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.

TCR/CD3-Induced activation and binding of Emt/Itk to linker of activated T cell complexes: requirement for the Src homology 2 domain

Expressed in mast and T cells/inducible T cell tyrosine kinase (Emt/Itk), a Tec family protein tyrosine kinase, is critical for the development and activation of T lymphocytes. The mechanism through which Emt/Itk mediates its effector functions is poorly understood. In this study, we show that the Emt/Itk Src homology 2 (SH2) domain is critical for the transphosphorylation and activation of Emt/Itk catalytic activity that is mediated by TCR/CD3 engagement. Furthermore, we find that the Emt/Itk SH2 domain is essential for the formation of TCR/CD3-inducible Emt/Itk-LAT complexes, whereas the SH3 domain and catalytic activity are not required. The Emt/Itk-linker of activated T cells (LAT) complexes are biologically important because Jurkat T cells with deficient LAT expression (JCaM2) fail to increase Emt/Itk tyrosine phosphorylation upon TCR/CD3 stimulation. Confocal microscopy reveals that in activated cells, LAT complexes colocalize with TCR/CD3. The present data suggest that upon TCR/CD3 engagement, the Emt/Itk SH2 domain mediates the formation of a molecular complex containing Emt/Itk, LAT, and TCR/CD3; this complex is essential for Emt/Itk activation and function.

Alteration of protein composition in mouse thymocytes by signals through T-cell receptor

To avoid destructive autoimmunity, T-cell precursors (thymocytes) expressing autoreactive T-cell receptor are deleted in the thymus via an apoptotic process by the signals from the T-cell receptor-CD3 complexes. In order to analyze the apoptotic mechanism, we established a cell-free system using the lysates from mouse thymocytes treated in vivo with anti-CD3 monoclonal antibody (mAb). The soluble cytosolic high molecular mass protein fraction from the anti-CD3-treated thymocytes revealed an activity that directly induces nuclear apoptotic morphological changes and DNA fragmentation. This fragmentation activity was not observed in the fraction from the thymocytes without anti-CD3 treatment. Proteins in both fractions were separated by two-dimensional electrophoresis. The silver-stained gels revealed differences in protein spots. These protein spots were identified by database searching of mass spectrometric (MS) and tandem mass spectrometric (MS/MS) data obtained from in-gel tryptic digests of the spots, using an integrated system of liquid chromatography/electrospray ionization/ion-trap mass spectrometry. In this study, the high mobility group protein HMG2 was identified as one of the cytosolic proteins that is increased by the signals from the T-cell receptor, and heterogeneous nuclear ribonucleoprotein A2/B1 and glyceraldehyde 3-phosphate dehydrogenase were found to be decreased by the signals.

Down-regulation of the T cell receptor CD3 zeta chain in rheumatoid arthritis (RA) and its influence on T cell responsiveness

T cells implicated in chronic inflammatory diseases such as RA respond weakly when stimulated in vitro with mitogen or antigen. The mechanism behind this hyporesponsiveness is unclear, but a depressed expression of the T cell receptor (TCR)-associated CD3zeta chain has been suggested. In the present work we describe a low expression of CD3zeta in synovial fluid (SF) T cells from RA patients compared with peripheral blood (PB) T cells, but no difference in CD3zeta expression between RA and healthy control PB T cells. In vitro studies demonstrated that granulocytes but not SF macrophages are able to down-regulate the expression of CD3zeta. Through stimulation with anti-CD3 antibodies we demonstrated that the TCR-dependent proliferative response was decreased in SF T cells compared with PB T cells. Stimulation with phorbol ester and ionomycin also resulted in a low proliferative response of SF T cells, indicating that both signal transduction through the TCR (stimulation with anti-CD3) and events further downstream in the signalling pathways (stimulation with phorbol ester and ionomycin) are affected. A similar depression of T cell activity was observed when induction of IL-2 and IL-4 was measured. However, SF T cells were not defective in the induction of interferon-gamma (IFN-gamma) when stimulated with phorbol myristate acetate (PMA)/ionomycin, in contrast to the diminished IFN-gamma response observed after stimulation with anti-CD3. This indicates that the hyporesponsiveness of SF T cells can not be generalized to all T cell functions. The differential response to external stimuli is likely to be of importance for the capacity of SF T cells to influence inflammatory reactions.

Regulated association between the tyrosine kinase Emt/Itk/Tsk and phospholipase-C gamma 1 in human T lymphocytes

The Emt/Itk/Tsk tyrosine kinase is involved in intracellular signaling events induced by several lymphocyte surface receptors. Modulation of TCR/CD3-induced phospholipase-C gamma 1 (PLC gamma 1) activity by the tyrosine kinase Emt/Itk/Tsk has been demonstrated based on studies of Itk-deficient murine T lymphocytes. Here we report a TCR/CD3-regulated association between Emt and PLC gamma 1 in both normal and leukemic T cells. In addition, this association was enhanced following independent ligation of the CD2, CD4, or CD28 costimulatory molecules, but not of CD5 or CD6 surface receptors, correlating to the induced tyrosine phosphorylation of Emt. Before Ab-induced T cell activation, we found that the Emt-SH3 domain was crucial for the constitutive Emt/PLC gamma 1 association; however, upon TCR/CD3 engagement, the Emt-SH2 domain was more efficient in mediating the enhanced Emt/PLC gamma 1 interaction. Furthermore, the PLC gamma 1-SH3 domain, but not the two PLC gamma 1-SH2 domains, contributed to formation of the protein complex. Thus, we describe a regulated interaction between Emt and PLC gamma 1, and based on our studies with individual Emt and PLC gamma 1 SH2/SH3 domains, we propose a mechanism for this association.

Age-related impairments in TCR/CD3 activation of ZAP-70 are associated with reduced tyrosine phosphorylations of zeta-chains and p59fyn/p56lck in human T cells

The expression and catalytic activity of the protein tyrosine kinase (PTK) ZAP-70 are needed for normal intracellular signaling through the T-cell receptor (TCR)/CD3 complex. However, the possible effect of aging on the catalytic activity of ZAP-70 in human peripheral blood T cells stimulated via the TCR/CD3 complex is unknown. The current studies show that T cells from a substantial proportion of elderly humans (12) exhibit significant reductions in the catalytic activity, but not expression of ZAP-70 when stimulated by ligation of the TCR/CD3 with cross-linked anti-CD3epsilon monoclonal antibody OKT3. In addition, the reduced catalytic activity of ZAP-70 in T cells from elderly subjects was not restored to the normal levels in response to ligation of CD4 receptors, suggesting defects in PTKs linked to both CD3 and CD4 receptors. Other experiments demonstrated that the age-related impairments of ZAP-70 activation in anti-CD3-stimulated T cells were accompanied by decreased tyrosine phosphorylations of zeta-chains and autophosphorylations of the PTKs p561ck/p59fyn. Moreover, the age-related defects in these early TCR/CD3-mediated phosphorylation events were readily detectable in both CD45RO+ memory and CD45RA+ naive T cells. Thus, these results suggest that defects in early TCR/CD3-mediated phosphorylation events among CD45RO+ memory and CD45RA+ naive T cells from certain elderly humans may con tribute to impaired induction of ZAP-70 catalytic activity.

[The role of vav protein in TCR-mediated signaling with MHC/peptide complexes leading to positive or negative selection of thymocytes]

On the basis of recent reports we discuss the role of Vav in TCR-dependent signaling pathways. The Vav protein is GDP/GTP exchange factor for Rac, which initiates transduction of signals in JNK pathway. Upon stimulation of TCR by antigenic peptides, Vav associates with Zap-70 in TCR/CD3 signaling complex and becomes phosphorylated on Tyr-174 by tyrosine kinase Lck. The function of Vav is modulated by substrates and products of PI3-kinase activated by interaction of CD28 on thymocytes with B7 on antigen presenting cells. The PI3-kinase substrates inhibit activation of Vav, while the products enhance phosphorylation and activation of Vav by Lck. It seems that Vav functions in key point of TCR-mediated signaling pathway, which is regulated by costimulatory molecule (CD28) necessary for negative selection. The Vav-mediated integration of signals results in positive or negative selection of thymocytes.

Molecular mechanisms of T lymphocyte activation: convergence of T cell antigen receptor and IL-1 receptor-induced signaling at the level of IL-2 gene transcription

Co-stimulation of murine EL-4 thymoma cells-carrying high numbers of TCR and type I IL-1 receptors (IL-1R)-with anti-CD3 antibodies and IL-1 resulted in synergistic enhancement of IL-2 synthesis. While the extracellular signal-regulated kinase (ERK) cascade was activated by both receptors, IL-1 preferentially stimulated Jun-N-terminal kinases (JNK) and p38 mitogen-activated kinase or microtubule-associated protein kinase (MAPK). Interruption of TCR- or IL-1R-stimulated ERK cascade by PD-98059, a specific inhibitor of MAP/ERK kinase (MEK), resulted in partial suppression of nuclear factor of activated T cells activation and in complete inhibition of IL-1-stimulated NFkappaB activation. Suppression of activation of both MEK and p38 MAPK resulted in significant inhibition of IL-2 gene expression. The results show that maximal activation of the IL-2 gene requires activation of at least two different protein kinase cascades, i.e. of the ERK and p38 pathways but presumably also that of JNK which converge at the level of the IL-2 promoter resulting in enhancement of its transcriptional activity.

The MHC class II ligand lymphocyte activation gene-3 is co-distributed with CD8 and CD3-TCR molecules after their engagement by mAb or peptide-MHC class I complexes

Previous studies indicated that signaling through lymphocyte activation gene-3 (LAG-3), a MHC class II ligand, induced by multivalent anti-receptor antibodies led to unresponsiveness to TCR stimulation. Here, lateral distribution of the LAG-3 molecules and its topological relationship (mutual proximity) to the TCR, CD8, CD4, and MHC class I and II molecules were studied in the plasma membrane of activated human T cells in co-capping experiments and conventional fluorescence microscopy. Following TCR engagement by either TCR-specific mAb or MHC-peptide complex recognition in T-B cell conjugates, LAG-3 was found to be specifically associated with the CD3-TCR complex. Similarly, following CD8 engagement LAG-3 and CD8 were co-distributed on the cell surface while only a low percentage of CD4-capped cells displayed LAG-3 co-caps. In addition, LAG-3 was found to be associated with MHC class II (i.e. DR, DP and DQ) and partially with MHC class I molecules. The supramolecular assemblies described here between LAG-3, CD3, CD8 and MHC class II molecules may result from an organization in raft microdomains, a phenomenon known to regulate early events of T cell activation.

Antigen-induced TCR-CD3 down-modulation does not require CD3delta or CD3gamma cytoplasmic domains, necessary in response to anti-CD3 antibody

We studied cytotoxic T lymphocyte (CTL) clones expressing cytoplasmic domain-deleted CD3delta and CD3gamma chains. These cells retained efficient antigen-specific cytolysis. Because the cytoplasmic domains of native CD3delta and CD3gamma chains contain a dileucine-based and a tyrosine-based motif thought to be important for receptor endocytosis, we compared TCR-CD3 down-modulation on the CTL clones expressing or not these domains. We found that antigen-induced TCR-CD3 down-modulation was not dependent on either the CD3delta or CD3gamma cytoplasmic domains. This contrasts with phorbol ester- and anti-CD3 mAb (soluble or plastic-coated)-induced TCR-CD3 down-modulation, that are respectively dependent on CD3gamma and on either CD3delta or CD3gamma cytoplasmic domains, suggesting that differences may exist between the mechanisms of TCR-CD3 down-modulation in response to the three stimuli. TCR-CD3 down-modulation in response to antigen was demonstrated by confocal microscopy to be associated with TCRbeta chain internalization, whether CD3delta and CD3gamma were native or truncated. Inhibition by the protein tyrosine kinase inhibitor PP1 of TCR-CD3 down-modulation in response to antigen was also similar whether CD3delta and CD3gamma cytoplasmic domains were present or not. These properties of receptor down-modulation are discussed with respect to the requirements for TCR engagement on antigen-presenting cells.

[Morphological study of CD3+ dendritic epidermal T cells in human skin and scar]

OBJECTIVE

To investigate the CD3/TCR-bearing T cell in the epidermis and explore the influence of the dendritic epidermal T cells on regeneration and differentiation of epidermal cells.

METHODS

The CD3/TCR-bearing T cells in epidermis of human skin and cicatrix were examined by means of histomorphology and immunohistochemistry.

RESULTS

It was found that the density of CD3/TCR-bearing T cells in human skin and scar tissue had great dissimilarity. CD3+ dendritic epidermal T cells increased in hyperplastic scar. Such result was not observed in atrophic scar.

CONCLUSION

Our findings indicate CD3+ dendritic epidermal T cells regulate the generation and differentiation of epidermal cells. CD3+ dendritic epidermal T cells may play an important role in the maintenance of the morphology of epidermal tissue.

Identification and functional characterization of the zeta-chain dimerization motif for TCR surface expression

We recognized a common dimerization motif between the transmembrane (TM) domain of zeta-chain family members and glycophorin A. We have shown that a glycine within the zeta-dimerization motif is critical for zeta-homodimerization and also for its association with the TCR/CD3 complex. Similarly, two residues within the CD3 delta gamma TM domains have proven to be critical for their interaction with the zeta-homodimer. A three-dimensional homology model of the zeta-chain TM domain highlights potential residues preferentially involved either in the zeta 2-CD3 or zeta 2-TCR alpha beta association, confirming our experimental findings. These results indicate that, for symmetrical reasons, the zeta-homodimer participates in the TCR/CD3 complex assembly by interacting with CD3 gamma delta TM domains, thereby masking their degradation signals located in the cytoplasmic tails.

Signals transduced by CD3epsilon, but not by surface pre-TCR complexes, are able to induce maturation of an early thymic lymphoma in vitro

Development of immature CD4-CD8- (double-negative) thymocytes to the CD4+CD8+ (double-positive) stage is linked to productive rearrangement of the TCRbeta locus by signals transduced through the pre-TCR. However, the mechanism whereby pre-TCR signaling is initiated remains unclear, in part due to the lack of an in vitro model system amenable to both biochemical and genetic analysis. In this study, we establish the thymic lymphoma Scid.adh as such a model system. Scid.adh responds to Ab engagement of surface IL-2Ra (TAC):CD3epsilon molecules (a signaling chimera that mimics pre-TCR signaling in vivo) by undergoing changes in gene expression observed following pre-TCR activation in normal thymocytes. These changes include down-regulation of CD25, recombinase-activating gene (RAG)-1, RAG-2, and pTalpha; and the up-regulation of TCRalpha germline transcripts. We term this complete set of changes in gene expression, in vitro maturation. Interestingly, Scid.adh undergoes only a subset of these changes in gene expression following Ab engagement of the pre-TCR. Our findings make two important points. First, because TAC:CD3epsilon stimulation of Scid.adh induces physiologically relevant changes in gene expression, Scid.adh is an excellent cellular system for investigating the molecular requirements for pre-TCR signaling. Second, Ab engagement of CD3epsilon signaling domains in isolation (TAC:CD3epsilon) promotes in vitro maturation of Scid.adh, whereas engagement of CD3epsilon molecules contained within the complete pre-TCR fails to do so. Our current working hypothesis is that CD3epsilon fails to promote in vitro maturation when in the context of an Ab-engaged pre-TCR because another pre-TCR subunit(s), possibly TCRzeta, qualitatively alters the CD3epsilon signal.

Costimulation of transduced T lymphocytes via T cell receptor-CD3 complex and CD28 leads to increased transcription of integrated retrovirus

Primary human T lymphocytes were transduced at high efficiency with the Moloney murine leukemia virus (Mo-MuLV) vector, LNC-mB7-1, in which an internal cytomegalovirus (CMV) promoter drives expression of the murine B7-1 cDNA. Compared with transduced T cells expanded in IL-2 or reactivated with soluble antibodies to CD3 or CD28, transgene expression was significantly increased after activation on immobilized anti-CD3 antibodies (CD3i) or by simultaneous activation on immobilized anti-CD3 and anti-CD28 antibodies (CD3i/CD28i). A similar pattern of transgene expression was observed in T cells transduced with Mo-MuLV LNC-EGFP. Proviral copy number was maintained in LNC-mB7-1-transduced T cells expanded in IL-2 or reactivated on CD3i/CD28i. Substantial increases in LNC-mB7-1 steady state mRNA in reactivated T lymphocytes, compared with those maintained in IL-2, correlated with increased transcription of the LNC-mB7-1 proviral DNA. Furthermore, T cells transduced with the Mo-MuLV ZIPPGK-mADA, in which the mADA cDNA is driven by an internal human phosphoglycerate kinase (PGK) promoter, showed increases in steady state ZIPPGK-mADA RNA on reactivation. High levels of transgene expression were evident irrespective of cell cycle position in both CD4+ and CD8+ lymphocytes. After reactivation, increases in LNC-mB7-1 mRNA were observed in the presence of the protein synthesis inhibitor cycloheximide, indicating that proteins involved in upregulating transgene expression preexisted in transduced lymphocytes. Induction of transgene expression on CD3i/CD28i showed a dose-dependent decrease in transgene expression when incubated with selective protein kinase inhibitors. These data provide new insights into the mechanisms governing transgene expression driven by Mo-MuLV constructs containing internal promoters in transduced primary T lymphocytes.

Requirement for Shc in TCR-mediated activation of a T cell hybridoma

Engagement of the TCR determines the fate of T cells to activate their functional programs, proliferate, or undergo apoptosis. The intracellular signal transduction pathways that dictate the specific outcome of receptor engagement have only been partially elucidated. The adapter protein, Shc, is involved in cytokine production, mitogenesis, transformation, and apoptosis in different cell systems. We found that Shc becomes phosphorylated on tyrosine residues upon stimulation of the TCR in DO11.10 hybridoma T cells; therefore, we investigated the role of Shc in activation-induced cell death in these cells by creating a series of stably transfected cell lines. Expression of Shc-SH2 (the SH2 domain of Shc) or Shc-Y239/240F (full-length Shc in which tyrosines 239 and 240 have been mutated to phenylalanine) resulted in the inhibition of activation-induced cell death and Fas ligand up-regulation after TCR cross-linking. Expression of wild-type Shc or Shc-Y317F had no significant effect. In addition, we found that Shc-SH2 and Shc-Y239/240F, but not Shc-Y317F, inhibited phosphorylation of extracellular signal-regulated protein kinase and production of IL-2 after TCR cross-linking. These results indicate an important role for Shc in the early signaling events that lead to activation-induced cell death and IL-2 production after TCR activation.

Virus-induced non-specific signals cause cell cycle progression of primed CD8(+) T cells but do not induce cell differentiation

In this report the significance of virus-induced non-specific T cell activation was re-evaluated using transgenic mice in which about half of the CD8(+) T cells expressed a TCR specific for amino acids 33-41 of lymphocytic choriomeningitis virus glycoprotein I. This allowed tracing of cells with known specificity and priming history in an environment also containing a normal heterogeneous CD8(+) population which served as an intrinsic control. Three parameters of T cell activation were analyzed: cell cycle progression, phenotypic conversion and cytolytic activity. Following injection of the IFN inducer poly(I:C), proliferation of memory (CD44(hi)) CD8(+) T cells but no phenotypic or functional activation was observed. Following injection of an unrelated virus [vesicular stomatitis virus (VSV)], naive TCR transgenic cells did not become significantly activated with respect to any of the parameters investigated. In contrast, memory TCR transgenic cells were found to proliferate extensively early after VSV infection (day 0-3), whereas limited proliferation was observed later (day 3-6) when proliferation of non-transgenic CD8(+) T cells is maximal. This aborted response did not result from anergy to TCR stimulation, as memory TCR transgenic cells proliferated vigorously upon stimulation with their nominal peptide. Despite the massive proliferation of memory cells observed early after VSV infection, no phenotypic or functional activation was observed. Together these findings indicate that both non-specific and antigen-specific signals contribute to the initial virus-induced proliferation of CD8(+) T cells, but for further proliferation and differentiation to take place, TCR-ligand interaction is required. The implications for maintenance of T cell memory is discussed.

Immobilization of glycosylphosphatidylinositol-anchored proteins inhibits T cell growth but not function

Accumulating evidence suggests that proteins tethered to the plasma membrane through glycosylphosphatidylinositol (GPI) anchors share common biological properties. In the present study we demonstrate that GPI-anchored proteins regulate T cell growth. Specifically, anti-TCR-induced proliferation was profoundly inhibited by co-immobilized mAb specific for Thy-1, CD48 and Ly6A/E. However, neither IL-2 production nor the effector function of cytotoxic T lymphocytes was impaired in these circumstances. Analysis of the IL-2 receptor (IL-2R) signaling pathway revealed that the association of IL-2R beta and gamma chains with the Janus kinases, JAK1 and JAK3, was not perturbed in the presence of mAb specific for GPI-linked proteins. However, in these conditions, IL-2-mediated recruitment of IL-2Ralpha, beta and gamma chains, resulting in the formation of the high-affinity hetero-trimeric IL-2R, was inhibited. The resulting phosphorylation of JAK1 and JAK3, indicative of their activation states, was correspondingly reduced. These results characterize a novel state of T cell physiology in which effector function is maintained, in the absence of clonal expansion. A physiological role for GPI-anchored proteins in the maintenance of cellular homeostasis and function is discussed.

Cytolytic T lymphocyte-associated antigen-4 and the TCR zeta/CD3 complex, but not CD28, interact with clathrin adaptor complexes AP-1 and AP-2

The negative signaling receptor cytolytic T lymphocyte-associated Ag-4 (CTLA-4) resides primarily in intracellular compartments such as the Golgi apparatus of T cells. However, little is known regarding the molecular mechanisms that influence this accumulation. In this study, we demonstrate binding of the clathrin adaptor complex AP-1 with the GVYVKM motif of the cytoplasmic domain of CTLA-4. Binding occurred primarily in the Golgi compartment of T cells, unlike with AP-2 binding that occurs mostly with cell surface CTLA-4. Although evidence was not found to implicate AP-1 binding in the retention of CTLA-4 in the Golgi, AP-1 appears to play a role in shuttling of excess receptor from the Golgi to the lysosomal compartments for degradation. In support of this, increased CTLA-4 synthesis resulted in an increase in CTLA-4/AP-1 binding and a concomitant increase in the appearance of CTLA-4 in the lysosomal compartment. At the same time, the level of intracellular receptor was maintained at a constant level, suggesting that CTLA-4/AP-1 binding represents one mechanism to ensure steady state levels of intracellular CTLA-4 in T cells. Finally, we demonstrate that the TCR zeta/CD3 complex (but not CD28) also binds to AP-1 and AP-2 complexes, thus providing a possible link between these two receptors in the regulation of T cell function.

Molecular mechanisms in the TCR (TCR alpha beta-CD3 delta epsilon, gamma epsilon) interaction with zeta 2 homodimers: clues from a 'phenotypic revertant' clone

The association between the TCRalphabeta-CD3gammaepsilondeltaepsilon hexamers and zeta2 homodimers in the endoplasmic reticulum (ER) constitutes a key step in TCR assembly and export to the T cell surface. Incompletely assembled TCR-CD3 complexes are degraded in the ER or the lysosomes. A previously described Jurkat variant (J79) has a mutation at position 195 on the TCR Calpha domain causing a phenylalanine to valine exchange. This results in a lack of association between TCRalphabeta-CD3gammaepsilondeltaepsilon hexamers and zeta2 homodimers. Two main hypotheses could explain this phenomenon in J79 cells: TCR-CD3 hexamers may be incapable of interacting with zeta2 due to a structural change in the TCR Calpha region; alternatively, TCR-CD3 hexamers may be incapable of interacting with zeta2 due to factors unrelated to either molecular complex. In order to assess these two possibilities, the TCR-CD3 membrane-negative J79 cells were treated with ethylmethylsulfonate and clones positive for TCR membrane expression were isolated. The characterization of the J79r58 phenotypic revertant cell line is the subject of this study. The main question was to assess the reason for the TCR re-expression. The TCR on J79r58 cells appears qualitatively and functionally equivalent to wild-type TCR complexes. Nucleotide sequence analysis confirmed the presence of the original mutation in the TCR Calpha region but failed to detect compensatory mutations in alpha, beta, gamma, delta, epsilon or zeta chains. Thus, mutated J79-TCR-CD3 complexes can interact with zeta2 homodimers. Possible mechanisms for the unsuccessful TCR-CD3 interaction with zeta2 homodimers are presented and discussed.

Type I interferons (IFNs) regulate tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) expression on human T cells: A novel mechanism for the antitumor effects of type I IFNs

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a proapoptotic member of the TNF family of type II membrane proteins, which constitutes one component of T cell cytotoxicity. In this study, we investigated the expression and function of TRAIL in human peripheral blood T (PBT) cells. Although freshly isolated PBT cells did not express a detectable level of TRAIL on their surface, a remarkable TRAIL expression was rapidly induced on the surface of both CD4(+) and CD8(+) PBT cells upon stimulation with anti-CD3 monoclonal antibody and type I interferons (IFNs). This enhancement of TRAIL expression was a unique feature of type I IFNs (IFN-alpha and IFN-beta), and neither type II IFN (IFN-gamma) nor various other cytokines enhanced TRAIL expression on anti-CD3-stimulated PBT cells. Type I IFNs have been used for clinical treatment of renal cell carcinomas (RCCs), and we found that most RCC cell lines were susceptible to TRAIL-induced apoptosis. Type I IFNs substantially augmented cytotoxic activity of anti-CD3-stimulated PBT cells against RCC cell lines in a TRAIL-dependent manner. These results indicate a unique feature of type I IFNs to regulate TRAIL-mediated T cell cytotoxicity, which may be involved in the antitumor effects of type I IFNs against various tumors.

TCRzeta is transported to and retained in the Golgi apparatus independently of other TCR chains: implications for TCR assembly

It is generally assumed that TCR assembly occurs in the endoplasmic reticulum (ER), and ER retention/degradation signals have been identified in several of the TCR chains. These signals are probably responsible for retention of incompletely assembled TCR complexes and free TCR chains in the ER. This study focused on the intracellular localization and transport of partially assembled TCR complexes as determined by confocal microscopy analyses. We found that none of the TCR chains except for TCRzeta were allowed to exit the ER in T cell variants in which the hexameric CD3gammaepsilonTi alphabetaCD3 deltaepsilon complex was not formed. Interestingly, TCRzeta was exported from the ER independently of other TCR chains and was predominantly located in a compartment identified as the Golgi apparatus. Furthermore, in the TCRzeta-negative cell line MA5.8, the hexameric CD3gammaepsilonTi alphabetaCD3 deltaepsilon complex was allowed to exit the ER and was also predominantly located in the Golgi apparatus. However, neither hexameric TCR complexes nor TCRzeta chains were efficiently expressed at the cell surface without the other. The observations that TCRzeta and hexameric TCR complexes are transported from the ER to the Golgi apparatus independently of each other and that these partial TCR complexes are unable to be efficiently expressed at the cell surface suggest that final TCR assembly occurs in the Golgi apparatus.