Degradation of T cell receptor (TCR)-CD3-zeta complexes after antigenic stimulation

Helicobacter pylori arginase inhibits T cell proliferation and reduces the expression of the TCR zeta-chain (CD3zeta)

Helicobacter pylori infects approximately half the human population. The outcomes of the infection range from gastritis to gastric cancer and appear to be associated with the immunity to H. pylori. Patients developing nonatrophic gastritis present a Th1 response without developing protective immunity, suggesting that this bacterium may have mechanisms to evade the immune response of the host. Several H. pylori proteins can impair macrophage and T cell function in vitro through mechanisms that are poorly understood. We tested the effect of H. pylori extracts and live H. pylori on Jurkat cells and freshly isolated human normal T lymphocytes to identify possible mechanisms by which the bacteria might impair T cell function. Jurkat cells or activated T lymphocytes cultured with an H. pylori sonicate had a reduced proliferation that was not caused by T cell apoptosis or impairment in the early T cell signaling events. Instead, both the H. pylori sonicate and live H. pylori induced a decreased expression of the CD3zeta-chain of the TCR. Coculture of live H. pylori with T cells demonstrated that the wild-type strain, but not the arginase mutant rocF(-), depleted L-arginine and caused a decrease in CD3zeta expression. Furthermore, arginase inhibitors reversed these events. These results suggest that H. pylori arginase is not only important for urea production, but may also impair T cell function during infection.

Recruitment of transferrin receptor to immunological synapse in response to TCR engagement

T cell receptor engagement by an APC induces the formation of a highly organized complex of surface receptors and intracellular signaling molecules, known as the immunological synapse, at the site of cell-cell contact. The transferrin receptor (TfR, CD71) is normally present in the plasma membrane and recycling endosomes. In this study, we show that, although the TfR is typically absent from lipid rafts at steady state, stimulation with a mitogenic mixture of anti-CD3 Abs of human Jurkat T cells leads to a rapid compartmentalization of the TfR into lipid rafts accompanying that of CD3epsilon and activated Lck. This change occurs very rapidly and is accompanied by an increase in the surface expression of the TfR, probably by translocation from an internal endosomal pool. TfR recruitment to lipid rafts was also observed in primary T cells treated with mitogenic anti-CD3 Abs and in Jurkat T cell-APC conjugates. The use of beads coated with Abs indicates that the surface and endosomal TfR pools redistribute to the contact site region in response to engagement of CD28 and CD3. In T cell-APC conjugates, the T cell TfR endosomal pool relocates beneath the contact site, whereas surface TfR localizes to the peripheral ring of the immunological synapse. In the presence of specific anti-TfR Abs, the total number of T cell-APC contacts and the percentage of conjugates with CD3 and Lck translocated to the contact site were reduced. Our results therefore suggest the involvement of the TfR in the formation of the immunological synapse.

Ubiquitin ligases and the immune response

Ubiquitin (Ub)-protein conjugation represents a novel means of posttranscriptional modification in a proteolysis-dependent or -independent manner. E3 Ub ligases play a key role in governing the cascade of Ub transfer reactions by recognizing and catalyzing Ub conjugation to specific protein substrates. The E3s, which can be generally classified into HECT-type and RING-type families, are involved in the regulation of many aspects of the immune system, including the development, activation, and differentiation of lymphocytes, T cell-tolerance induction, antigen presentation, immune evasion, and virus budding. E3-promoted ubiquitination affects a wide array of biological processes, such as receptor downmodulation, signal transduction, protein processing or translocation, protein-protein interaction, and gene transcription, in addition to proteasome-mediated degradation. Deficiency or mutation of some of the E3s like Cbl, Cbl-b, or Itch, causes abnormal immune responses such as autoimmunity, malignancy, and inflammation. This review discusses our current understanding of E3 Ub ligases in both innate and adaptive immunity. Such knowledge may facilitate the development of novel therapeutic approaches for immunological diseases.

Control of Immune Responses by Trafficking Cell Surface Proteins, Vesicles and Lipid Rafts to and from the Immunological Synapse

Supramolecular clusters at the immunological synapse provide a mechanism for structuring complex communication networks between cells of the immune system. Regulating intra- and intercellular trafficking of proteins and lipids to and from the immunological synapse provides an additional level of complexity in determining the functional outcome of immune cell interactions. An emergent principle is that molecules requiring tightly regulated cell surface expression, e.g. negative regulators of cell activation or molecules promoting cytotoxicity, are trafficked to the immunological synapse from intracellular secretory as required lysosomes. Many molecules required for the early stages of the intercellular communication are already present at the cell surface, sometimes in lipid rafts, and are rapidly translocated laterally to the intercellular contact. Our understanding of these events critically depends on utilizing appropriate technologies for probing supramolecular recognition in live cells. Thus, we also present here a critical discussion of the technologies used to study lipid rafts and, more broadly, a map of the spatial and temporal dimensions covered by current live cell physical techniques, highlighting where advances are needed to exceed current spatial and temporal boundaries.

Differences in the Kinetics, Amplitude, and Localization of ERK Activation in Anergy and Priming Revealed at the Level of Individual Primary T Cells by Laser Scanning Cytometry

One of the potential mechanisms of peripheral tolerance is the unresponsiveness of T cells to secondary antigenic stimulation as a result of the induction of anergy. It has been widely reported that antigenic unresponsiveness may be due to uncoupling of MAPK signal transduction pathways. However, such signaling defects in anergic T cell populations have been mainly identified using immortalized T cell lines or T cell clones, which do not truly represent primary Ag-specific T cells. We have therefore attempted to quantify signaling events in murine primary Ag-specific T cells on an individual cell basis, using laser-scanning cytometry. We show that there are marked differences in the amplitude and cellular localization of phosphorylated ERK p42/p44 (ERK1/2) signals when naive, primed and anergic T cells are challenged with peptide-pulsed dendritic cells. Primed T cells display more rapid kinetics of phosphorylation and activation of ERK than naive T cells, whereas anergic T cells display a reduced ability to activate ERK1/2 upon challenge. In addition, the low levels of pERK found in anergic T cells are distributed diffusely throughout the cell, whereas in primed T cells, pERK appears to be targeted to the same regions of the cell as the TCR. These data suggest that the different consequences of Ag recognition by T cells are associated with distinctive kinetics, amplitude, and localization of MAPK signaling.

E3 ligases in T cell anergy--turning immune responses into tolerance

Peripheral tolerance is an important strategy used by the immune system to prevent self-reactive lymphocytes from attacking host tissues. A variety of mechanisms contribute to peripheral tolerance, among them activation-induced cell death, suppression by regulatory T cells, and T cell anergy or unresponsiveness. Recent work has led to a better understanding of the cell-intrinsic program that establishes T cell anergy. A major insight is that during the induction phase of anergy, incomplete stimulation (T cell receptor stimulation without costimulation) leads via calcium influx to an altered gene expression program that includes up-regulation of several E3 ubiquitin ligases. When the anergic T cells contact antigen-presenting cells, intracellular signaling proteins are monoubiquitinated and targeted for lysosomal degradation, thus decreasing intracellular signaling and also resulting in decreased stability of the T cell-antigen-presenting cell contact. We propose a molecular program leading to T cell anergy and discuss other proteins that may play a role.

The Cbl Family and Other Ubiquitin Ligases

Regulation of tyrosine kinase-mediated cellular activation through antigen receptors is of great biological and practical significance. The evolutionarily conserved Cbl family ubiquitin ligases have emerged as key negative regulators of activated tyrosine kinase-coupled receptors, and their impaired function switches a normal immune response into autoimmunity. Cbl proteins facilitate the ubiquitinylation of activated tyrosine kinases and other signaling proteins and of the signaling chains of receptors themselves; monoubiquitin tag promotes sorting of activated receptors and associated proteins into internal vesicles of the multivesicular body, facilitating their lysosomal degradation, whereas polyubiquitin tag promotes proteasomal degradation. Notably, increased expression of Cbl proteins and other ubiquitin ligases is a component of anergic signaling program in T cells. Thus, controlled destruction of the signaling apparatus has emerged as a key to fine-tuning antigen receptor signaling. Further studies of this pathway are likely to elucidate the pathogenesis of autoimmune diseases and offer new therapeutic targets.

Activation-induced polarized recycling targets T cell antigen receptors to the immunological synapse; involvement of SNARE complexes

The mechanism by which T cell antigen receptors (TCR) accumulate at the immunological synapse has not been fully elucidated. Since TCRs are continuously internalized and recycled back to the cell surface, we investigated the role of polarized recycling in TCR targeting to the immunological synapse. We show here that the recycling endosomal compartment of T cells encountering activatory antigen-presenting cells (APCs) polarizes towards the T cell-APC contact site. Moreover, TCRs in transit through recycling endosomes are targeted to the immunological synapse. Inhibition of T cell polarity, constitutive TCR endocytosis, or recycling reduces TCR accumulation at the immunological synapse. Conversely, increasing the amount of TCRs in recycling endosomes before synapse formation enhanced their accumulation. Finally, we show that exocytic t-SNAREs from T cells cluster at the APC contact site and that tetanus toxin inhibits TCR accumulation at the immunological synapse, indicating that vesicle fusion mediated by SNARE complexes is involved in TCR targeting to the immunological synapse.

Constitutive and Ligand-Induced TCR Degradation

Modulation of TCR expression levels is a central event during T cell development and activation, and it probably plays an important role in adjusting T cell responsiveness. Conflicting data have been published on down-regulation and degradation rates of the individual TCR subunits, and several divergent models for TCR down-regulation and degradation have been suggested. The aims of this study were to determine the rate constants for constitutive and ligand-induced TCR degradation and to determine whether the TCR subunits segregate or are processed as an intact unit during TCR down-regulation and degradation. We found that the TCR subunits in nonstimulated Jurkat cells were degraded with rate constants of approximately 0.0011 min(-1), resulting in a half-life of approximately 10.5 h. Triggering of the TCR by anti-TCR Abs resulted in a 3-fold increase in the degradation rate constants to approximately 0.0033 min(-1), resulting in a half-life of approximately 3.5 h. The subunits of the TCR complex were down-regulated from the cell surface and degraded with identical kinetics, and most likely remained associated during the passage throughout the endocytic pathway from the cell surface to the lysosomes. Similar results were obtained in studies of primary human Vbeta8+ T cells stimulated with superantigen. Based on these results, the simplest model for TCR internalization, sorting, and degradation is proposed.

Analysis of regulatory CD8 T cells in Qa-1-deficient mice

The mouse protein Qa-1 (HLA-E in humans) is essential for immunological protection and immune regulation. Although Qa-1 has been linked to CD8 T cell-dependent suppression, the physiological relevance of this observation is unclear. We generated mice deficient in Qa-1 to develop an understanding of this process. Qa-1-deficient mice develop exaggerated secondary CD4 responses to foreign and self peptides. Enhanced responses to proteolipid protein self peptide were associated with resistance of Qa-1-deficient CD4 T cells to Qa-1-restricted CD8 T suppressor activity and increased susceptibility to experimental autoimmune encephalomyelitis. These findings delineate a Qa-1-dependent T cell-T cell inhibitory interaction that prevents the pathogenic expansion of autoreactive CD4 T cell populations and consequent autoimmune disease.

Recruitment of the Actin-binding Protein HIP-55 to the Immunological Synapse Regulates T Cell Receptor Signaling and Endocytosis

Actin cytoskeleton dynamics critically regulate T cell activation. We found that the cytoplasmic adaptor HIP-55, a Src/Syk-kinases substrate and member of the drebrin/Abp1 family of actin-binding proteins, localized to the T cell-antigen-presenting cell (APC) contact site in an antigen-dependent manner. Using green fluorescent protein fusion proteins, both Src homology 3 (SH3) and actin binding domains were found necessary for recruitment at the T cell-APC interface. HIP-55 was not implicated in conjugate formation and actin polymerization but regulated distal signaling events through binding and activation of hematopoietic progenitor kinase 1 (HPK1), a germinal center kinase (GCK) family kinase involved in negative signaling in T cells. Using RNA interference and overexpression experiments, the HIP-55-HPK1 complex was found to negatively regulate nuclear factor of activated T cell (NFAT) activation by the T cell antigen receptor. Moreover, we show that HIP-55, which partly co-localized with early endocytic compartments, promoted both basal and ligand-dependent T cell receptor (TCR) down-modulation, resulting in a decreased TCR expression. SH3 and actin-depolymerizing factor homology domains were required for this function. As controls, the expression of CD28 and the glycosylphosphatidylinositol-linked protein CD59 was not affected by HIP-55 overexpression. These results suggest that, in addition to binding to HPK1, HIP-55 might negatively regulate TCR signaling through down-regulation of TCR expression. Our findings show that HIP-55 is a key novel component of the immunological synapse that modulates T cell activation by connecting actin cytoskeleton and TCRs to gene activation and endocytic processes.

Architectural Changes in the TCR:CD3 Complex Induced by MHC:Peptide Ligation

A hallmark of T cell activation is the ligation-induced down-modulation of the TCR:CD3 complex. However, little is known about the molecular events that drive this process. The CD3 zeta-chain has been shown to play a unique role in regulating the assembly, transport, and cell surface expression of the TCR:CD3 complex. In this study we have investigated the relationship between CD3zeta and the TCRalphabetaCD3epsilondeltagamma complex after ligation by MHC:peptide complexes. Our results show that there is a significant increase in free surface CD3zeta, which is not associated with the TCR:CD3 complex, after T cell stimulation. This may reflect dissociation of CD3zeta from the TCRalphabetaCD3epsilondeltagamma complex or transport of intracellular CD3zeta directly to the cell surface. We also show that MHC:peptide ligation also results in exposure of the TCR-associated CD3zeta NH2 terminus, which is ordinarily buried in the complex. These observations appears to be dependent on Src family protein tyrosine kinases, which are known to be critical for efficient T cell activation. These data suggest a mechanism by which ligated TCR may be differentiated from unligated TCR and selectively down-modulated.

Calcineurin imposes T cell unresponsiveness through targeted proteolysis of signaling proteins

Sustained calcium signaling induces a state of anergy or antigen unresponsiveness in T cells, mediated through calcineurin and the transcription factor NFAT. We show here that Ca(2+)-induced anergy is a multistep program that is implemented at least partly through proteolytic degradation of specific signaling proteins. Calcineurin increased mRNA and protein of the E3 ubiquitin ligases Itch, Cbl-b and GRAIL and induced expression of Tsg101, the ubiquitin-binding component of the ESCRT-1 endosomal sorting complex. Subsequent stimulation or homotypic cell adhesion promoted membrane translocation of Itch and the related protein Nedd4, resulting in degradation of two key signaling proteins, PKC-theta and PLC-gamma1. T cells from Itch- and Cbl-b-deficient mice were resistant to anergy induction. Anergic T cells showed impaired calcium mobilization after TCR triggering and were unable to maintain a mature immunological synapse, instead showing late disorganization of the outer ring containing lymphocyte function-associated antigen 1. Our results define a complex molecular program that links gene transcription induced by calcium and calcineurin to a paradoxical impairment of signal transduction in anergic T cells.

Surface T-cell antigen receptor expression and availability for long-term antigenic signaling

It is important to understand how T-cell antigen receptor (TCR) engagement and signaling are regulated throughout an immune response. This review examines the dynamics of surface TCR expression and signaling capacity during thymic and effector T-cell development. Although the TCR can undergo vast changes in surface expression, T cells remain capable of sustaining TCR engagement for long periods of time. This may be achieved by a combination of mechanisms that involve (a) controlling the quantity of surface TCR available for ligand interaction and (b) controlling the quality of surface TCR expression during T-cell activation. TCR signaling itself appears to be one of the main quantitative modulators of surface TCR expression, and it can cause both downregulation and upregulation at different times of T-cell activation. Recent studies indicate that the degree of upregulation is tunable by the strength of antigenic stimulation. There is evidence that qualitatively distinct forms of the TCR exist, and their potential role in sustained antigenic signaling is also discussed. A goal of future studies will be to better characterize these modulations in surface TCR expression and to clarify their impact on the regulation of immune responses.

Use of anti-CD3/CD28 mAb coupled magnetic beads permitting subsequent phenotypic analysis of activated human T cells by indirect immunofluorescence

Functional analysis of T lymphocytes requires in vitro stimulation of these cells under experimental conditions that mimic as closely as possible physiological in vivo stimulation and that involve antigen/T cell receptor (TCR)-mediated activation. Because of the low frequency of antigen-specific T cells in human clinical samples, stimulation with a combination of anti-CD3 and anti-CD28 monoclonal antibodies (mAbs) is a preferred method. Interaction of these mAbs with their ligand results in modulation of the mAb-ligand complex from the cell surface. However, as a result of incomplete modulation, CD3/CD28 mAb complexes often remain at the cell surface, thereby precluding subsequent indirect immunofluorescence and flow cytometry analysis using mouse immunoglobulin (Ig)-specific antibodies. This is of importance in situations in which no specific fluorochrome-conjugated mAbs are available, such as in screening procedures of Ig-containing hybridoma culture supernatants. We propose here the use of CD3/CD28 mAbs, linked to magnetic beads allowing standardization of the activation conditions, optimal activation of T cells and complete modulation of antigen-antibody complexes from the cell surface.

The Immunological Synapse Balances T Cell Receptor Signaling and Degradation

The immunological synapse is a specialized cell-cell junction between T cell and antigen-presenting cell surfaces. It is characterized by a central cluster of antigen receptors, a ring of integrin family adhesion molecules, and temporal stability over hours. The role of this specific organization in signaling for T cell activation has been controversial. We use in vitro and in silico experiments to determine that the immunological synapse acts as a type of adaptive controller that both boosts T cell receptor triggering and attenuates strong signals.

Regulation of membrane trafficking and subcellular organization of endocytic compartments revealed with FM1-43 in resting and activated human T cells

FM1-43, a fluorescent styryl dye that penetrates into and stains membranes, was used to investigate kinetics of constitutive endocytosis and to visualize the fate of endocytic organelles in resting and activated human T lymphocytes. The rate of dye accumulation was strongly temperature dependent and approximately 10-fold higher in activated than in resting T cells. Elevation of cytosolic free Ca2+ concentration with thapsigargin or ionomycin further accelerated the rate of FM1-43 accumulation associated with cytosolic actin polymerization. Direct modulation of actin polymerization affected membrane trafficking. Actin condensation beneath the plasma membrane with calyculin A abolished FM1-43 internalization, whereas actin depolymerization with cytochalasin D had no effect. Photoconversion of DAB by FM1-43 revealed altered endocytic compartment targeting associated with T cell activation. Internalized cargo was carried to lysosome-like compartments in resting T cells and to multivesicular bodies (MVB) in activated T cells. Externalization of exosomes from MVB occurred commonly in activated but not in resting T cells. T cell exosomes contained raft-associated CD3 proteins, GM1 glycosphingolipids, and phosphatidylserine at the outer membrane leaflet. The present study demonstrates the utility of FM1-43 as a marker of membrane trafficking in T cells and reveals possible mechanisms of its modulation during T cell activation.

Another View of T Cell Antigen Recognition: Cooperative Engagement of Glycolipid Antigens by Va14Ja18 Natural TCR

Va14Ja18 natural T (iNKT) cells rapidly elicit a robust effector response to different glycolipid Ags, with distinct functional outcomes. Biochemical parameters controlling iNKT cell function are partly defined. However, the impact of iNKT cell receptor beta-chain repertoire and how alpha-galactosylceramide (alpha-GalCer) analogues induce distinct functional responses have remained elusive. Using altered glycolipid ligands, we discovered that the Vb repertoire of iNKT cells impacts recognition and Ag avidity, and that stimulation with suboptimal avidity Ag results in preferential expansion of high-affinity iNKT cells. iNKT cell proliferation and cytokine secretion, which correlate with iNKT cell receptor down-regulation, are induced within narrow biochemical thresholds. Multimers of CD1d1-alphaGalCer- and alphaGalCer analogue-loaded complexes demonstrate cooperative engagement of the Va14Ja18 iNKT cell receptor whose structure and/or organization appear distinct from conventional alphabeta TCR. Our findings demonstrate that iNKT cell functions are controlled by affinity thresholds for glycolipid Ags and reveal a novel property of their Ag receptor apparatus that may have an important role in iNKT cell activation.

Sustained exposure to bacterial antigen induces interferon-γ-dependent T cell receptor ζ down-regulation and impaired T cell function

T cell antigen receptor zeta chain down-regulation and impaired in vitro T cell function have been described in cancer and autoimmune and infectious diseases. However, the immunological basis for this phenomenon is unknown. Sustained exposure to antigen and chronic systemic inflammation, factors shared by the various pathologies, might account for this phenomenon. We developed an in vivo experimental system that mimics these conditions and show that sustained exposure of mice to bacterial antigens was sufficient to induce T cell antigen receptor zeta chain down-regulation and impair T cell function, provided an interferon-gamma-dependent T helper type 1 immune response developed. This indicates zeta chain down-regulation could be a physiological response that attenuates an exacerbated immune response. However, it can act as a 'double-edged sword', impairing immune responses to chronic diseases.

TCR comodulation of nonengaged TCR takes place by a protein kinase C and CD3 gamma di-leucine-based motif-dependent mechanism

One of the earliest events following TCR triggering is TCR down-regulation. However, the mechanisms behind TCR down-regulation are still not fully known. Some studies have suggested that only directly triggered TCR are internalized, whereas others studies have indicated that, in addition to triggered receptors, nonengaged TCR are also internalized (comodulated). In this study, we used transfected T cells expressing two different TCR to analyze whether comodulation took place. We show that TCR triggering by anti-TCR mAb and peptide-MHC complexes clearly induced internalization of nonengaged TCR. By using a panel of mAb against the Ti beta chain, we demonstrate that the comodulation kinetics depended on the affinity of the ligand. Thus, high-affinity mAb (K(D) = 2.3 nM) induced a rapid but reversible comodulation, whereas low-affinity mAb (K(D) = 6200 nM) induced a slower but more permanent type of comodulation. Like internalization of engaged TCR, comodulation was dependent on protein tyrosine kinase activity. Finally, we found that in contrast to internalization of engaged TCR, comodulation was highly dependent on protein kinase C activity and the CD3 gamma di-leucine-based motif. Based on these observations, a physiological role of comodulation is proposed and the plausibility of the TCR serial triggering model is discussed.

Continuous T cell receptor signaling required for synapse maintenance and full effector potential

Although signals through the T cell receptor (TCR) are essential for the initiation of T helper cell activation, it is unclear what function such signals have during the prolonged T cell-antigen-presenting cell contact. Here we simultaneously tracked TCR-CD3 complex and phosphoinositide 3-kinase activity in single T cells using three-dimensional video microscopy. Despite rapid internalization of most of the TCR-CD3, TCR-dependent signaling was still evident up to 10 h after conjugate formation. Blocking this interaction caused dissolution of the synapse and proportional reductions in interleukin 2 production and cellular proliferation. Thus TCR signaling persists for hours, has a cumulative effect and is necessary for the maintenance of the immunological synapse.

Cooperative interactions of simian immunodeficiency virus Nef, AP-2, and CD3-zeta mediate the selective induction of T-cell receptor-CD3 endocytosis

The Nef proteins of human immunodeficiency virus and simian immunodeficiency virus (SIV) bind the AP-1 and AP-2 clathrin adaptors to downmodulate the expression of CD4 and CD28 by recruiting them to sites of AP-2 clathrin-dependent endocytosis. Additionally, SIV Nef directly binds the CD3-zeta subunit of the CD3 complex and downmodulates the T-cell receptor (TCR)-CD3 complex. We report here that SIV mac239 Nef induces the endocytosis of TCR-CD3 in Jurkat T cells. SIV Nef also induces the endocytosis of a chimeric CD8-CD3-zeta protein containing only the CD3-zeta cytoplasmic domain (8-zeta), in the absence of other CD3 subunits. Thus, the interaction of SIV Nef with CD3-zeta likely mediates the induction of TCR-CD3 endocytosis. In cells expressing SIV Nef and 8-zeta, both proteins colocalize with AP-2, indicating that Nef induces 8-zeta internalization via this pathway. Surprisingly, deletion of constitutively strong AP-2 binding determinants (CAIDs) in SIV Nef had little effect on its ability to induce TCR-CD3, or 8-zeta endocytosis, even though these determinants are required for the induction of CD4 and CD28 endocytosis via this pathway. Fluorescent microscopic analyses revealed that while neither the mutant SIV Nef protein nor 8-zeta colocalized with AP-2 when expressed independently, both proteins colocalized with AP-2 when coexpressed. In vitro binding studies using recombinant SIV Nef proteins lacking CAIDs and recombinant CD3-zeta cytoplasmic domain demonstrated that SIV Nef and CD3-zeta cooperate to bind AP-2 via a novel interaction. The fact that Nef uses distinct AP-2 interaction surfaces to recruit specific membrane receptors demonstrates how Nef independently selects distinct types of target receptors and recruits them to AP-2 for endocytosis.

Prevalence of T cell receptor zeta chain deficiency in systemic lupus erythematosus

T cells from patients with systemic lupus erythematosus (SLE) display antigen receptor-mediated signaling aberrations associated with defective T cell receptor (TCR) zeta chain expression. We determined the prevalence of TCR zeta chain deficiency in SLE from a large cohort of unselected racially diverse patients with different levels of clinical disease activity as determined by SLE Disease Activity Index (SLEDAI). Our data show that the occurrence of TCR zeta chain deficiency is 78% in SLE patients. There was no relationship between the deficiency of TCR zeta chain and the SLEDAI scores or theapy. TCR zeta chain deficiency was also not associated with age, race or gender and persisted over a 3 year follow-up period. Thus, there is a high prevalence of TCR zeta chain deficiency in SLE patients that is independent of disease activity, and persists over time indicating an important role for TCR zeta chain deficiency in SLE pathogenesis.

TCR dynamics in human mature T lymphocytes lacking CD3 gamma

The contribution of CD3gamma to the surface expression, internalization, and intracellular trafficking of the TCR/CD3 complex (TCR) has not been completely defined. However, CD3gamma is believed to be crucial for constitutive as well as for phorbol ester-induced internalization. We have explored TCR dynamics in resting and stimulated mature T lymphocytes derived from two unrelated human congenital CD3gamma-deficient (gamma(-)) individuals. In contrast to gamma(-) mutants of the human T cell line Jurkat, which were selected for their lack of membrane TCR and are therefore constitutively surface TCR negative, these natural gamma(-) T cells constitutively expressed surface TCR, mainly through biosynthesis of new chains other than CD3gamma. However, surface (but not intracellular) TCR expression in these cells was less than wild-type cells, and normal surface expression was clearly CD3gamma-dependent, as it was restored by retroviral transduction of CD3gamma. The reduced surface TCR expression was likely caused by an impaired assembly or membrane transport step during recycling, whereas constitutive internalization and degradation were apparently normal. Ab binding to the mutant TCR, but not phorbol ester treatment, caused its down-modulation from the cell surface, albeit at a slower rate than in normal controls. Kinetic confocal analysis indicated that early ligand-induced endocytosis was impaired. After its complete down-modulation, TCR re-expression was also delayed. The results suggest that CD3gamma contributes to, but is not absolutely required for, the regulation of TCR trafficking in resting and Ag-stimulated mature T lymphocytes. The results also indicate that TCR internalization is regulated differently in each case.

The diversity of immunological synapses

Immunological synapses (ISs) are specialised signalling domains characterised by complex molecular clustering and segregation at the contact site between cells of the immune system. T lymphocytes form different ISs depending on their state of activation and on the antigen-presenting cells with which they interact. The structural features of the various ISs are better established than the functions they carry out. Recent advances point to the importance of taking into account diversity in both the structures and the functions of IS.

Regulator T cells: specific for antigen and/or antigen receptors?

Adaptive immune responses are regulated by many different molecular and cellular effectors. Regulator T cells are coming to their rights again, and these T cells seem to have ordinary alpha/beta T-cell receptors (TCRs) and to develop in the thymus. Autoimmune responses are tightly regulated by such regulatory T cells, a phenomenon which is beneficial to the host in autoimmune situations. However, the regulation of autoimmune responses to tumour cells is harmful to the host, as this regulation delays the defence against the outgrowth of neoplastic cells. In the present review, we discuss whether regulatory T cells are specific for antigen and/or for antigen receptors. Our interest in these phenomena comes from the findings that T cells produce many more TCR-alpha and TCR-beta chains than are necessary for surface membrane expression of TCR-alphabeta heterodimers with CD3 complexes. Excess TCR chains are degraded by the proteasomes, and TCR peptides thus become available to the assembly pathway of major histocompatibility complex class I molecules. Consequently, do T cells express two different identification markers on the cell membrane, the TCR-alphabeta clonotype for recognition by B-cell receptors and clonotypic TCR-alphabeta peptides for recognition by T cells?

A role for the cytoplasmic tail of the pre-T cell receptor (TCR) alpha chain in promoting constitutive internalization and degradation of the pre-TCR

Engagement of the alpha beta T cell receptor (TCR) by its ligand results in the down-modulation of TCR cell surface expression, which is thought to be a central event in T cell activation. On the other hand, pre-TCR signaling is a key process in alpha beta T cell development, which appears to proceed in a constitutive and ligand-independent manner. Here, comparative analyses on the dynamics of pre-TCR and TCR cell surface expression show that unligated pre-TCR complexes expressed on human pre-T cells behave as engaged TCR complexes, i.e. they are rapidly internalized and degraded in lysosomes and proteasomes but do not recycle back to the cell surface. Thus, pre-TCR down-regulation takes place constitutively without the need for extracellular ligation. By using TCR alpha/p Tau alpha chain chimeras, we demonstrate that prevention of recycling and induction of degradation are unique pre-TCR properties conferred by the cytoplasmic domain of the pT alpha chain. Finally, we show that pre-TCR internalization is a protein kinase C-independent process that involves the combination of src kinase-dependent and -independent pathways. These data suggest that constitutive pre-TCR down-modulation regulates pre-TCR surface expression levels and hence the extent of ligand-independent signaling through the pre-TCR.

Forced expression of the Fc receptor gamma-chain renders human T cells hyperresponsive to TCR/CD3 stimulation

High level expression of Fc epsilon RI gamma chain replaces the deficient TCR zeta-chain and contributes to altered TCR/CD3-mediated signaling abnormalities in T cells of patients with systemic lupus erythematosus. Increased responsiveness to Ag has been considered to lead to autoimmunity. To test this concept, we studied early signaling events and IL-2 production in fresh cells transfected with a eukaryotic expression vector encoding the Fc epsilon RI gamma gene. We found that the overexpressed Fc epsilon RI gamma chain colocalizes with the CD3 epsilon chain on the surface membrane of T cells and that cross-linking of the new TCR/CD3 complex leads to a dramatic increase of intracytoplasmic calcium concentration, protein tyrosine phosphorylation, and IL-2 production. We observed that overexpression of Fc epsilon RI gamma is associated with increased phosphorylation of Syk kinase, while the endogenous TCR zeta-chain is down-regulated. We propose that altered composition of the CD3 complex leads to increased T cell responsiveness to TCR/CD3 stimulation and sets the biochemical grounds for the development of autoimmunity.

Human CD8 T cells of the peripheral blood contain a low CD8 expressing cytotoxic/effector subpopulation

Heterogeneity of lymphocyte populations demonstrates the diversity of cellular immune responses and provide a better understanding of the immune system. CD3+ CD8+ T cells exhibit a low CD8 expressing (CD8low) population in flow cytometric analysis of peripheral blood T cells. In healthy donors, this population consists of 0.2-7.0% of all CD8 T cells. The majority of the CD8low T cell population showed an elevated expression of CD25, CD45RA, and CD95L, and low levels of CD28, CD62L and CD45RO. Circulating CD8low T cells resemble cytotoxic effector cells because they express cytolytic mediators and are able to execute cytotoxicity. A restricted T cell receptor profile with increased Vbeta9, Vbeta14 and Vbeta23 expression was observed and the CD8low T cell population contain Epstein-Barr virus-specific T cells. Therefore, the CD8low population represent a subset of activated CD8 effector T cells, resulting most probably from a continuous and/or balanced immune response to intracellular pathogens.

Effector CD8+CD45RO-CD27-T cells have signalling defects in patients with squamous cell carcinoma of the head and neck

A subset of circulating T cells (CD8(+)CD45RO(-)CD27(-)) with a naïve phenotype, but mediating effector function, is considered to play an important role in host antitumour defence. To investigate the attributes of these effector T cells in patients with squamous cell carcinoma (SCC) of the head and neck cancer, venous blood was obtained from 39 individuals with cancer and 45 normal controls (NC). Peripheral blood mononuclear cells were isolated, stained with labelled monoclonal antibodies specific for CD8, CD45RO, CD45RA, CD62L, CD27, TCR-zeta as well as isotype controls and examined by multicolour flow cytometry. Annexin V binding to CD8(+) T cells and PMA/ionomycin-induced IFN-gamma expression were also evaluated in patients and NC. The proportions of CD45RA(+)CD45RO(-) (naïve) and CD45RA(-)CD45RO(+) (memory) cells were found to be comparable within the CD8(+) T-cell subset. However, relative to NC, the frequency of effector CD8(+)CD45RO(-)CD27(-) cells was strikingly increased in all SCC patients regardless of the disease status (P=0.0003). The proportion of these cells was found to increase with age in both patients and NC. In NC, stimulated IFN-gamma expression was largely restricted to CD8(+)CD45RO(-)CD27(+) cells, while in patients CD8(+)CD45RO(-)CD27(-) expressed IFN-gamma after ex vivo stimulation. Expression of the TCR-associated zeta chain was decreased or absent in freshly isolated CD8(+)CD45RO(-)CD27(-) T cells in patients (P<0.0001). Annexin V was found to bind to a higher proportion of circulating CD8(+) T cells in patients than NC (P<0.006), and significantly more Annexin V(+) T cells were present in the effector (P<0.0059) than the naïve subset within the CD8(+)CD45RO(-) compartment. The data indicate that the expanded CD8(+)CD45RO(-)CD27(-) T cells, which contain precursors of IFN-gamma-producing T cells, are zeta-negative and sensitive to apoptosis in the circulation of patients with HNC.

Defective expression of the T-cell receptor-CD3 zeta chain in T-cell acute lymphoblastic leukaemia

This study analysed the T-cell receptor (TCR)-CD3 zeta complex and the signal transduction apparatus of T-acute lymphoblastic leukaemia (T-ALL) blasts, and investigated the function of the ubiquitin-proteasome system. In all nine T-ALL samples studied, the leukaemic cells showed a marked reduction in the expression of the zeta chain, while a variety of tyrosine kinases (p56lck, ZAP70 and SYK) were normally present. There was no expression of the FcepsilonRIgamma chain. To confirm that this aberration was specific to immature T-ALL blasts, we investigated two patients with lymphoproliferative disorders of granular lymphocytes (LDGL), characterized by the expansion of mature T lymphocytes and found normal zeta chain expression. The reduction of the zeta chain protein was not reversible after 72 h stimulation with the anti-CD3 monoclonal antibody and interleukin 2, either alone or in combination. Northern blot analysis indicated that the reduced protein expression did not correspond to a defect at the mRNA level, nor were mutations in the coding region of the zeta chain found. We, therefore, hypothesized that the observed reduction of protein expression in T-ALL blasts could be secondary to an increased degradation at the proteasome level. Following selective inhibition of the proteasome, a marked increase of the zeta chain expression was observed. Moreover, an increase in the surface expression of CD3 was also documented. Taken together, these results indicate that the expression of the zeta subunit of the TCR-CD3 complex is consistently reduced in T-ALL blasts and that degradation of the protein is mediated by the proteasome system.

Regulation of constitutive TCR internalization by the zeta-chain

The ability of a T cell to be activated is critically regulated by the number of TCRs expressed on the plasma membrane. Cell surface TCR expression is influenced by dynamic processes such as synthesis and transport of newly assembled receptors, endocytosis of surface TCR, and recycling to the plasma membrane of internalized receptors. In this study, the internalization of fluorescently labeled anti-TCR Abs was used to analyze constitutive endocytosis of TCRs on T cells, and to investigate the role of the zeta-chain in this process. We found that cell surface TCRs lacking zeta were endocytosed more rapidly than completely assembled receptors, and that reexpression of full-length zeta led to a dose-dependent decrease in the rate of TCR internalization. Rapid TCR internalization was also observed with CD4(+)CD8(+) thymocytes from zeta-deficient mice, whereas TCR internalization on thymocytes from CD3-delta deficient animals was slow, similar to that of wild-type thymocytes. This identifies a specific role for zeta in the regulation of constitutive receptor internalization. Furthermore, chimeric zeta molecules containing non-native intracellular amino acid sequences also led to high levels of TCR expression and reduced TCR cycling. These effects were dependent solely on the length of the intracellular tail, ruling out a role for intracellular zeta-specific interactions with other molecules as a mechanism for regulating TCR internalization. Rather, these findings strongly support a model in which the zeta-chain stabilizes TCR residency on the cell surface, and functions to maintain cell surface receptor expression by sterically blocking internalization sequences in other TCR components.

c-Cbl and Cbl-b regulate T cell responsiveness by promoting ligand-induced TCR down-modulation

How Cbl family proteins regulate T cell responses is unclear. We found that c-Cbl Cbl-b double knock-out (dKO) T cells became hyperresponsive upon anti-CD3 stimulation, even though the major T cell antigen receptor (TCR) signaling pathways were not enhanced. The dKO T cells did not down-modulate surface TCR after ligand engagement, which resulted in sustained TCR signaling. However, these cells showed normal ligand-independent TCR internalization, and trafficking of internalized TCR to the lysosome compartment after ligand engagement was reduced. These findings show that Cbl family proteins negatively regulate T cell activation by promoting clearance of engaged TCR from the cell surface, a process that is apparently essential for the termination of TCR signals.

Tumour-derived soluble MIC ligands impair expression of NKG2D and T-cell activation

Engagement of the NKG2D receptor by tumour-associated ligands may promote tumour rejection by stimulating innate and adaptive lymphocyte responses. In humans, NKG2D is expressed on most natural killer cells, gammadelta T cells and CD8alphabeta T cells. Ligands of NKG2D include the major histocompatibility complex class I homologues MICA and MICB, which function as signals of cellular stress. These molecules are absent from most cells and tissues but can be induced by viral and bacterial infections and are frequently expressed in epithelial tumours. MIC engagement of NKG2D triggers natural killer cells and costimulates antigen-specific effector T cells. Here we show that binding of MIC induces endocytosis and degradation of NKG2D. Expression of NKG2D is reduced markedly on large numbers of tumour-infiltrating and matched peripheral blood T cells from individuals with cancer. This systemic deficiency is associated with circulating tumour-derived soluble MICA, causing the downregulation of NKG2D and in turn severe impairment of the responsiveness of tumour-antigen-specific effector T cells. This mode of T-cell silencing may promote tumour immune evasion and, by inference, compromise host resistance to infections.

Soluble peptide-MHC monomers cause activation of CD8+ T cells through transfer of the peptide to T cell MHC molecules

T cell receptor (TCR)-mediated activation of CD4(+) T cells is known to require multivalent engagement of the TCR by, for example, oligomeric peptide-MHC complexes. In contrast, for CD8(+) T cells, there is evidence for TCR-mediated activation by univalent engagement of the TCR. We have here compared oligomeric and monomeric L(d) and K(b) peptide-MHC complexes and free peptide as stimulators of CD8(+) T cells expressing the 2C TCR. We found that the monomers are indeed effective in activating naive and effector CD8(+) T cells, but through an unexpected mechanism that involves transfer of peptide from soluble monomers to T cell endogenous MHC (K(b)) molecules. The result is that T cells, acting as antigen-presenting cells, are able to activate other naive T cells.

The synapse assembly model

A framework for quantitative analysis of the mechanisms underlying immunological synapse assembly has been recently developed. This model uses partial differential equations to describe the binding interactions of receptors and ligands, with the constraint that they are embedded in apposed deformable membranes linked to a cytoskeletal complex.

Regulation of surface expression of the human pre-T cell receptor complex

Considerable progress has recently been made in defining the role that pre-antigen receptor complexes, namely the pre-T and pre-B cell receptors, play in lymphocyte development. It is now established that these receptors direct, in a similar way, the survival, expansion, clonality and further differentiation of pre-T and pre-B lymphocytes, respectively. However, less is known about the mechanisms which ensure that only minute amounts of pre-TCR and pre-BCR reach the plasma membrane of developing lymphocytes. In this review, we discuss the implications of recent experimental approaches which address the developmental regulation of human pre-TCR expression and the molecular mechanisms that control surface pre-TCR expression levels.

Activated TCRs remain marked for internalization after dissociation from pMHC

To assess the roles of serial engagement and kinetic proofreading in T cell receptor (TCR) internalization, we have developed a mathematical model of this process. Our determination of TCR down-regulation for an array of TCR mutants, interpreted in the context of the model, has provided new information about peptide-induced TCR internalization. The amount of TCR down-regulation increases to a maximum value and then declines as a function of the half-life of the bond between the TCR and peptide-major histocompatibility complex (pMHC). The model shows that this behavior, which reflects competition between serial engagement and kinetic proofreading, arises only if it is postulated that activated TCRs remain marked for internalization after dissociation from pMHC. The model also predicts that because of kinetic proofreading, the range of TCR-pMHC-binding half-lives required for T cell activation depends on the concentrations and localization of intracellular signaling molecules. We show here that kinetic proofreading provides an explanation for the different requirements for activation observed in naïve and memory T cells.

TCR/CD3 down-modulation and zeta degradation are regulated by ZAP-70

TCR down-modulation following binding to MHC/peptide complexes is considered to be instrumental for T cell activation because it allows serial triggering of receptors and the desensitization of stimulated cells. We studied CD3/TCR down-modulation and zeta degradation in T cells from two ZAP-70-immunodeficient patients. We show that, at high occupancy of the TCR, down-modulation of the CD3/TCR is comparable whether T cells express or do not express ZAP-70. However, if TCR occupancy was low, we found that CD3/TCR was down-regulated to a lesser extent in ZAP-70-negative than in ZAP-70-positive T cells. We studied CD3/TCR down-modulation in P116 (a ZAP-70-negative Jurkat cell-derived clone) and in P116 transfected with genes encoding the wild-type or a kinase-dead form of ZAP-70. Down-modulation of the TCR at high occupancy did not require ZAP-70, whereas at low TCR occupancy down-modulation was markedly reduced in the absence of ZAP-70 and in cells expressing a dead kinase mutant of ZAP-70. Thus, the presence of ZAP-70 alone is not sufficient for down-modulation; the kinase activity of this molecule is also required. The degradation of zeta induced by TCR triggering is also severely impaired in T cells from ZAP-70-deficient patients, P116 cells, and P116 cells expressing a kinase-dead form of ZAP-70. This defect in TCR-induced zeta degradation is observed at low and high levels of TCR occupancy. Our results identify ZAP-70, a tyrosine kinase known to be crucial for T cell activation, as a key player in TCR down-modulation and zeta degradation.

Down-regulation of the CD3-zeta chain in sentinel node biopsies from breast cancer patients

BACKGROUND

In several neoplastic diseases, immunosuppression has been shown to correlate with disease stage, progression, and outcome. As the prognosis for metastatic breast cancer is still pessimistic, additional strategies are being sought to improve survival. Local immunosuppression in sentinel node biopsies from 24 evaluable breast cancer patients was studied as a possible way of selecting patients for immunotherapy.

METHOD

Sentinel node biopsy was performed in 24 out of 25 women operated on for primary breast cancer (one was not evaluable). Specimens were snap-frozen and double-stained for the zeta-chain of the T-cell receptor. The degree of down-regulation of the zeta-chain was evaluated in three different lymph-node areas: primary follicles, secondary follicles, and paracortex.

RESULTS

Down-regulation of varying degrees was noted in all 24 sentinel node biopsies. A high degree of down-regulation (more than 50% of T-cells not expressing zeta-chain) was seen in the primary follicles in six patients (25%), in the secondary follicles in 13 patients (72%), and in the paracortex in 19 patients (79%).

CONCLUSION

Local down-regulation of an immune function parameter was seen in sentinel node biopsies from breast cancer patients. In addition to possible prognostic implications, the sentinel node might be an appropriate location for detecting early-stage immunological down-regulation, which might open a possibility of selecting patients who could benefit from immunotherapy.

A therapeutic CD4 monoclonal antibody inhibits TCR-zeta chain phosphorylation, zeta-associated protein of 70-kDa Tyr319 phosphorylation, and TCR internalization in primary human T cells

The molecular mechanisms mediating the inhibitory effects of a humanized CD4 mAb YHB.46 on primary human CD4(+) T cells were investigated. Preincubation of T cells with soluble YHB.46 caused a general inhibition of TCR-stimulated protein tyrosine phosphorylation events, including a reduction in phosphorylation of p95(vav), linker for activation of T cells, and Src homology 2 domain-containing leukocyte protein of 76-kDa signaling molecules. A marked reduction in activation of the Ras/mitogen-activated protein kinase pathway was also observed. Examination of the earliest initiation events of TCR signal transduction showed that YHB.46 inhibited TCR-zeta chain phosphorylation together with recruitment and tyrosine phosphorylation of the zeta-associated protein of 70-kDa tyrosine kinase, particularly at Tyr(319), as well as reduced recruitment of p56(lck) to the TCR-zeta and zeta-associated protein of 70-kDa complex. These inhibitory events were associated with inhibition of TCR endocytosis. Our results show that the YHB.46 mAb is a powerful inhibitor of the early initiating events of TCR signal transduction.

Constitutive endocytosis and degradation of the pre-T cell receptor

The pre-T cell receptor (TCR) signals constitutively in the absence of putative ligands on thymic stroma and signal transduction correlates with translocation of the pre-TCR into glycolipid-enriched microdomains (rafts) in the plasma membrane. Here, we show that the pre-TCR is constitutively routed to lysosomes after reaching the cell surface. The cell-autonomous down-regulation of the pre-TCR requires activation of the src-like kinase p56(lck), actin polymerization, and dynamin. Constitutive signaling and degradation represents a feature of the pre-TCR because the gammadeltaTCR expressed in the same cell line does not exhibit these features. This is also evident by the observation that the protein adaptor/ubiquitin ligase c-Cbl is phosphorylated and selectively translocated into rafts in pre-TCR- but not gammadeltaTCR-expressing cells. A role of c-Cbl-mediated ubiquitination in pre-TCR degradation is supported by the reduction of degradation through pharmacological inhibition of the proteasome and through a dominant-negative c-Cbl ubiquitin ligase as well as by increased pre-TCR surface expression on immature thymocytes in c-Cbl-deficient mice. The pre-TCR internalization contributes significantly to the low surface level of the receptor on developing T cells, and may in fact be a requirement for optimal pre-TCR function.

Regulation of T cell receptor CD3zeta chain expression by L-arginine

L-Arg plays a central role in the normal function of several organ systems including the immune system. L-Arg can be depleted by arginase I produced by macrophages and hepatocytes in several disease states such as trauma and sepsis and following liver transplantation. The decrease in L-Arg levels induces a profound decrease in T cell function through mechanisms that have remained unclear. The data presented here demonstrate that Jurkat T cells cultured in medium without L-Arg (L-Arg-free RPMI) have a rapid decrease in the expression of the T cell antigen receptor zeta chain (CD3zeta), the principal signal transduction element in this receptor, and a decrease in T cell proliferation. This phenomenon is completely reversed by the replenishment of L-Arg but not other amino acids. These changes are not caused by cell apoptosis; instead, the diminished expression of CD3zeta protein is paralleled by a decrease in CD3zeta mRNA. This change in CD3zeta mRNA expression is not caused by a decrease in the transcription rate but rather by a significantly shorter CD3zeta mRNA half-life. This mechanism is sensitive to cycloheximide. Therefore, the regulation of L-Arg concentration in the microenvironment could represent an important mechanism to modulate the expression of CD3zeta and the T cell receptor and consequently of T cell function.

Ligand-induced TCR down-regulation is not dependent on constitutive TCR cycling

TCR internalization takes place both in resting T cells as part of constitutive TCR cycling, after PKC activation, and during TCR triggering. It is still a matter of debate whether these pathways represent distinct pathways. Thus, some studies have indicated that ligand-induced TCR internalization is regulated by mechanisms distinct from those involved in constitutive internalization, whereas other studies have suggested that the ligand-induced TCR internalization pathway is identical with the constitutive pathway. To resolve this question, we first identified requirements for constitutive TCR cycling. We found that in contrast to PKC-induced TCR internalization where both CD3gamma-S(126) and the CD3gamma leucine-based internalization motif are required, constitutive TCR cycling required neither PKC nor CD3gamma-S(126) but only the CD3gamma leucine-based motif. Having identified these requirements, we next studied ligand-induced internalization in cells with abolished constitutive TCR cycling. We found that ligand-induced TCR internalization was not dependent on constitutive TCR internalization. Likewise, constitutive internalization and recycling of the TCR were independent of an intact ligand-induced internalization of the TCR. In conclusion, ligand-induced TCR internalization and constitutive cycling of the TCR represents two independent pathways regulated by different mechanisms.

TCR activation of human T cells induces the production of exosomes bearing the TCR/CD3/zeta complex

We show in this study that human T cells purified from peripheral blood, T cell clones, and Jurkat T cells release microvesicles in the culture medium. These microvesicles have a diameter of 50-100 nm, are delimited by a lipidic bilayer membrane, and bear TCR beta, CD3epsilon, and zeta. This microvesicle production is regulated because it is highly increased upon TCR activation, whereas another mitogenic signal, such as PMA and ionomycin, does not induce any release. T cell-derived microvesicles also contain the tetraspan protein CD63, suggesting that they originate from endocytic compartments. They contain adhesion molecules such as CD2 and LFA-1, MHC class I and class II, and the chemokine receptor CXCR4. These transmembrane proteins are selectively sorted in microvesicles because CD28 and CD45, which are highly expressed at the plasma membrane, are not found. The presence of phosphorylated zeta in these microvesicles suggests that the CD3/TCR found in the microvesicles come from the pool of complexes that have been activated. Proteins of the transduction machinery, tyrosine kinases of the Src family, and c-Cbl are also observed in the T cell-derived microvesicles. Our data demonstrate that T lymphocytes produce, upon TCR triggering, vesicles whose morphology and phenotype are reminiscent of vesicles of endocytic origin produced by many cell types and called exosomes. Although the exact content of T cell-derived exosomes remains to be determined, we suggest that the presence of TCR/CD3 at their surface makes them powerful vehicles to specifically deliver signals to cells bearing the right combination of peptide/MHC complexes.

Intercellular transfer of antigen-presenting cell determinants onto T cells: molecular mechanisms and biological significance

Upon physiological stimulation, receptors with tyrosine kinase activity (RTK) are rapidly internalized together with their soluble ligands. T cell activation is the consequence of recognition by the T cell receptor (TCR) of specific peptide-major histocompatibility protein complexes (peptide-MHC) present at the membrane of antigen-presenting cells (APC). The TCR belongs to the RTK family and is known to be endocytosed upon ligand recognition. It differs from most other RTK in that its ligand, the peptide-MHC complex, is membrane bound and the TCR-ligand interaction is quite weak. Recent experiments have shown that the TCR ligand becomes internalized by T cells upon stimulation. Here we review current knowledge on the molecular mechanisms by which the membrane-bound MHC molecules can be transferred onto T cells, and propose hypotheses on the role this phenomenon could play in physio-pathological situations involving T cells.

Impact of antigen presentation on TCR modulation and cytokine release: implications for detection and sorting of antigen-specific CD8+ T cells using HLA-A2 wild-type or HLA-A2 mutant tetrameric complexes

Soluble MHC class I molecules loaded with antigenic peptides are available either to detect and to enumerate or, alternatively, to sort and expand MHC class I-restricted and peptide-reactive T cells. A defined number of MHC class I/peptide complexes can now be implemented to measure T cell responses induced upon Ag-specific stimulation, including CD3/CD8/zeta-chain down-regulation, pattern, and quantity of cytokine secretion. As a paradigm, we analyzed the reactivity of a Melan-A/MART-1-specific and HLA-A2-restricted CD8(+) T cell clone to either soluble or solid-phase presented peptides, including the naturally processed and presented Melan-A/MART-1 peptide AAGIGILTV or the peptide analog ELAGIGILTV presented either by the HLA-A2 wild-type (wt) or mutant (alanineright arrowvaline aa 245) MHC class I molecule, which reduces engagement of the CD8 molecule with the HLA-A2 heavy chain. Soluble MHC class I complexes were used as either monomeric or tetrameric complexes. Soluble monomeric MHC class I complexes, loaded with the Melan-A/MART-1 peptide, resulted in CD3/CD8 and TCR zeta-chain down-regulation, but did not induce measurable cytokine release. In general, differences pertaining to CD3/CD8/zeta-chain regulation and cytokine release, including IL-2, IFN-gamma, and GM-CSF, were associated with 1) the format of Ag presentation (monomeric vs tetrameric MHC class I complexes), 2) wt vs mutant HLA-A2 molecules, and 3) the target Ag (wt vs analog peptide). These differences are to be considered if T cells are exposed to recombinant MHC class I Ags loaded with peptides implemented for detection, activation, or sorting of Ag-specific T cells.

Expression of the signal transduction molecule zeta in peripheral and tumour-associated lymphocytes in Hodgkin's disease in relation to the Epstein-Barr virus status of the tumour cells

We investigated whether the described immune evasion of Epstein-Barr virus (EBV)-infected malignant Hodgkin and Reed-Sternberg (HRS) cells in Hodgkin's disease (HD) is paralleled by a disturbed expression of the signal transduction molecule zeta associated with CD3 and CD16 in tumour-associated T lymphocytes (TAL). Flow cytometric analysis revealed a significantly lower zeta expression in CD3+/4+, CD3+/8+ and CD16+ patient peripheral blood lymphocytes (PBL; n = 10) compared with normal donor PBLs (n = 11). When patient PBLs were compared with the corresponding TAL, the latter showed a significantly higher (CD3+/4+) or equal (CD3+/8+) zeta expression. The EBV status of the tumours did not correlate with zeta expression in the TAL. Immunohistochemical staining revealed zeta-positive lymphocytes among the adjacent bystander cells of the HRS cells in all analysed tumours (n = 8), irrespective of tumour EBV status. In conclusion, these results do not support downregulation of zeta in TAL as a critical mechanism contributing specifically to the immune escape of EBV+ HRS cells.

T cell receptor signaling precedes immunological synapse formation

The area of contact between a T cell and an antigen-presenting cell (APC) is known as the immunological synapse. Although its exact function is unknown, one model suggests that it allows for T cell receptor (TCR) clustering and for sustained signaling in T cells for many hours. Here we demonstrate that TCR-mediated tyrosine kinase signaling in naïve T cells occurred primarily at the periphery of the synapse and was largely abated before mature immunological synapses had formed. These data suggest that many hours of TCR signaling are not required for T cell activation. These observations challenge current ideas about the role of immunological synapses in T cell activation.