Rho GTPases link cytoskeletal rearrangements and activation processes induced via the tetraspanin CD82 in T lymphocytes

Activation of T lymphocytes requires the engagement of the T-cell receptor and costimulation molecules through cell-to-cell contacts. The tetraspanin CD82 has previously been shown to act as a cytoskeleton-dependent costimulation molecule. We show here that CD82 engagement leads to the tyrosine phosphorylation and association of both the Rho GTPases guanosine exchange factor Vav1 and adapter protein SLP76, suggesting that Rho GTPases participate in CD82 signaling. Indeed, broad inactivation of all Rho GTPases, or a specific blockade of RhoA, Rac1 or Cdc42, inhibited the morphological changes linked to CD82 engagement but failed to modulate the inducible association of CD82 with the actin network. Rho GTPase inactivation, as well as actin depolymerization, reduced the ability of CD82 to phosphorylate Vav and SLP76 and to potentiate the phosphorylation of two early TcR signaling intermediates: the tyrosine kinases ZAP70 and membrane adapter LAT. Taken together, this suggests that an amplification loop, via early Vav and SLP76 phosphorylations and Rho-GTPases activation, is initiated by CD82 association with the cytoskeleton, which permits cytoskeletal rearrangements and costimulatory activity. Moreover, the involvement of CD82 in the formation of the immunological synapse is strongly suggested by its accumulation at the site of TcR engagement. This novel link between a tetraspanin and the Rho GTPase cascade could explain why tetraspanins, which are known to form heterocomplexes, are involved in cell activation, adhesion, growth and metastasis.

How H13 histocompatibility peptides differing by a single methyl group and lacking conventional MHC binding anchor motifs determine self-nonself discrimination

The mouse H13 minor histocompatibility (H) Ag, originally detected as a barrier to allograft transplants, is remarkable in that rejection is a consequence of an extremely subtle interchange, P4(Val/Ile), in a nonamer H2-D(b)-bound peptide. Moreover, H13 peptides lack the canonical P5(Asn) central anchor residue normally considered important for forming a peptide/MHC complex. To understand how these noncanonical peptide pMHC complexes form physiologically active TCR ligands, crystal structures of allelic H13 pD(b) complexes and a P5(Asn) anchored pD(b) analog were solved to high resolution. The structures show that the basis of TCRs to distinguish self from nonself H13 peptides is their ability to distinguish a single solvent-exposed methyl group. In addition, the structures demonstrate that there is no need for H13 peptides to derive any stabilization from interactions within the central C pocket to generate fully functional pMHC complexes. These results provide a structural explanation for a classical non-MHC-encoded H Ag, and they call into question the requirement for contact between anchor residues and the major MHC binding pockets in vaccine design.

Differential effects of physiologically relevant hypoxic conditions on T lymphocyte development and effector functions

Direct measurements revealed low oxygen tensions (0.5-4.5% oxygen) in murine lymphoid organs in vivo. To test whether adaptation to changes in oxygen tension may have an effect on lymphocyte functions, T cell differentiation and functions at varying oxygen tensions were studied. These studies show: 1) differentiated CTL deliver Fas ligand- and perforin-dependent lethal hit equally well at all redox conditions; 2) CTL development is delayed at 2.5% oxygen as compared with 20% oxygen. Remarkably, development of CTL at 2.5% oxygen is more sustained and the CTL much more lytic; and 3) hypoxic exposure and TCR-mediated activation are additive in enhancing levels of hypoxia response element-containing gene products in lymphocyte supernatants. In contrast, hypoxia inhibited the accumulation of nonhypoxia response element-containing gene products (e.g., IL-2 and IFN-gamma) in the same cultures. This suggests that T cell activation in hypoxic conditions in vivo may lead to different patterns of lymphokine secretion and accumulation of cytokines (e.g., vascular endothelial growth factor) affecting endothelial cells and vascular permeabilization. Thus, although higher numbers of cells survive and are activated during 20% oxygen incubation in vitro, the CTL which develop at 2.5% oxygen are more lytic with higher levels of activation markers. It is concluded that the ambient 20% oxygen tension (plus 2-ME) is remarkably well suited for immunologic specificity and cytotoxicity studies, but oxygen dependence should be taken into account during the design and interpretation of results of in vitro T cell development assays and gene expression studies in vivo.

TCR dynamics on the surface of living T cells

T lymphocyte activation by specific antigen requires prolonged TCR occupancy and sustained signaling. This is accomplished by the formation of a specialized signaling domain, the immunological synapse, at the T cell-antigen-presenting cell contact site. Surface receptors and signaling components are progressively recruited into this domain where they are organized in defined three-dimensional structures. To better understand how TCR are supplied to the signaling domain during the activation process, we measured (using confocal microscopy and photo-bleaching recovery techniques) lateral mobility of GFP-tagged TCR on living Jurkat cell surface. We show that: (i) surface-expressed TCR exhibit an intrinsic, actin cytoskeleton-independent, lateral mobility which allows them to passively diffuse over the entire T cell surface within approximately 60 min and (ii) non-stimulated TCR rapidly enter the signaling domain. Our results indicate that TCR lateral mobility per se is sufficient to ensure TCR supply to the immunological synapse in the course of sustained T cell activation.

CD8beta endows CD8 with efficient coreceptor function by coupling T cell receptor/CD3 to raft-associated CD8/p56(lck) complexes

The extraordinary sensitivity of CD8+ T cells to recognize antigen impinges to a large extent on the coreceptor CD8. While several studies have shown that the CD8beta chain endows CD8 with efficient coreceptor function, the molecular basis for this is enigmatic. Here we report that cell-associated CD8alphabeta, but not CD8alphaalpha or soluble CD8alphabeta, substantially increases the avidity of T cell receptor (TCR)-ligand binding. To elucidate how the cytoplasmic and transmembrane portions of CD8beta endow CD8 with efficient coreceptor function, we examined T1.4 T cell hybridomas transfected with various CD8beta constructs. T1.4 hybridomas recognize a photoreactive Plasmodium berghei circumsporozoite (PbCS) peptide derivative (PbCS (4-azidobezoic acid [ABA])) in the context of H-2K(d), and permit assessment of TCR-ligand binding by TCR photoaffinity labeling. We find that the cytoplasmic portion of CD8beta, mainly due to its palmitoylation, mediates partitioning of CD8 in lipid rafts, where it efficiently associates with p56(lck). In addition, the cytoplasmic portion of CD8beta mediates constitutive association of CD8 with TCR/CD3. The resulting TCR-CD8 adducts exhibit high affinity for major histocompatibility complex (MHC)-peptide. Importantly, because CD8alphabeta partitions in rafts, its interaction with TCR/CD3 promotes raft association of TCR/CD3. Engagement of these TCR/CD3-CD8/lck adducts by multimeric MHC-peptide induces activation of p56(lck) in rafts, which in turn phosphorylates CD3 and initiates T cell activation.

Early thymocyte development is regulated by modulation of E2A protein activity

The E2A gene encodes the E47 and E12 basic helix-loop-helix (bHLH) transcription factors. T cell development in E2A-deficient mice is partially arrested before lineage commitment. Here we demonstrate that E47 expression becomes uniformly high at the point at which thymocytes begin to commit towards the T cell lineage. E47 protein levels remain high until the double positive developmental stage, at which point they drop to relatively moderate levels, and are further downregulated upon transition to the single positive stage. However, stimuli that mimic pre-T cell receptor (TCR) signaling in committed T cell precursors inhibit E47 DNA-binding activity and induce the bHLH inhibitor Id3 through a mitogen-activated protein kinase kinase-dependent pathway. Consistent with these observations, a deficiency in E2A proteins completely abrogates the developmental block observed in mice with defects in TCR rearrangement. Thus E2A proteins are necessary for both initiating T cell differentiation and inhibiting development in the absence of pre-TCR expression. Mechanistically, these data link pre-TCR mediated signaling and E2A downstream target genes into a common pathway.

Importance of the T cell receptor alpha-chain transmembrane distal region for assembly with cognate subunits

Antigen recognition by alphabeta T lymphocytes is mediated via the multisubunit TCR complex consisting of invariant CD3gamma,delta,epsilon and zeta chains associated with clonotypic TCRalpha and beta molecules. Charged amino acids located centrally within the TCRalpha transmembrane region are necessary and sufficient for assembly with the CD3deltaepsilon heterodimer. Previously, we have shown that deletion of 6-12 amino acids from the carboxy terminus of the TCRalpha-chain dramatically abrogates surface TCR expression, suggesting that the distal portion of the TCRalpha transmembrane region contains information that regulates the assembly and/or intracellular transport of TCR complexes. We have examined in more detail the molecular basis for reduced TCR expression in T cells bearing truncated TCRalpha chains. We found that in contrast to wild-type (wt), variant TCRalpha proteins missing the last nine C-terminal amino acids did not associate with core CD3gamma,delta,epsilon chains and were not assembled into disulphide-linked alphabeta heterodimers. The stability of newly synthesised wt and variant TCRalpha molecules was similar, showing that the abrogated surface TCR expression was not a consequence of impaired protein survival. Nevertheless, truncated TCRalpha chains still assembled with the chaperon protein calnexin in the endoplasmic reticulum, indicating that the distal portion of the TCRalpha transmembrane region is not essential for calnexin interaction. These data document a role for the distal portion of the TCRalpha transmembrane region in the assembly of TCR complexes and provide a molecular basis for reduced TCR expression in cells bearing truncated TCRalpha chains.

Novel CD28-responsive enhancer activated by CREB/ATF and AP-1 families in the human interleukin-2 receptor alpha-chain locus

The interaction of interleukin-2 (IL-2) with its receptor (IL-2R) critically regulates the T-cell immune response, and the alpha chain CD25/IL-2Ralpha is required for the formation of the high-affinity receptor. Tissue-specific, inducible expression of the IL-2Ralpha gene is regulated by at least three positive regulatory regions (PRRI, PRRII, and PRRIII), but none responded to CD28 engagement in gene reporter assays although CD28 costimulation strongly amplifies IL-2Ralpha gene transcription. By DNase I hypersensitivity analysis, we have identified a novel TCR-CD3- and CD28-responsive enhancer (CD28rE) located 8.5 kb 5' of the IL-2Ralpha gene. PRRIV/CD28rE contains a functional CRE/TRE element required for CD28 signaling. The T-cell-specific, CD28-responsive expression of the IL-2Ralpha gene appears controlled through PRRIV/CD28rE by cooperation of CREB/ATF and AP-1 family transcription factors.

Adenosine deaminase deficiency increases thymic apoptosis and causes defective T cell receptor signaling

Adenosine deaminase (ADA) deficiency in humans results in a severe combined immunodeficiency (SCID). This immunodeficiency is associated with severe disturbances in purine metabolism that are thought to mediate lymphotoxicity. The recent generation of ADA-deficient (ADA(-/-)) mice has enabled the in vivo examination of mechanisms that may underlie the SCID resulting from ADA deficiency. We demonstrate severe depletion of T and B lymphocytes and defects in T and B cell development in ADA(-/-) mice. T cell apoptosis was abundant in thymi of ADA(-/-) mice, but no increase in apoptosis was detected in the spleen and lymph nodes of these animals, suggesting that the defect is specific to developing thymocytes. Studies of mature T cells recovered from spleens of ADA(-/-) mice revealed that ADA deficiency is accompanied by TCR activation defects of T cells in vivo. Furthermore, ex vivo experiments on ADA(-/-) T cells demonstrated that elevated adenosine is responsible for this abnormal TCR signaling. These findings suggest that the metabolic disturbances seen in ADA(-/-) mice affect various signaling pathways that regulate thymocyte survival and function. Experiments with thymocytes ex vivo confirmed that ADA deficiency reduces tyrosine phosphorylation of TCR-associated signaling molecules and blocks TCR-triggered calcium increases.

Production of TNF-alpha by human V gamma 9V delta 2 T cells via engagement of Fc gamma RIIIA, the low affinity type 3 receptor for the Fc portion of IgG, expressed upon TCR activation by nonpeptidic antigen

Human lymphocytes expressing the gammadelta TCR represent a minor T cell subpopulation found in blood. The majority of these cells express Vgamma9Vdelta2 determinants and respond to nonpeptidic phosphoantigens. Several studies have shown that, in vivo, the percentage of Vgamma9Vdelta2 T cells dramatically increases during pathological infection, leading to the hypothesis that they play an important role in the defense against pathogens. However, the specific mechanisms involved in this response remain poorly understood. It has been established that Vgamma9Vdelta2 T cells display potent cytotoxic activity against virus-infected and tumor cells, thereby resembling NK cells. In this study, we show that, upon stimulation by nonpeptidic Ags, Vgamma9Vdelta2 T cells express FcgammaRIIIA (CD16), a receptor that is constitutively expressed on NK cells. CD16 appears to be an activation Ag for Vgamma9Vdelta2 T cells. Indeed, ligation of CD16 on Vgamma9Vdelta2 T cells leads to TNF-alpha production. This TNF-alpha production, which is dependent (like that induced via the TCR-CD3 complex) on the activation of the p38 and extracellular signal-regulated kinase-2 mitogen-activated protein kinases, can be modulated by CD94 NK receptors. Therefore, it appears that Vgamma9Vdelta2 T cells can be physiologically activated by two sequential steps via two different cell surface Ags: the TCR-CD3 complex and the FcgammaRIIIA receptor, which are specific cell surface Ags for T lymphocytes and NK cells, respectively. This strongly suggests that, in the general scheme of the immune response, Vgamma9Vdelta2 T cells represent an important subpopulation of cells that play a key role in the defense against invading pathogens.

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.

Molecular modelling and endoplasmic reticulum retention of mutated TCR/CD3 complexes

T cell receptor (TCR)/CD3 complex assembly takes place in the endoplasmic reticulum (ER). Normal TCR/CD3 complexes egress from the ER to the cis-Golgi, where the interaction with zeta2 homodimers occurs. This interaction leads to further uncontrolled transport of TCR/CD3/zeta molecules to the cell surface. The purpose of the present experiments was to determine firstly the basis for the impact of the Phe195/216 --> Val mutations on TCR/CD3 expression in Jurkat cells, and secondly why mutated J79-cell TCRalphabeta/CD3 hexamers are prevented from interacting with zeta2 homodimers. We found that Phe --> Val mutations cause serious perturbations in a so far undefined hydrophobic area formed by the two Phe195/216 on beta-strand F and aromatic/large hydrophobic amino acids on neighboring beta-strands B and A in Calpha and Cbeta domains, respectively. In addition, TCR/CD3 hexamers and zeta2 homodimers colocalize in normal Jurkat T cells, in revertant J79r58 cells, and in J79 cells transfected with wild-type TCRalpha cDNA but not in J79 mutant cells (confocal microscopy). Furthermore, mutated TCR/CD3 complexes seem to be actively retained in the ER in J79 cells but not in revertant J79r58 cells by a nondominant mechanism. We propose that a hitherto undefined ER-retention molecule controls both the protein structure and egress of TCR/CD3 complexes from the ER of alphabeta and gammadelta T cells.

Early TCR αβ Expression Promotes Maturation of T Cells Expressing FcεRIγ Containing TCR/CD3 Complexes

In a previous study we presented data indicating that the expanded population of CD4(-)CD8(-) (DN) alphabeta T cells in TCRalpha-chain-transgenic mice was partially if not entirely derived from gammadelta T cell lineage cells. The development of both gammadelta T cells and DN alphabeta T cells is poorly understood; therefore, we thought it would be important to identify the immediate precursors of the transgene-induced DN alphabeta T cells. We have in this report studied the early T cell development in these mice and we show that the transgenic TCRalpha-chain is expressed by precursor thymocytes already at the CD3(-)CD4(-)CD8(-) (triple negative, TN) CD44(+)CD25(-) stage of development. Both by using purified precursor populations in reconstitution experiments and by analyzing fetal thymocyte development, we demonstrated that early TN precursors expressing endogenous TCRbeta-chains matured into DN alphabeta T cells at several stages of development. The genes encoding the gamma-chain of the high affinity receptor for IgE (FcepsilonRIgamma) and the CD3zeta protein were found to be reciprocally expressed in TN thymocytes such that during development the FcepsilonRIgamma expression decreased whereas CD3zeta expression increased. Furthermore, in a fraction of the transgene-induced DN alphabeta T cells the FcepsilonRIgamma protein colocalized with the TCR/CD3 complex. These data suggest that similarly to gammadelta T cells and NKT cells, precursors expressing the TCR early in the common alphabetagammadelta developmental pathway may use the FcepsilonRIgamma protein as a signaling component of the TCR/CD3 complex.

Endoplasmic reticulum (ER)-associated degradation of T cell receptor subunits. Involvement of ER-associated ubiquitin-conjugating enzymes (E2s)

Degradation of proteins from the endoplasmic reticulum is fundamental to quality control within the secretory pathway, serves as a way of regulating levels of crucial proteins, and is utilized by viruses to enhance pathogenesis. In yeast two ubiquitin-conjugating enzymes (E2s), UBC6p and UBC7p are implicated in this process. We now report the characterization of murine homologs of these E2s. MmUBC6 is an integral membrane protein that is anchored via its hydrophobic C-terminal tail to the endoplasmic reticulum. MmUBC7, which is not an integral membrane protein, shows significant endoplasmic reticulum colocalization with MmUBC6. Overexpression of catalytically inactive MmUBC7 significantly delayed degradation from the endoplasmic reticulum of two T cell antigen receptor subunits, alpha and CD3-delta, and suggests a role for the ubiquitin conjugating system at the initiation of retrograde movement from the endoplasmic reticulum. These findings also implicate, for the first time, a specific E2 in degradation from the endoplasmic reticulum in mammalian cells.

Prolonged exposure of T cells to TNF down-regulates TCR zeta and expression of the TCR/CD3 complex at the cell surface

A role for TNF-alpha in the pathogenesis of chronic inflammatory disease is now firmly established. Paradoxically, TNF also has potent immunomodulatory effects on CD4(+) T lymphocytes, because Ag-specific proliferative and cytokine responses are suppressed following prolonged exposure to TNF. We explored whether TNF attenuated T cell activation by uncoupling proximal TCR signal transduction pathways using a mouse T cell hybridoma model. Chronic TNF exposure induced profound, but reversible, T cell hyporesponsiveness, with TNF-treated T cells requiring TCR engagement with higher peptide concentrations for longer periods of time for commitment to IL-2 production. Subsequent experiments revealed that chronic TNF exposure led to a reversible loss of TCRzeta chain expression, in part through a reduction in gene transcription. Down-regulation of TCRzeta expression impaired TCR/CD3 assembly and expression at the cell surface and uncoupled membrane-proximal tyrosine phosphorylation events, including phosphorylation of the TCRzeta chain itself, CD3epsilon, ZAP-70 protein tyrosine kinase, and linker for activation of T cells (LAT). Intracellular Ca(2+) mobilization was also suppressed in TNF-treated T cells. We propose that TNF may contribute to T cell hyporesponsiveness in chronic inflammatory and infectious diseases by mechanisms that include down-regulation of TCRzeta expression. We speculate that by uncoupling proximal TCR signals TNF could also interrupt mechanisms of peripheral tolerance that are dependent upon intact TCR signal transduction pathways.

Role of diacylglycerol kinase alpha in the attenuation of receptor signaling

Diacylglycerol kinase (DGK) is suggested to attenuate diacylglycerol-induced cell responses through the phosphorylation of this second messenger to phosphatidic acid. Here, we show that DGKalpha, an isoform highly expressed in T lymphocytes, translocates from cytosol to the plasma membrane in response to two different receptors known to elicit T cell activation responses: an ectopically expressed muscarinic type I receptor and the endogenous T cell receptor. Translocation in response to receptor stimulation is rapid, transient, and requires calcium and tyrosine kinase activation. DGKalpha-mediated phosphatidic acid generation allows dissociation of the enzyme from the plasma membrane and return to the cytosol, as demonstrated using a pharmacological inhibitor and a catalytically inactive version of the enzyme. The NH(2)-terminal domain of the protein is shown to be responsible for receptor-induced translocation and phosphatidic acid-mediated membrane dissociation. After examining induction of the T cell activation marker CD69 in cells expressing a constitutively active form of the enzyme, we present evidence of the negative regulation that DGKalpha exerts on diacylglycerol-derived cell responses. This study is the first to describe DGKalpha as an integral component of the signaling cascades that link plasma membrane receptors to nuclear responses.

A redundant role of the CD3 gamma-immunoreceptor tyrosine-based activation motif in mature T cell function

At least four different CD3 polypeptide chains are contained within the mature TCR complex, each encompassing one (CD3gamma, CD3delta, and CD3epsilon) or three (CD3zeta) immunoreceptor tyrosine-based activation motifs (ITAMs) within their cytoplasmic domains. Why so many ITAMs are required is unresolved: it has been speculated that the different ITAMs function in signal specification, but they may also serve in signal amplification. Because the CD3zeta chains do not contribute unique signaling functions to the TCR, and because the ITAMs of the CD3-gammadeltaepsilon module alone can endow the TCR with normal signaling capacity, it thus becomes important to examine how the CD3gamma-, delta-, and epsilon-ITAMs regulate TCR signaling. We here report on the role of the CD3gamma chain and the CD3gamma-ITAM in peripheral T cell activation and differentiation to effector function. All T cell responses were reduced or abrogated in T cells derived from CD3gamma null-mutant mice, probably because of decreased expression levels of the mature TCR complex lacking CD3gamma. Consistent with this explanation, T cell responses proceed undisturbed in the absence of a functional CD3gamma-ITAM. Loss of integrity of the CD3gamma-ITAM only slightly impaired the regulation of expression of activation markers, suggesting a quantitative contribution of the CD3gamma-ITAM in this process. Nevertheless, the induction of an in vivo T cell response in influenza A virus-infected CD3gamma-ITAM-deficient mice proceeds normally. Therefore, if ITAMs can function in signal specification, it is likely that either the CD3delta and/or the CD3epsilon chains endow the TCR with qualitatively unique signaling functions.

Controlled cell deformation produces defined areas of contact between cells and ligand-coated surfaces

A method which allows precise control of the time of initiation and the area of contact of T cells with immobilized ligands has been developed. Cells are trapped in an asymmetric film that can be quantitatively thinned by reducing the film's capillary pressure. Ligands adsorbed to the base of the apparatus are forced into close contact with the cells as the air-liquid interface is drawn down. Using interference microscopy and microbeads to indicate the film height, the amount of thinning can be controlled to within 1 microm. In this study, this system was used to produce contact areas of 182 and 356 microm2 between T cells and anti-CD3 coated surfaces. These contact areas were measured using fluorescent dye exclusion microscopy. This apparatus can be used for quantitative studies of T cell activation, as is reported in Patrick et al., J. Immunol. Method. 24:97-108, 2000.

T cells in mice expressing a transgenic human TCR beta chain get positively selected but cannot be activated in the periphery by signaling through TCR

TCR-CD3 complex-mediated signaling is crucial for both developmental selection and antigenic activation of T cells. We report that mice expressing a recombined human TCRbeta chain (Tg), which have normal development of T cells, mounted very weak responses to immunization with protein antigens as well as the HA307-319 peptide recognized by the human T cell clone HA1.7 from which the transgene is derived. An anti-CD3epsilon mAb triggered equivalent proliferation from Tg and non-Tg T cells, but an anti-human TCRbeta mAb induced proliferation poorly in Tg T cells in contrast to human T cells or HA1.7. In Tg mice, T cells expressing endogenous TCR were CD44(high), whereas most transgene-expressing T cells remained CD44(low), suggesting that transgene-expressing cells are not activated in the periphery to participate in immune responses. However, anti-human TCRbeta could induce some activation markers on T cells and cross-linking of the Tg TCR by plate-coated anti-human TCRbeta efficiently induced T cell proliferation. Human TCRbeta-mediated Tg T cell activation could be rescued by exogenous IL-2, as well as by the calcium ionophore A23187, but not by phorbol esters. Thus, this human TCRbeta chain functions efficiently for positive selection of mouse T cells, but not for their peripheral activation, probably because of a lack of oligomerization leading to defects in signaling for calcium flux and IL-2 induction. The data thus suggest an early point of separation of signaling pathways between positive selection and peripheral activation of T cells.

Tec kinase signaling in T cells is regulated by phosphatidylinositol 3-kinase and the Tec pleckstrin homology domain

Tec, the prototypical member of the Tec family of tyrosine kinases, is abundantly expressed in T cells and other hemopoietic cell types. Although the functions of Itk and Txk have recently been investigated, little is known about the role of Tec in T cells. Using antisense oligonucleotide treatment to deplete Tec protein from primary T cells, we demonstrate that Tec plays a role in TCR signaling leading to IL-2 gene induction. Interestingly, Tec kinases are the only known family of tyrosine kinases containing a pleckstrin homology (PH) domain. Using several PH domain mutants overexpressed in Jurkat T cells, we show that the Tec PH domain is required for Tec-mediated IL-2 gene induction and TCR-mediated Tec tyrosine phosphorylation. Furthermore, we show that Tec colocalizes with the TCR after TCR cross-linking, and that both the Tec PH and Src homology (SH) 2 domains play a role in this association. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, abolishes Tec-mediated IL-2 gene induction and Tec tyrosine phosphorylation, and partially suppresses Tec colocalization with the activated TCR. Thus, our data implicate the Tec kinase PH domain and phosphatidylinositol 3-kinase in Tec signaling downstream of the TCR.

The quantity of TCR signal determines positive selection and lineage commitment of T cells

It is generally accepted that the avidity of TCR for self Ag/MHC determines the fate of immature thymocytes. However, the contribution of the quantity of TCR signal to T cell selection has not been well established, particularly in vivo. To address this issue, we analyzed DO-TCR transgenic CD3zeta-deficient (DO-Tg/zetaKO) mice in which T cells have a reduced TCR on the cell surface. In DO-Tg/zetaKO mice, very few CD4 single positive (SP) thymocytes developed, indicating that the decrease in TCR signaling resulted in a failure of positive selection of DO-Tg thymocytes. Administration of the peptide Ag to DO-Tg/zetaKO mice resulted in the generation of functional CD4 SP mature thymocytes in a dose-dependent manner, and, unexpectedly, DO-Tg CD8 SP cells emerged at lower doses of Ag. TCR signal-dependent, sequential commitment from CD8(+) SP to CD4(+) SP was also shown in a class I-restricted TCR-Tg system. These in vivo analyses demonstrate that the quantity of TCR signal directly determines positive and negative selection, and further suggest that weak signal directs positively selected T cells to CD8 lineage and stronger signal to CD4 lineage.

Increase of NK-T cells in aged depressed patients not treated with antidepressive drugs

BACKGROUND

A change in number and/or activity of natural killer cells has repeatedly been reported in depressive illness. Much less attention has yet been given to the subgroup of natural killer cells that are positive for the T-cell marker CD3 (NK-T cells). These cells possibly have important immunoregulatory properties.

METHODS

We compared number and percentage of NK-T cells (defined as CD3(+) and CD16(+) and/or CD56(+) by two-color flow cytometry) in the peripheral blood of control subjects and two groups of elderly depressive subjects using or not using antidepressive drugs.

RESULTS

The number and percentage of NK-T cells were strongly elevated in elderly depressive subjects not using antidepressive drugs, as compared with control subjects and elderly depressive subjects using antidepressive drugs.

CONCLUSIONS

Depressive illness in a geriatric population is associated with a substantial increase of NK-T cells. This increase was absent in a depressive group using antidepressive drugs.

Enhanced antigen-specific antitumor immunity with altered peptide ligands that stabilize the MHC-peptide-TCR complex

T cell responsiveness to an epitope is affected both by its affinity for the presenting MHC molecule and the affinity of the MHC-peptide complex for TCR. One limitation of cancer immunotherapy is that natural tumor antigens elicit relatively weak T cell responses, in part because high-affinity T cells are rendered tolerant to these antigens. We report here that amino acid substitutions in a natural MHC class I-restricted tumor antigen that increase the stability of the MHC-peptide-TCR complex are significantly more potent as tumor vaccines. The improved immunity results from enhanced in vivo expansion of T cells specific for the natural tumor epitope. These results indicate peptides that stabilize the MHC-peptide-TCR complex may provide superior antitumor immunity through enhanced stimulation of specific T cells.

Interelationship between CD3 and CD28 pathways in a murine T cell thymoma

It is well known that the CD28 costimulatory signal is important to complement T cell receptor (TCR)/CD3-initiated T cell activation, but the mechanism by which these two distinct signaling pathways are integrated is not clearly understood. In our laboratory, we dispose of a murine T cell hybridoma transfected with human CD28 molecule which is able to produce IL-2 in response to stimulation, suggesting that the signal transduction machinery coupled to the CD28 molecule is capable of triggering effector functions. Nevertheless, the action of three immunosuppressive agents previously shown in our model, suggested an interaction between the CD3 and CD28 pathways. We confirmed here this hypothesis by transfecting the cDNA of the human CD28 molecule in the BW5147 thymoma which lacks CD3 surface expression. Stimulation of the human CD28 did not lead to IL-2 secretion while the restoration of the TCR/CD3 complex re-established the functionality of this costimulatory molecule. These data demonstrate that the IL-2 production induced by the CD28 activation pathway is dependent of the TCR/CD3 complex cell surface expression and suggest the formation of a functional membrane complex between the CD3 and CD28 molecules. The molecular basis of the functional dependence of CD28 signaling on the TCR/CD3 complex is presently unknown. Nonetheless, we showed that some early events induced by CD28 stimulation, such as PI3-kinase association, are independent of the TCR/CD3 complex expression.