRPs) comprising a repeated GPO sequence, namely GCO(GPO)(10)GCOGNH(2) and GKO(GPO)(10)GKOGNH(2), which also bind to receptor GPVI. These peptides (monomeric or cross-linked forms) gave a high affinity inhibition of 10-20% for concentrations between 10 ng/ml and 5 microg/ml, followed by a second phase of inhibition at concentrations greater than 5 microg/ml. It was shown also that the inhibition of FITC-CVX binding by CRPs was independent on the time of preincubation of platelets with CRPs, and the same percentage of inhibition was seen with various concentrations of convulxin. Confocal microscopy of the distribution of FITC-CVX binding sites on platelets showed an homogeneous distribution of FITC-CVX bound to GPVI, although some limited clustering may exist.

Triggering the TCR complex causes the downregulation of nonengaged receptors by a signal transduction-dependent mechanism

Downregulation of the TCR complex is believed to be intimately tied to T cell activation, allowing serial triggering of receptors and desensitization of stimulated cells. We studied transfected and transgenic T cells expressing CD3zeta chimeras to demonstrate that ligand engagement of the TCR or chimeras causes comodulation of nonengaged receptors. Comodulation required protein tyrosine kinase activity but not trans-phosphorylation of nonengaged receptors. The TCR appears to be downregulated by at least two mechanisms. One mechanism requires direct engagement, independent of signaling. The second requires signaling and downregulates nontriggered receptors. These results shed new light on the process of TCR downregulation and indicate that the number of downregulated TCRs cannot be assumed to equal the number of engaged receptors.

The CD3 epsilon subunit of the TCR contains endocytosis signals

Ligand binding to TCR induces its internalization and cell surface down-modulation. These phenomena contribute to the extinction of activation signals. Due to the multicomponent nature of the TCR-CD3 complex, its internalization may be mediated by one or several of its subunits. Although it has been reported that CD3 gamma and CD3 delta contain endocytosis motifs involved in the internalization of the TCR-CD3 complex, other subunits could also be involved in this process. For instance, CD3 epsilon and CD zeta display amino acid sequences reminiscent of internalization motifs. To investigate whether CD3 epsilon bears endocytosis signals, we have analyzed the internalization capacity of a panel of deletion and point mutants of CD3 epsilon that were expressed on the cell surface independently of other TCR-CD3 subunits. Here we report that CD3 epsilon displays endocytosis determinants. These data indicate that CD3 epsilon could contribute to the internalization and cell surface down-regulation of TCR-CD3 complexes. Moreover, the existence of endocytosis signals in this polypeptide could serve to retrieve unassembled CD3 epsilon subunits or partial CD3 complexes from the plasma membrane, thus restricting the expression on the cell surface to fully functional TCR-CD3 complexes.

Cooperative activation of TCRs by enterotoxin superantigens

Staphylococcus enterotoxin superantigens are potent T cell activators. To gain new insights into the mechanism of T cell activation induced by these superantigens, we investigated the recruitment of signaling molecules in this process. Here, we show that enterotoxin superantigen activation can be transmitted to TCR-CD3 complexes that did not interact with their ligand. Indeed, by studying cells expressing two distinct TCRs, we found that enterotoxin superantigens induced tyrosine phosphorylation of TCRzeta subunits, the recruitment and tyrosine phosphorylation of the protein tyrosine kinase ZAP-70, and an increase in protein tyrosine kinase activity of both directly stimulated and unstimulated TCR-CD3 complexes. As the involvement of unstimulated TCR-CD3 complexes in signal transduction would increase the number of signaling molecules and, therefore, the efficiency of T cell activation, these data provide a novel explanation for the ability of enterotoxin superantigens to potently activate T lymphocytes.

Peptide antigen or superantigen-induced down-regulation of TCRs involves both stimulated and unstimulated receptors

T cell activation by peptide/MHC complexes, superantigens, or mAbs induces the down-regulation of cell surface TCRs. We addressed the question of whether TCR down-modulation affects only TCRs that had directly interacted with their ligand or whether down-modulation could also affect TCRs that had not interacted with their ligand. To this end, we generated T cells coexpressing equal levels of two different TCRs by transfecting the appropriate cDNAs into cells of the human T cell line, Jurkat. Each set of TCRs can be distinguished by means of anti-Vbeta mAbs and can be stimulated separately with peptide Ag, bacterial superantigens, or mAbs. We found that activation of these cells with each of these stimuli down-modulated not only directly stimulated TCR complexes but also unstimulated ones. Comodulation of stimulated and unstimulated receptors may reflect functional interactions between surface TCRs that could take place during Ag or superantigen recognition by T cells without the need for ligand cross-linking. Consistent with this idea, both stimulated and unstimulated receptors colocalized in patches on the cell surface after activation.

Peptide antigen or superantigen-induced down-regulation of TCRs involves both stimulated and unstimulated receptors

T cell activation by peptide/MHC complexes, superantigens, or mAbs induces the down-regulation of cell surface TCRs. We addressed the question of whether TCR down-modulation affects only TCRs that had directly interacted with their ligand or whether down-modulation could also affect TCRs that had not interacted with their ligand. To this end, we generated T cells coexpressing equal levels of two different TCRs by transfecting the appropriate cDNAs into cells of the human T cell line, Jurkat. Each set of TCRs can be distinguished by means of anti-Vbeta mAbs and can be stimulated separately with peptide Ag, bacterial superantigens, or mAbs. We found that activation of these cells with each of these stimuli down-modulated not only directly stimulated TCR complexes but also unstimulated ones. Comodulation of stimulated and unstimulated receptors may reflect functional interactions between surface TCRs that could take place during Ag or superantigen recognition by T cells without the need for ligand cross-linking. Consistent with this idea, both stimulated and unstimulated receptors colocalized in patches on the cell surface after activation.

Differential cytosolic tail dependence and intracellular fate of T-cell receptors internalized upon activation with superantigen or phorbol ester

Activation of T lymphocytes by T-cell receptor (TCR) ligands such as peptide/MHC complexes, superantigens or anti-TCR mAbs, or by pharmacological activators of protein kinase C such as phorbol esters, results in the internalization and cell surface downregulation of TCRs. We investigated the role of internalization motifs located in the cytosolic region of CD3 gamma in the internalization of TCR complexes induced by enterotoxin superantigens, anti-TCR mAbs or phorbol esters. To this end, a series of CD3 gamma mutants were expressed in a CD3 gamma-deficient variant of the human T-cell line Jurkat. We found that serine126 and the di-leucine motif (Leu131-Leu132) are required for phorbol-ester-induced TCR downregulation, but they are not necessary for enterotoxin superantigen or antibody-induced TCR downregulation. Moreover, the tyrosine-based motifs (residues 138 to 141 and 149 to 152) are not required either for phorbol aster or for superantigen or antibody-induced TCR downregulation. Confocal microscopy analysis reveals that TCR complexes accumulate in an early endocytic/recycling compartment upon activation of cells with phorbol esters, whereas TCRs internalized upon activation with superantigen or anti-TCR mAbs are routed to lysosomes. Consistent with this intracellular localization, TCRs internalized in response to phorbol ester are not degraded and can be reexpressed on the cell surface. In contrast, TCRs internalized upon superantigen activation are degraded.

An activated epidermal growth factor receptor/Lck chimera restores early T cell receptor-mediated calcium response in a CD45-deficient T cell line

In T cells, cell surface expression of CD45, a transmembrane tyrosine phosphatase, is required for T cell receptor (TCR) signal transduction. Indirect evidence suggests that CD45 function in TCR signaling involves the dephosphorylation of the C-terminal negative regulatory site of p56(lck), Tyr-505. To evaluate the importance of CD45-mediated dephosphorylation of p56(lck) Tyr-505 in TCR signaling, we established CD45(-) Jurkat cell lines expressing various forms of a chimera containing the extracellular and transmembrane domains of the epidermal growth factor receptor (EGFR) fused to p56(lck). We report that an activated EGFR/Lck chimera is able to reconstitute a Ca2+ response after CD3 stimulation in the absence of CD45 expression. In addition, the wild-type and kinase inactive versions of the EGFR/Lck chimera fail to restore early signaling. Restoration of the response by EGFR/LckF505 required EGF binding to the chimeric kinase. Altogether, these results provide the first direct evidence that the lack of efficient dephosphorylation of p56(lck) Tyr-505 is, in part, responsible for the unresponsiveness of CD45(-) cells. They also indicate that a second event is required for p56(lck) function in TCR signaling in addition to its dephosphorylation at Tyr-505.

Combination of human cytomegalovirus recombinant immediate-early protein (IE1) with 80 nm cationic biovectors: protection from proteolysis and potentiation of presentation to CD4+ T-cell clones in vitro

We have shown in a previous study that the proliferative CD4+ T-cell response to the regulatory immediate-early protein IE1 was a major component of the overall anti viral response in human cytomegalovirus (HCMV) seropositive blood donors. This viral antigen may be valuable in subunit vaccine design, since anti IE1 CD4+ T cells might provide help for production of antibodies and cytotoxic T lymphocytes (CTL) responses, and could take part in the control of viral infection. Preliminary to the elaboration of future vaccine formulations, we developed immunogenic complexes resulting from the combination of a purified recombinant protein derived from the fusion of Escherichia coli glutathione-S-transferase (GST) and a large C-terminal fragment (e4) of IE1, with new 80 nm cationic synthetic particles called Biovectors. We have shown that the antigen GST-e4 was stably complexed to vectors and that, contrary to the soluble form, it was protected from proteolysis in cell culture medium. By confocal microscopy we observed that the synthetic vectors were internalized by lymphoblastoid B cells, providing a significant enhancement of antigen delivery in antigen presenting cells (APC). Indeed, we demonstrated that the previous combination of antigen with particles, significantly enhanced the proliferation of specific CD4+ T-cell clones directed against IE1 in vitro, when either HLA-matched isolated peripheral blood mononuclear cells or EBV transformed B cell lines were used as APC. The relevance of these observations to the use of these new vectors for vaccine design against HCMV is discussed.

The Staphylococcus aureus enterotoxin B superantigen induces specific T cell receptor down-regulation by increasing its internalization

Superantigens are able to stimulate T lymphocyte populations expressing T cell antigen receptors (TCR) belonging to particular V beta families. Moreover, the presence of these superantigens may induce long term unresponsiveness (anergy) of these sensitive cells. Some bacterial toxins are potent superantigens. We have analyzed in vitro the capacity of some Staphylococcus aureus enterotoxin superantigens to modulate T cell antigen receptor expression and the cellular mechanisms involved. Staphylococcus enterotoxin B (SEB) induced rapid down-regulation of surface T cell antigen receptors in V beta 3-expressing T lymphocytes, as assessed by flow cytometry. This phenomenon was a consequence of the direct interaction between the toxin and the TCR since it was observed in the absence of cells expressing major histocompatibility complex class II molecules. The cellular mechanism involved in SEB-induced down-regulation of TCR was further investigated. Immunofluorescence and confocal microscopy experiments showed that toxin B induced intracellular accumulation of TCR.CD3 in endocytic vesicles. Moreover, SEB induced an increase in T cell receptor endocytosis as measured using radiolabeled Fab fragments of an anti-CD3 monoclonal antibody. Taken together, our observations indicate that Staphylococcus enterotoxin B superantigen induced changes in the dynamics of surface T cell receptors, which resulted in the fast reduction of membrane receptor numbers.

Jacalin, a lectin that inhibits in vitro HIV-1 infection, induces intracellular calcium increase via CD4 in cells lacking the CD3/TcR complex

The lectin jacalin interacts with the CD4 cell surface antigen; this lectin inhibits in vitro infection by human immunodeficiency virus type 1 without preventing virus binding on the host cell. The infection process is known to involve cellular events triggered by the binding of the viral external glycoprotein gp120 to CD4. Herein we demonstrate that jacalin induces cell signaling directly through the CD4 antigen and that independently of the CD3/TcR complex. The capacity of jacalin to trigger cell signals through the CD4 molecule is discussed in relation to its ability to inhibit HIV infection.

Evidence for an extended structure of the T-cell co-receptor CD8 alpha as deduced from the hydrodynamic properties of soluble forms of the extracellular region

We expressed soluble forms of the human T-cell coreceptor CD8 alpha extracellular region, CD8 alpha 161, and the amino-terminal immunoglobulin-like domain, CD8 alpha 114, in Chinese hamster ovary cells and Escherichia coli, respectively. Both molecules were readily purified to homogeneity in milligram amounts and were recognized by a large panel of monoclonal antibodies. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis indicated that approximately 70% of CD8 alpha 161 was secreted as a disulfide-linked dimer, but CD8 alpha 114 was not disulfide-linked. To investigate the structural features of CD8 alpha 161 and CD8 alpha 114 under native conditions, we performed gel filtration and sucrose gradient sedimentation analysis. In spite of being partially or totally noncovalently bound, both recombinant molecules were stably associated homodimers, as no monomers could be detected at a fairly low protein concentration (approximately 1 microM). This suggests that the CD8 alpha amino-terminal domain alone strongly contributes to chain association. Determination of the Stokes radius (Rs) and sedimentation coefficient (s20,w) gave results consistent with CD8 alpha 114 having a globular shape and CD8 alpha 161 being an asymmetric molecule. Taking into account the contribution of hydration to the frictional coefficient, we obtained for CD8 alpha 161 an axial ratio of approximately 5, when modeled as a prolate ellipsoid. These results indicate that the elongated structure of CD8 alpha 161 is essentially contributed by the hinge region and help to explain how the CD8 alpha is able to bridge the distance between the T-cell surface and its binding site in the alpha 3 domain of major histocompatibility complex class I molecules on the target cell.

A soluble form of the human CD8 alpha chain expressed in the baculovirus system: biochemical characterization and binding to MHC class I

We have generated a soluble form of the CD8 molecule consisting of the entire extracellular domains of the human alpha chain, by expressing a mutated CD8 alpha cDNA in SF9 cells infected with a recombinant baculovirus. The truncated molecule was secreted into the medium mostly as a disulfide-linked homodimer in which a single cysteine residue in the hinge-like region (Cys143) was sufficient to assure covalent bonding. Soluble CD8 purified to homogeneity appears to be monodisperse as assessed by gel filtration analysis and contains only O-linked carbohydrates. To determine whether recombinant CD8 can interact with MHC class I molecules, we developed an assay that measures binding of MHC class I-bearing cell lines to purified CD8 adsorbed to plastic plates. The level of binding of cells to immobilized CD8 depended on the amount of CD8 bound to the plate and correlated with the levels of cell surface MHC class I expression. The binding was specifically inhibited by monoclonal antibodies directed either against CD8 or MHC class I molecules. This assay therefore provides a way to measure CD8 binding to MHC class I independently of other cell-cell interactions and should allow direct structure-function studies.

Both T cell receptor (TcR)-CD3 complex and CD2 increase the tyrosine kinase activity of p56lck. CD2 can mediate TcR-CD3-independent and CD45-dependent activation of p56lck

T cell activation by triggering the T cell receptor (TcR)-CD3 complex leads to a dramatic increase in tyrosine phosphorylation of multiple cellular proteins. To date, there has been no direct evidence on the identity of the tyrosine kinase activity implicated in this signaling pathway. In this study, we demonstrate that activation of human T cells with anti-CD3 monoclonal antibody increases tyrosine kinase activity of p56lck. This extends our previous findings which demonstrated the involvement of p56lck kinase activity in the CD2 signal transduction pathway. The results from peripheral blood lymphocytes and Jurkat cell line showed in both cases an early and transient change in the specific activity of p56lck, followed by a shift to a higher apparent molecular mass. Therefore, to test directly the role of TcR-CD3 in CD2-induced activation of p56lck, we utilized mutant variants of the Jurkat cell line lacking in cell surface TcR-CD3. We found that cell surface expression of TcR-CD3 is not required for the activation of p56lck via CD2 but is necessary for the appearance of the reduced-electrophoretic-mobility form of p56lck observed after CD2 triggering. By isolating CD45- mutants from Jurkat cells, we observed that surface expression of the tyrosine phosphatase CD45 is required in order to increase p56lck activity following CD2 stimulation, while CD4-induced activation of the kinase remained unchanged. These data provide evidence for a specific functional linkage between CD2 and p56lck, in which CD45 may play an essential role.

Engagement of major histocompatibility complex class I and class II molecules up-regulates intercellular adhesion of human B cells via a CD11/CD18-independent mechanism

We have studied the role of major histocompatibility complex (MHC) molecules in the regulation of intercellular adhesion of human B cells. We found that molecules able to bind to MHC class II molecules, such as monoclonal antibodies or staphylococcal enterotoxins, induced rapid and sustained homotypic adhesion of Epstein-Barr virus (EBV)-transformed B cell lines as well as peripheral blood B lymphocytes. Moreover, anti-MHC class I monoclonal antibodies also stimulated intercellular adherence. Adhesion induced upon MHC engagement was faster and stronger than that triggered by phorbol esters. It needed active metabolism, but divalent cations were not required. Monoclonal antibodies directed against LFA-1 (CD11a/CD18) or its ligand ICAM-1 (CD54) did not inhibit MHC class II-induced homotypic adhesion of various EBV-transformed B cell lines, nor of a variant of the B cell line Raji expressing very low LFA-1 surface levels. Moreover, EBV-transformed B cells from a severe lymphocyte adhesion deficiency patient, lacking surface CD11/CD18, also aggregated in response to anti-MHC class I or class II monoclonal antibodies. Together these data indicate that engagement of MHC molecules may transduce signals to B cells resulting in up-regulation of intercellular adhesion, via an LFA-1-independent mechanism. This may play a role in the stabilization of T cell/antigen-presenting cell conjugates at the moment of antigen recognition.

The tyrosine kinase activity of p56lck is increased in human T cells activated via CD2

An early biochemical event associated with T cell activation is tyrosine phosphorylation. We have previously shown that p56lck, a lymphocyte-specific protein tyrosine kinase, is hyperphosphorylated on serine and tyrosine residues 15 minutes after activation via CD2 with a concomitant shift to a higher molecular mass. We now demonstrate that the tyrosine kinase activity of p56lck is increased within seconds following CD2 triggering. This activity decreases thereafter correlating with the appearance of changes in phosphorylation previously described. These results suggest that p56lck may play an important role in the CD2 activation pathway.

Characterization of functional GTP binding proteins in Jurkat T cell mutants lacking either CD3-Ti or CD2 surface receptors

G proteins are membrane-bound molecules involved in coupling of surface receptors with signal transduction effector systems in multiple cell types including T lymphocytes. Given that mature T cells which lack antigen receptors (CDl-Ti) are refractory to stimulation through CD2 or other accessory molecules, T cell receptor components likely play a critical role in coupling surface receptors with signal transduction effectors. It has recently been proposed that modulation of T cell receptor components with MAbs results in a physical loss or functional inactivation of G protein(s). In view of the importance of the T cell activation process, we herein examined G proteins in untreated or antibody-modulated Jurkat T cells as well as in genetic variants lacking either CD3-Ti or CD2 surface receptors. 43- and 41-kDa G protein alpha chains are ADP ribosylated with cholera (CTX) and pertussis (PTX) toxins, respectively, in wild type and receptor minus cell populations. In the wild type Jurkat cell line as well as in CD3- and CD2- variants, AlF4- can activate the G protein(s) presumably associated with phospholipase C to generate polyphosphoinositide turnover as well as an increase in cytoplasmic free calcium ions. Furthermore, G protein(s) linked to adenylylcyclase, a pathway which inhibits T lymphocyte activation, can be directly activated with CTX in the absence of CD3-Ti or CD2 on the membrane. Importantly, AlF4- can also induce polyphosphoinositide turnover in Jurkat cells whose T cell receptor proteins have been modulated with anti-CD3 MAb. These data provide functional and biochemical evidence that at least certain G proteins are intact in the absence of surface expression of CD3-Ti or CD2 molecules and imply that CD3-Ti desensitization is not singularly due to G protein loss.

Lysine 271 in the transmembrane domain of the T-cell antigen receptor beta chain is necessary for its assembly with the CD3 complex but not for alpha/beta dimerization

The T-cell antigen receptor (TcR) complex present on most T-cells is formed by a clone-specific disulfide-linked alpha/beta heterodimer noncovalently associated to the CD3 complex, the latter composed of five invariant polypeptides: gamma, delta, epsilon, zeta/zeta, or zeta/eta. The presence of conserved, oppositely charged, amino acids in the predicted transmembrane domains of all the subunits of the TcR.CD3 complex suggests that these residues may have a critical function in the assembly and/or stabilization of the complex. In order to analyze the role of the transmembrane-charged amino acids in the association and cell surface expression of the TcR.CD3 complex, we have carried out site-directed mutagenesis of Lys271 in the transmembrane domain of the TcR beta chain and analyzed the capacity of the altered chain to assemble in a TcR beta-negative T-cell line. Here we show that substitution of this positively charged residue by alanine or glutamine does not prevent cytoplasmic association of alpha and beta chains to form disulfide-linked heterodimers, but does abolish formation of an alpha/beta.CD3 complex and, consequently, its expression on the cell surface.

The lymphocyte-specific protein tyrosine kinase p56lck is hyperphosphorylated on serine and tyrosine residues within minutes after activation via T cell receptor or CD2

Human T cells can be activated and induced to proliferate through either the antigen-specific receptor complex (TcR-CD3) or the CD2 surface molecule. Following stimulation, both serine and tyrosine phosphorylation of cellular protein have been demonstrated to occur. p56lck, a protein tyrosine kinase associated to the inner face of the plasma membrane, is almost exclusively expressed in lymphoid cells, especially T cells. Within minutes after activation of a human T cell-derived line (Jurkat) via stimulation of either the TcR-CD3 complex or the CD2 glycoprotein, we observed a hyperphorphosylation of p56lck. A concomitant shift to a higher molecular weight in sodium dodecyl sulfate-polyacrylamide gel was also observed. Similar changes were obtained with phorbol 12-myristate 13-acetate. Tryptic phosphopeptide analysis of the hyperphosphorylated form of p56lck yielded new phosphorylated sites in serine residues and an increased tyrosine phosphorylation. These results suggest that p56lck may be intimately connected to the signaling pathway in T cell activation.

The structural biology of CD2

The CD2 molecule is a 50-55KD transmembrane glycoprotein expressed on the vast majority of thymocytes and virtually all peripheral T lymphocytes. Its functions are two-fold: adhesion and activation. CD2 serves to facilitate conjugate formation between the T-lineage cell and its cognate partner via intermolecular interaction of CD2 and LFA-3 on the former and latter cells, respectively. Perturbation of the CD2 extracellular segment by certain combinations of anti-CD2 MAbs or LFA-3 and a single anti-CD2 MAb activate T-lineage function. These CD2-mediated activation events also synergize with signals mediated through the TCR to augment T-cell response. Based on microchemical analysis of immunoaffinity-purified human CD2 and cDNA and genomic cloning of mouse and human molecules, considerable structural information is now available. The mature surface human CD2 molecule consists of 327 amino acids: a 185 aa extracellular segment; a 25 aa hydrophobic transmembrane segment; and a 117 aa cytoplasmic domain rich in prolines and basic residues. The CD2 gene is comprised of five exons which span approximately 12 Kb on chromosome 1. A similar protein structure and gene exon organization is found for the mouse CD2 homologue. The CD2 adhesion domain is approximately 103 aa in length and is encoded by a single exon (exon 2). This domain is resistant to proteolysis, even though it lacks any intrachain disulfides and, like the entire extracellular segment protein expressed in a baculovirus system, binds to its cellular ligand, LFA-3. The latter occurs with a micromolar Kd. This relatively low affinity suggests that multivalent interactions among CD2 monomers on the T cells and individual LFA-3 structures on the cognate partner are important in enhancing the avidity of the T-cell interaction with its target or stimulator cell. The affinity of the CD2 extracellular segment for LFA-3 is not affected by truncations in the CD2 cytoplasmic domain, implying that ligand binding is not regulated by intracellular mechanisms. Given that CD2 mRNA expression and surface CD2 copy number are increased by more than one order of magnitude post-TCR stimulation, it is more likely that adhesion via CD2 is modulated by alteration in surface copy number. Analysis of early transduction events occurring via CD3-Ti (TCR) and CD2 including single channel Ca2+ patch-clamp recordings on living human T lymphocytes indicate a virtual identity of signals.(ABSTRACT TRUNCATED AT 400 WORDS)

CD3Ti+ human thymocyte-derived clones displaying a differential response to activation via CD3Ti and CD2

Previous studies indicated that, unlike peripheral T-cells, freshly isolated thymocytes show little or no proliferation to activation signals via either the antigen/MHC receptor complex (CD3Ti) or the CD2 structure, unless exogenous IL-2 or phorbol esters are added. To investigate these differences in more detail, we have studied the response of clonal populations of mature thymocyte subsets as well as peripheral T-cell clones to activation via either CD3Ti or CD2. Here we report the characterization of three clones belonging to different subsets of mature thymocytes: CD3+ CD4+ (Ti alpha/beta), CD3+ CD8+ (Ti alpha/beta), and CD3+ CD4- CD8- (Ti gamma/delta). All three clones could be induced to proliferate to insolubilized anti-CD3 mAb. In contrast, activating anti-CD2 mAbs, which induced proliferation in all peripheral T-cell clones tested, did not induce an appreciable proliferation of the thymocyte clones. The latter required additional signals provided by the phorbol ester PMA. However, anti-CD2 mAbs were able to induce early activation events such as phosphoinositide turnover and [Ca2+]i increase to an extent similar to the ones elicited by anti-CD3 mAb. These results further support previous findings suggesting that mature thymocytes are not functionally identical to peripheral T-cells.

Triggering of T-lymphocytes via either T3-Ti or T11 surface structures opens a voltage-insensitive plasma membrane calcium-permeable channel: requirement for interleukin-2 gene function

Stimulation of human T-lymphocytes via either the surface T3-Ti antigen-major histocompatibility complex receptor complex or the T11 molecule results in clonal proliferation through a calcium-dependent mechanism. To investigate this signal transduction, plasma membrane calcium-permeable channels were characterized in T-lymphocytes by means of whole cell or single channel patch-clamp recordings. Stimulation of T-lymphocytes via either structure results in opening of an identical set of voltage-insensitive plasma membrane Ca2+-permeable channels through the action of a diffusible second messenger. Previous work with excised inside-out patches suggests that inositol 1,4,5-trisphosphate is the activating second messenger of the voltage-insensitive T-cell Ca2+-permeable channel. Since there is a significant increase in phosphoinositide turnover after stimulation via either the T3-Ti or T11 pathway, it is suggested that triggering of either structure opens a common set of channels through this mechanism. Furthermore, currents flowing through Ca2+-permeable channels are apparently autoregulated, as inward conductance is abolished by elevation of Ca2+ concentration in the bathing solution. In particular, the steady-state rise in interleukin-2 (T-cell growth factor) mRNA is dependent on the rise of [Ca2+]i resulting from ion movement across this channel.

Expression of a functional CD3-Ti antigen/MHC receptor in the absence of surface CD2. Analysis with clonal Jurkat cell mutants

To investigate the requirement for CD2 expression in activation of T lymphocytes via the CD3-Ti antigen/MHC receptor complex, we produced and characterized a series of CD2- Jurkat variants. These mutants lack detectable surface CD2 as determined by indirect immunofluorescence, immunoprecipitation analysis, and specific radiolabeled antibody binding assay, but nevertheless, expressed normal numbers of CD3-Ti receptors. As expected, the combination of anti-CD2 antibodies, termed anti-T112 and anti-T113, which are mitogenic for resting T lymphocytes, failed to stimulate activation of these variants. In contrast, triggering of their CD3-Ti components resulted in the normal set of T lymphocyte-associated activation events, including phosphoinositide turnover, elevation in intracellular free calcium, early gene-induction events, and IL-2 production. Assuming that the Jurkat cell line is representative of normal cycling human T lymphocytes, we conclude that the presence of the CD2 molecule on the plasma membrane is not in itself a requirement for an operational CD3-Ti-alpha/beta receptor.

Interdependence of CD3-Ti and CD2 activation pathways in human T lymphocytes

Human T lymphocytes can be activated through either the antigen/MHC receptor complex T3-Ti (CD3-Ti) or the T11 (CD2) molecule to proliferate via an IL-2 dependent mechanism. To investigate the relationship of these pathways to one another, we generated and characterized Jurkat mutants which selectively express either surface CD3-Ti or CD2. Here we show that CD3-Ti- mutants fail to be stimulated by either pathway to increase phosphoinositide turnover, mobilize calcium or induce the IL-2 gene. The activation capacity of these mutants via CD2 as well as CD3-Ti can be restored following reconstitution of surface CD3-Ti expression upon appropriate DNA transfer (e.g. Ti beta subunit cDNA into Ti beta- Jurkat variants). Collectively, these results demonstrate that CD3-Ti and CD2 pathways are interdependent and that phosphoinositide turnover is linked to the CD3-Ti complex.

The T 11 (CD 2) cDNA encodes a transmembrane protein which expresses T 11(1), T 11(2) and T 11(3) epitopes but which does not independently mediate calcium influx: analysis by gene transfer in a baculovirus system

To determine whether a full-length human T 11 (CD 2) cDNA encodes a protein which expresses all three T 11 epitopes (T 11(1), T 11(2), T 11(3] and independently triggers activation in a cell other than a T lymphocyte, a baculovirus expression system was employed. Here we show that a recombinant T 11 cDNA-containing baculovirus can induce high-level expression of T 11(1), T 11(2) and T 11(3) epitopes on the surface of gut epithelial SF9 cells. However, in this environment the T 11 protein cannot be triggered to transduce a signal resulting in an elevation of the cytosolic free Ca2+ as is known to occur in T lymphocytes. These results support the notion that T 11 functions in a coordinate fashion with other intracellular lymphoid components.

Ion channels activated by specific Ti or T3 antibodies in plasma membranes of human T cells

T lymphocytes are activated to proliferate via a surface membrane receptor recognizing the antigen/major histocompatibility complex. This membrane component is comprised of at least five polypeptide subunits, collectively termed the Ti-T3 receptor complex. A transient increase in cytosolic free calcium occurs as an early event in the T-cell activation process and is necessary for induction of the endogenous IL-2 and certain other genes. Monoclonal antibodies specific to epitopes of either the Ti or the T3 components were shown to be effective agonists, also leading to such transient rises in cytosolic free calcium and activating the lymphocytes. Here we show, using micropipette-supported bilayers formed from membranes of the human T-cell line REX, that Ti- or T3-specific antibodies cause opening of ligand gated ion channels. Both types of specific antibodies yielded similar histograms of conductance amplitudes which show a channel with a conductance of 2-3 pS in symmetrical 100 mM CaCl2 solutions. These channels allow the passage of calcium and barium ions and are blocked by lanthanum ions, suggesting that they are specific for calcium. We propose that these channels, by allowing the entry of external calcium, may account for a large fraction of the rise in intracellular calcium observed upon triggering of the Ti-T3 receptor.

Functional and molecular aspects of human T lymphocyte activation via T3-Ti and T11 pathways

Two pathways of human lymphocyte activation are known to exist on T-lineage cells. The first involves the T-lymphocyte receptor for antigen (T3-Ti) which operates in conjunction with gene products of the MHC complex and is a molecular complex composed of 5 polypeptide chains. Both the 49KD alpha and 43KD beta chains are immunoglobulin-like and thus contain variable domains responsible for ligand binding. In contrast, the 20-25KD T3 gamma, delta and epsilon chains are monomorphic structures presumably involved in transmembrane signalling. The alpha and beta subunits are disulfide bonded to each other and held in noncovalent association with the T3 chains. The second pathway involves the 50KD T11 sheep erythrocyte binding protein. The T11 pathway is operational during early intrathymic ontogeny, prior to T3-Ti receptor expression. Under physiologic conditions, T3-Ti and T11 pathways appear to function in series with T11, representing a more "nuclear proximal" structure. However, each pathway, independently of the other, can activate the phosphoinositol cascade and lead to elevation in cytosolic free calcium. The latter is critical for transcriptional activation of the endogenous IL-2 gene. The ability of the T3-Ti complex to regulate T11-mediated activation is discussed with reference to its possible role in thymic selection.