Essential role for ZAP-70 in both positive and negative selection of thymocytes

p38 MAP kinase activity modulates alpha beta T cell development

Reciprocal interactions between bone marrow derived precursor cells and the thymic environment lead to the generation of the complete repertoire of diverse and functional T cells. We have previously shown that p38 MAP kinase is activated in response to intrathymic signals in thymocytes. In this report we provide evidence that p38 MAP kinase activity is essential for pre-TCR-mediated transition of thymocytes from the CD4- CD8- to the CD4+ CD8+ stage of development. In the absence of p38 MAP kinase activity differentiation of alpha beta T cells, but not gamma delta T cells, was blocked.

Differential involvement of p38 MAP kinase pathway and Bax translocation in the mitochondria-mediated cell death in TCR- and dexamethasone-stimulated thymocytes

Mitochondria play a central role in many apoptotic reactions. Although mitochondrial apoptotic changes and caspase activation have been demonstrated in the apoptotic thymocytes, cell death signal through mitochondria in TCR-stimulated thymocytes has not been fully understood. In this study, we show that TCR stimulation induced disruption of mitochondrial transmembrane potential (Delta Psi(m)), the cytochrome c release from mitochondira, capase-3 activation, and the cell death of thymocytes. Bongkrekic acid, an inhibitor of Delta Psi(m) disruption, blocked the cytochrome c release from mitochondria and the following caspase-3-mediated cell death. Furthermore, a pro-apoptotic Bcl-2 family protein, Bax, but not Bad or Bid, was translocated from cytosol to mitochondria in TCR-stimulated thymocytes. This translocation and the following apoptotic changes were inhibited by SB203580, a p38 kinase inhibitor, in a specific manner. These results suggest that activated p38 kinase pathway by TCR stimulation induces translocation of Bax to mitochondria, causing Delta Psi(m) disruption, and the release of cytochrome c, which finally induces caspase-3-mediated apoptosis in thymocytes.

The Src-like adaptor protein downregulates the T cell receptor on CD4+CD8+ thymocytes and regulates positive selection

In this report, we show that the Src-like adaptor protein (SLAP) plays an important role in thymocyte development. SLAP expression is developmentally regulated; it is low in CD4-CD8- thymocytes, it peaks in the CD4+CD8+ subset, and it decreases to low levels in more mature cells. Disruption of the SLAP gene leads to a marked upregulation of TCR and CD5 expression at the CD4+CD8+ stage. The absence of SLAP was also developmentally significant because it enhanced positive selection in mice expressing the DO11.10 transgenic T cell receptor. Moreover, SLAP deletion at least partially rescued the development of ZAP-70-deficient thymocytes. These results demonstrate that SLAP participates in a novel mechanism of TCR downregulation at the CD4+CD8+ stage and regulates positive selection.

T cell development and T cell responses in mice with mutations affecting tyrosines 292 or 315 of the ZAP-70 protein tyrosine kinase

After stimulation of the T cell receptor (TCR), the tyrosine residues 292 and 315 in interdomain B of the protein tyrosine kinase ZAP-70 become phosphorylated and plausibly function as docking sites for Cbl and Vav1, respectively. The two latter proteins have been suggested to serve as substrates for ZAP-70 and to fine-tune its function. To address the role of these residues in T cell development and in the function of primary T cells, we have generated mice that express ZAP-70 molecules with Tyr to Phe substitution at position 292 (Y292F) or 315 (Y315F). When analyzed in a sensitized TCR transgenic background, the ZAP-70 Y315F mutation reduced the rate of positive selection and delayed the occurrence of negative selection. Furthermore, this mutation unexpectedly affected the constitutive levels of the CD3-zeta p21 phosphoisoform. Conversely, the ZAP-70 Y292F mutation upregulated proximal events in TCR signaling and allowed more T cells to produce interleukin 2 and interferon gamma in response to a given dose of antigen. The observation that ZAP-70 Y292F T cells have a slower rate of ligand-induced TCR downmodulation suggests that Y292 is likely involved in regulating the duration activated TCR reside at the cell surface. Furthermore, we showed that Y292 and Y315 are dispensable for the TCR-induced tyrosine phosphorylation of Cbl and Vav1, respectively. Therefore, other molecules present in the TCR signaling cassette act as additional adaptors for Cbl and Vav1. The present in vivo analyses extend previous data based on transformed T cell lines and suggest that residue Y292 plays a role in attenuation of TCR signaling, whereas residue Y315 enhances ZAP-70 function.

Requirement for tyrosine residues 315 and 319 within zeta chain-associated protein 70 for T cell development

Engagement of the T cell antigen receptor (TCR) induces the transphosphorylation of the zeta chain-associated protein of 70,000 Mr (ZAP-70) protein tyrosine kinase (PTK) by the CD4/8 coreceptor associated Lck PTK. Phosphorylation of Tyr 493 within ZAP-70's activation loop results in the enzymatic activation of ZAP-70. Additional tyrosines (Tyrs) within ZAP-70 are phosphorylated that play both positive and negative regulatory roles in TCR function. Phosphorylation of Tyr residues (Tyrs 315 and 319) within the Interdomain B region of the ZAP-70 PTK plays important roles in the generation of second messengers after TCR engagement. Here, we demonstrate that phosphorylation of these two Tyr residues also play important roles in mediating the positive and negative selection of T cells in the thymus.

Regulation of NK cell-mediated cytotoxicity by the adaptor protein 3BP2

Stimulation of lymphocytes through multichain immune recognition receptors activates multiple signaling pathways. Adaptor proteins play an important role in integrating these pathways by their ability to simultaneously bind multiple signaling components. Recently, the 3BP2 adaptor protein has been shown to positively regulate the transcriptional activity of T cells. However, the mechanisms by which signaling components are involved in this regulation remain unclear, as does a potential role for 3BP2 in the regulation of other cellular functions. Here we describe a positive regulatory role for 3BP2 in NK cell-mediated cytotoxicity. We also identify p95(vav) and phospholipase C-gamma isoforms as binding partners of 3BP2. Our results show that tyrosine-183 of 3BP2 is specifically involved in this interaction and that this residue critically influences 3BP2-dependent function. Therefore, 3BP2 regulates NK cell-mediated cytotoxicity by mobilizing key downstream signaling effectors.

Human natural killer cell receptors and signal transduction

Natural killer (NK) cells express numerous receptors, which continually engage with ligands on cell surfaces. Until 1995, only a handful of these receptors were characterized and the molecular basis of NK cell activation was obscure. Recently, considerable advances have been made in characterizing the receptor repertoire on human NK cells. Both activating and inhibitory receptors can transduce positive or negative signals to regulate NK cell cytotoxicity and cytokine release responses. The inhibitory receptors normally predominate in this balance of signals. Certain tumor cells and virally infected cells that lack major histocompatibility complex (MHC) class I molecules, however, can rapidly trigger NK cell activation. The basis of this activation is the loss of negative signals that are normally transmitted by MHC class I-binding inhibitory receptors, and the corresponding domination of activating receptor signals. While ligand specificity for a number of the recently described receptors is still a mystery, their signal transduction properties have begun to be defined. The dynamic crosstalk between these receptors ultimately governs the NK cell activation state. Although the complexities of NK cell signalling are only marginally understood, several overall themes have been defined by characterizing the roles of distinct pathways during NK cell responses.

Insights into T-cell development from studies using transgenic and knockout mice

The generation of immunocompetent lymphocytes is a complex process that utilizes a multitude of cell surface receptors and intracellular signaling pathways. Moreover, specific cell-cell interactions and specialized microenvironments are required, so that purely in vitro experimental systems are limited in their ability to explain the complexity of T-cell development. In vivo models have been used extensively in the study of T-cell development. In the present review we summarize but a few of the seminal discoveries that have been made in this field using transgenic and knockout mouse models. In addition to demonstrating the wealth of information that can be gained, we also discuss some of the present limitations of this technology. Novel advances that allow the conditional and inducible modification of the genome and knock-in mutations promise to lead to an even more rapid advancement in our knowledge of T-cell development.

Immune system dysfunction and autoimmune disease in mice lacking Emk (Par-1) protein kinase

Emk is a serine/threonine protein kinase implicated in regulating polarity, cell cycle progression, and microtubule dynamics. To delineate the role of Emk in development and adult tissues, mice lacking Emk were generated by targeted gene disruption. Emk(-/-) mice displayed growth retardation and immune cell dysfunction. Although B- and T-cell development were normal, CD4(+)T cells lacking Emk exhibited a marked upregulation of the memory marker CD44/pgp-1 and produced more gamma interferon and interleukin-4 on stimulation through the T-cell receptor in vitro. In addition, B-cell responses to T-cell-dependent and -independent antigen challenge were altered in vivo. As Emk(-/-) animals aged, they developed splenomegaly, lymphadenopathy, membranoproliferative glomerulonephritis, and lymphocytic infiltrates in the lungs, parotid glands and kidneys. Taken together, these results demonstrate that the Emk protein kinase is essential for maintaining immune system homeostasis and that loss of Emk may contribute to autoimmune disease in mammals.

Defective development of NK1.1+ T-cell antigen receptor alphabeta+ cells in zeta-associated protein 70 null mice with an accumulation of NK1.1+ CD3- NK-like cells in the thymus

Development of natural killer 1.1+ (NK1.1+) CD3+ (NK1.1+ T) cells was analyzed in zeta-associated protein 70 (ZAP-70) null ((-/-)) mice. Both NK1.1+ TCRalphabeta+ and NK1.1+ TCRgammadelta+ cell populations were absent in the thymus and spleen. By contrast, the number of NK1.1+ CD3- cells was increased in these tissues. The NK1.1+ CD3- thymocytes in ZAP-70(-/-) mice had surface phenotypes in common with NK or NK1.1+ T cells. However, some of them were discordant either with NK cells or with NK1.1+ T cells. The NK1.1+ CD3- cells produced interferon-gamma upon stimulation with NK1.1 cross-linking in the presence of interleukin-2 and exhibited a substantial cytotoxicity against YAC-1 cells. Moreover, the generation of NK1.1+ T cells with invariant Valpha14Jalpha281 chains was induced from the NK1.1+ CD3- thymocytes following stimulation with phorbol myristate acetate and ionomycin in a neonatal thymic organ culture. An introduction of TCRalpha and beta transgenes to the ZAP-70(-/-) mice resulted in generation of an NK1.1+ TCRalphabeta(dim) population, whereas no substantial CD4+ CD8- or CD4- CD8+ population that expressed the introduced TCRalphabeta was generated in the mainstream T lineage. These findings demonstrate that ZAP-70 kinase is indispensable for the development of NK1.1+ T cells and that the unique NK1.1+ CD3- thymocytes in ZAP-70(-/-) mice contain immediate precursors of NK1.1+ T cells.

Regulation of the helix-loop-helix proteins, E2A and Id3, by the Ras-ERK MAPK cascade

Activation of mitogen-activated protein kinase (MAPK) pathways leads to cellular differentiation and/or proliferation in a wide variety of cell types, including developing thymocytes. The basic helix-loop-helix (bHLH) proteins E12 and E47 and an inhibitor HLH protein, Id3, play key roles in thymocyte differentiation. We show here that E2A DNA binding is lowered in primary immature thymocytes consequent to T cell receptor (TCR)-mediated ligation. Whereas expression of E2A mRNA and protein are unaltered, Id3 transcripts are rapidly induced upon signaling from the TCR. Activation of Id3 transcription is regulated in a dose-dependent manner by the extracellular signal-regulated kinase (ERK) MAPK module. These observations directly connect the ERK MAPK cascade and HLH proteins in a linear pathway.

IgD receptor-mediated signal transduction in T cells

Upregulation of immunoglobulin D-specific receptors (IgD-R) on CD4+ T cells may facilitate their interaction with specific carbohydrate moieties uniquely associated with membrane IgD on B cells. Previous studies have shown that upregulation of IgD-R facilitates cognate T-B cell interactions by mediating bidirectional signaling resulting in increased antibody responses and clonal expansion of antigen-specific T cells. Murine T hybridoma cells, 7C5, constitutively express IgD-R, as has been confirmed by staining with biotinylated IgD. Earlier studies have shown that inhibitors of protein tyrosine kinase (PTK) completely prevented upregulation of IgD-R in response to oligomeric IgD, suggesting that cross-linking of IgD-R may induce signal transduction and functional consequences through one or more PTK activation pathways, leading to upregulation of IgD-R. In the present study we show that cross-linking of IgD-R by oligomeric IgD indeed results in (a) T cell activation as seen by tyrosine phosphorylation of several intracellular proteins, (b) tyrosine phosphorylation of p56 Lck and PLC-gamma in 7C5 T hybridoma cells, and (c) phosphorylation of an approximately 29-kDa band that exhibits strong affinity for IgD. We analyzed tyrosine phosphorylation of p56 Lck and PLC-gamma in BALB/c splenic T cells that were exposed to oligomeric IgD both in vivo and in vitro. In vitro cross-linking as well as in vivo followed by in vitro cross-linking of IgD-R resulted in enhanced phosphorylation of p56 Lck and moderate tyrosine phosphorylation of PLC-gamma. These results suggest that interactions between IgD-R and IgD mediate signal transduction and support our previous findings that IgD-R+ T cells enhance cognate T cell-B cell interactions and antibody production.

Distinct T cell developmental consequences in humans and mice expressing identical mutations in the DLAARN motif of ZAP-70

The protein tyrosine kinase, ZAP-70, is pivotally involved in transduction of Ag-binding signals from the TCR required for T cell activation and development. Defects in ZAP-70 result in SCID in humans and mice. We describe an infant with SCID due to a novel ZAP-70 mutation, comparable with that which arose spontaneously in an inbred mouse colony. The patient inherited a homozygous missense mutation within the highly conserved DLAARN motif in the ZAP-70 kinase domain. Although the mutation only modestly affected protein stability, catalytic function was absent. Despite identical changes in the amino acid sequence of ZAP-70, the peripheral T cell phenotypes of our patient and affected mice are distinct. ZAP-70 deficiency in this patient, as in other humans, is characterized by abundant nonfunctional CD4(+) T cells and absent CD8(+) T cells. In contrast, ZAP-70-deficient mice lack both major T cell subsets. Although levels of the ZAP-70-related protein tyrosine kinase, Syk, may be sufficiently increased in human thymocytes to rescue CD4 development, survival of ZAP-70-deficient T cells in the periphery does not appear to be dependent on persistent up-regulation of Syk expression.

NF-kappa B is required for the positive selection of CD8+ thymocytes

To examine the role of NF-kappaB in T cell development, we analyzed thymocyte ontogeny in transgenic (mutant I-kappaBalpha (mI-kappaBalpha)) mice that express a superinhibitory form of the NF-kappaB inhibitory protein, I-kappaBalpha (I-kappaBalpha(A32/36)), under the control of the T cell-specific CD2 promoter and enhancer. Thymi from mI-kappaBalpha mice contained increased numbers of double-positive (DP) and decreased numbers of both CD4(+) and CD8(+) single-positive cells, consistent with a block in DP thymocyte maturation. In addition, expression of CD69, a marker of positive selection, was decreased on DP thymocytes from the mI-kappaBalpha mice. To test directly whether NF-kappaB was required for positive or negative selection, we generated mI-kappaBalpha mice expressing the H-Y or 2C alphabeta TCR transgenes. Expression of the I-kappaBalpha(A32/36) transgene caused a block in the positive selection of CD8(+) single-positive cells in both strains of TCR transgenic animals. In contrast, negative selection was unaffected by expression of the I-kappaBalpha(A32/36) transgene. Taken together, these results identified a NF-kappaB-dependent transcriptional pathway that is selectively required for the positive selection of CD8(+) thymocytes.

Possible involvement of cyclophilin B and caspase-activated deoxyribonuclease in the induction of chromosomal DNA degradation in TCR-stimulated thymocytes

TCR engagement of immature CD4(+)CD8(+) thymocytes induces clonal maturation (positive selection) as well as clonal deletion (negative selection) in the thymus. However, the cell death execution events of thymocytes during the negative selection process remain obscure. Using a cell-free system, we identified two different DNase activities in the cytosol of in vivo anti-TCR-stimulated murine thymocytes: one that induced chromosomal DNA fragmentation, which was inhibited by an inhibitor of caspase-activated DNase, and another that induced plasmid DNA degradation, which was not inhibited by an inhibitor of caspase-activated DNase. We purified the protein to homogeneity that induced plasmid DNA degradation from the cytosol of anti-CD3-stimulated thymocytes and found that it is identical with cyclophilin B (Cyp B), which was reported to locate in endoplasmic reticulum. Ab against Cyp B specifically inhibited the DNA degradation activity in the cytosol of anti-CD3-stimulated thymocytes. Furthermore, recombinant Cyp B induced DNA degradation of naked nuclei, but did not induce internucleosomal DNA fragmentation. Finally, we demonstrated that TCR engagement of a murine T cell line (EL4) with anti-CD3/CD28 resulted in the release of Cyp B from the microsome fraction to the cytosol/nuclear fraction. Our data strongly suggest that both active caspase-activated DNase and Cyp B may participate in the induction of chromosomal DNA degradation during cell death execution of TCR-stimulated thymocytes.

RasGRP is essential for mouse thymocyte differentiation and TCR signaling

The Ras signaling pathway plays a critical role in thymopoiesis and T cell activation, but the mechanism of Ras regulation is controversial. At least one mode of Ras regulation in T cells involves the messenger diacylglycerol (DAG). RasGRP, a Ras activator with a DAG-binding C1 domain, is expressed in T cells and thymocytes. Here we show that thymi of RasGRP-null mutant mice have approximately normal numbers of immature thymocytes but a marked deficiency of mature, single-positive (CD4+CD8- and CD4-CD8+) thymocytes. In Ras signaling and proliferation assays, mutant thymocytes showed a complete lack of response to DAG analogs or T cell receptor (TCR) stimulation by antibodies. Thus, TCR and DAG are linked through RasGRP to Ras signaling.

Differential requirement of ZAP-70 for CD2-mediated activation pathways of mature human T cells

This study addresses the role of the tyrosine kinase ZAP-70 in CD2-mediated T cell activation. Patients lacking ZAP-70 have few mature CD8+ T cells and high numbers of CD4+ T cells that are nonfunctional upon TCR triggering. Such a patient with a homozygous deletion in the zap-70 gene that resulted in the complete absence of ZAP-70 protein expression has been identified. Expression of the tyrosine kinases Lck, Fyn, and Syk was normal. The patient's T cells were activated with two different pairs of mitogenic mAbs. CD2-induced phosphorylation of the zeta-chain and influx of Ca2+ was defective in the ZAP-70-deficient T cells, whereas CD2-induced phosphorylation of several other proteins, including Syk, was not affected. CD2-induced proliferation as well as production of TNF-alpha and IFN-gamma was abrogated in ZAP-70-deficient T cells, whereas PMA plus ionomycin induced normal activation of these cells. Together, this study shows that CD2-activation triggers ZAP-70-dependent and -independent pathways. Deletion of ZAP-70 affected CD2- and CD3-mediated proliferation and cytokine production in a similar way, suggesting that one of the different CD2 pathways converges with a CD3 pathway at or upstream of the activation of ZAP-70.

Combined natural killer cell and dendritic cell functional deficiency in KARAP/DAP12 loss-of-function mutant mice

KARAP/DAP12 is a transmembrane polypeptide with an intracytoplasmic immunoreceptor tyrosine-based activation motif (ITAM). KARAP/DAP12 is associated with several activating cell surface receptors in hematopoietic cells. Here, we report that knockin mice bearing a nonfunctional KARAP/DAP12 ITAM present altered innate immune responses. Although in these mice NK cells are present and their repertoire of inhibitory MHC class I receptors is intact, the NK cell spectrum of natural cytotoxicity toward tumor cell targets is restricted. KARAP/DAP12 loss-of-function mutant mice also exhibit a dramatic accumulation of dendritic cells in muco-cutaneous epithelia, associated with an impaired hapten-specific contact sensitivity. Thus, despite its homology with CD3zeta and FcRgamma, KARAP/DAP12 plays a specific role in innate immunity, emphasizing the nonredundancy of these ITAM-bearing polypeptides in hematopoietic cells.

Induction of NK1.1(+) alpha beta TCR(+) T cells by bypassing TCR signals in ZAP-70 deficient mice

The mechanism of development of a unique subset of T cells, thymic NK1.1(+) alpha beta T cells, has been poorly understood. We found that the development of thymic NK1.1(+) alpha beta T cells was defective in mice deficient in ZAP-70. Instead, an accumulation of NK1.1(+) TCR beta(-) NK-like population was detected in the thymus and spleen of the ZAP-70 deficient (ZAP -/-) mouse. In the present report, we examined whether biochemical treatments that replace TCR-mediated positive selection signals could restore the generation of thymic NK1.1(+) alpha beta T cells in ZAP -/- mice using the thymus organ culture. We found that a higher concentration of phorbol ester (PMA) than that required for CD4(+) T cell generation and ionomycin induced the generation of NK1.1(+) alpha beta T cells. Phenotypic analysis of the induced NK1.1(+) alpha beta T cell population suggested that these cells expressed CD8 but not CD4 molecules, which is a different characteristic from ordinary thymic NK1.1(+) alpha beta T cells. These results suggest that differential signaling is required for the generation of mainstream T cells and thymic NK1.1(+) alpha beta T cells.

Signaling via the T cell antigen receptor induces phosphorylation of Stat1 on serine 727

The Stat1 transcription factor plays a pivotal role in both, the antiviral and antigrowth actions of interferons. Stat1 acquires the ability to bind DNA by becoming phosphorylated on Tyr(701). However, to effectively stimulate gene transcription, it must also be phosphorylated on Ser(727). We show that engagement of T cell antigen receptor (TCR)/CD3 complex in either Jurkat cells or peripheral blood lymphocytes stimulates phosphorylation of Ser(727) but not Tyr(701) of Stat1. This process does not require the expression of tyrosine kinases Lck and Zap-70. Interestingly, pretreatment of T cells with the Src kinase inhibitor PP1 completely abrogated CD3-mediated serine phosphorylation of Stat1, whereas inhibitors to MEK1 and phosphatidylinositol 3-kinase had no effect. Phosphorylation of Ser(727) of Stat1 in T cells is not restricted to TCR/CD3 but also results when cells are stimulated via the costimulatory molecule CD28. The combination of CD3 and CD28 did not augment phosphorylation of Stat1 Ser(727). Surprisingly, Stat1-mediated transcriptional activity in response to IFN-alpha was enhanced with CD3 stimulation, whereas CD3 alone had little effect. These findings suggest that Stat1 is a signaling molecule in TCR signaling and may play a role in T cell function.

Alternative antigen receptor (TCR) signaling in T cells derived from ZAP-70-deficient patients expressing high levels of Syk

ZAP-70-deficient patients present with nonfunctional CD4+ T cells in the periphery. We find that a subset of primary ZAP-70-deficient T cells, expressing high levels of the related protein-tyrosine kinase Syk, can proliferate in vitro. These cells (denoted herein as Syk(hi)/ZAP-70(-) T cells) provide a unique model in which the contribution of Syk to TCR-mediated responses can be explored in a nontransformed background. Importantly, CD3-induced responses, such as tyrosine phosphorylation of cellular substrates (LAT, SLP76, and PLC-gamma1), as well as calcium mobilization, which are defective in T cells expressing neither ZAP-70 nor Syk, are observed in Syk(hi)/ZAP-70(-) T cells. However, Syk(hi)/ZAP-70(-) T cells differ from control T cells with respect to the T cell antigen receptor (TCR)-mediated activation of the MAPK cascades: extracellular signal-regulated kinase activity and recruitment of the JNK and p38 stress-related MAPK pathways are diminished. This distinct phenotype of Syk(hi)/ZAP-70(-) T cells is associated with a profound decrease in CD3-mediated interleukin 2 secretion and proliferation relative to control T cells. Thus, ZAP-70 and Syk appear to play distinct roles in transducing a TCR-mediated signal.

A new look at Syk in alpha beta and gamma delta T cell development using chimeric mice with a low competitive hematopoietic environment

The Syk protein tyrosine kinase (PTK) is essential for B, but not T or NK, cell development, although certain T cell subsets (i.e., gamma delta T cells of intestine and skin) appear to be dependent on Syk. In this report, we have re-evaluated the role of Syk in T cell development in hematopoietic chimeras generated by using Syk-deficient fetal liver hematopoietic stem cells (FL-HSC). We found that Syk-/- FL-HSC were vastly inferior to wild-type FL-HSC in reconstituting T cell development in recombinant-activating gene 2 (RAG2)-deficient mice, identifying an unexpected and nonredundant role for Syk in this process. This novel function of Syk in T cell development was mapped to the CD44-CD25+ stage. According to previous reports, development of intestinal gamma delta T cells was arrested in Syk-/- -->RAG2-/- chimeras. In striking contrast, when hosts were the newly established alymphoid RAG2 x common cytokine receptor gamma-chain (RAG2/gamma c) mice, Syk-/- chimeras developed intestinal gamma delta T cells as well as other T cell subsets (including alpha beta T cells, NK1.1+ alpha beta T cells, and splenic and thymic gamma delta T cells). However, all Syk-deficient T cell subsets were reduced in number, reaching about 25-50% of controls. These results attest to the utility of chimeric mice generated in a low competitive hematopoietic environment to evaluate more accurately the impact of lethal mutations on lymphoid development. Furthermore, they suggest that Syk intervenes in early T cell development independently of ZAP-70, and demonstrate that Syk is not essential for the intestinal gamma delta T cell lineage to develop.

Successive expression and activation of NFAT family members during thymocyte differentiation

Differentiation of immature CD4(+)CD8(+) thymocytes to mature CD4(+) or CD8(+) T cells is induced by positive selection and appears to involve calcineurin-dependent activation of NFAT, a family of transcription factors. NFATx is predominantly expressed in CD4(+)CD8(+) thymocytes, whereas NFATp and NFATc are expressed at much lower levels in the thymus than in mature T cells. However, how or when each NFAT member is involved in the differentiation pathway is unclear. Using an in vitro model system where isolated CD4(+)CD8(+) thymocytes can survive and differentiate into semi-mature CD4-lineage T cells, we suggest that low calcineurin activity sustained for approximately 20 h is required for cell survival and differentiation. Accordingly, the DNA binding activity of NFAT slowly increased during the stimulation of 20 h to induce the differentiation. NFATx significantly contributed to the early rise, but the late increase was mostly due to NFATc activation. Meanwhile, the expression of NFATx mRNA decreased and that of NFATc mRNA increased. The DNA-binding activity of NFATp was detectable but low throughout the stimulation. NFATp became dominantly active after the semi-mature T cells differentiated into mature and activated CD4 T cells. These findings suggest that NFATx and NFATc successively play roles in T cell development.

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.

Signaling pathways engaged by NK cell receptors: double concerto for activating receptors, inhibitory receptors and NK cells

Despite the absence of antigen-specific receptors at their surface, NK cells can selectively eliminate virus-infected cells, tumor cells and allogenic cells. A dynamic and precisely coordinated balance between activating and inhibitory receptors governs NK cell activation programs. Multiple activating and inhibitory NK cell surface molecules have been described, a group of them acting as receptors for MHC class I molecules. In spite of their heterogeneity, activating NK cell receptors present remarkable structural and functional homologies with T cell- and B cell-antigen receptors. Inhibitory NK cell receptors operate at early stages of activating cascades by recruiting protein tyrosine phosphatases via intra- cytoplasmic motifs (ITIM), a strategy which is widely conserved in hematopoietic and non-hematopoietic cells.

Characterisation and specificity of two single-chain Fv antibodies directed to the protein tyrosine kinase Syk

In order to obtain single chain Fv fragments (scFv) specific for the protein tyrosine kinase Syk, we screened a human synthetic phage-display library. Two glutathione S-transferase (GST):Syk fusion proteins containing both SH2 domains of Syk were used to perform three rounds of selection of the library. Among the scFv fragments resulting from the third round of selection, the ones specific for the GST portion of the fusion proteins were eliminated by performing enzyme-linked immunosorbent assay tests on GST:Syk versus GST coated plates, and the monoclonal scFv fragments binding only to the GST:Syk coated plates with high affinities were further analysed. We report here the in vitro characterisation of G4G11 and G6G2 anti-Syk scFvs. G4G11 shows the best performance in immunoprecipitation and immunofluorescence experiments, and G6G2 is able to detect Syk in immunoprecipitation, immunofluorescence and on Western blots. Both scFvs are also able to detect the phosphorylated form of Syk, and neither of them binds to Zap-70, the other member of the Syk family of protein tyrosine kinases.

Tyrosine kinase SYK: essential functions for immunoreceptor signalling

The tyrosine kinase SYK plays critical roles in signalling through immune receptors. Gene-targeting studies have identified the cell types that require SYK for development and function, and the receptors that use SYK as well as their downstream signalling effectors. There is also evidence of a role for SYK in non-immune cells and in the maintenance of vascular integrity.

MEK activity regulates negative selection of immature CD4+CD8+ thymocytes

CD4+CD8+ thymocytes are either positively selected and subsequently mature to CD4 single positive (SP) or CD8 SP T cells, or they die by apoptosis due to neglect or negative selection. This clonal selection is essential for establishing a functional self-restricted T cell repertoire. Intracellular signals through the three known mitogen-activated protein (MAP) kinase pathways have been shown to selectively guide positive or negative selection. Whereas the c-Jun N-terminal kinase and p38 MAP kinase regulate negative selection of thymocytes, the extracellular signal-regulated kinase (ERK) pathway is required for positive selection and T cell lineage commitment. In this paper, we show that the MAP/ERK kinase (MEK)-ERK pathway is also involved in negative selection. Thymocytes from newborn TCR transgenic mice were cultured with TCR/CD3epsilon-specific Abs or TCR-specific agonist peptides to induce negative selection. In the presence of the MEK-specific pharmacological inhibitors PD98059 or UO126, cell recovery was enhanced and deletion of DP thymocytes was drastically reduced. Furthermore, development of CD4 SP T cells was blocked, but differentiation of mature CD8 SP T cells proceeded in the presence of agonist peptides when MEK activity was blocked. Thus, our data indicate that the outcome between positively and negatively selecting signals is critically dependent on MEK activity.

Differential T-cell antigen receptor signaling mediated by the Src family kinases Lck and Fyn

Src family tyrosine kinases play a key role in T-cell antigen receptor (TCR) signaling. They are responsible for the initial tyrosine phosphorylation of the receptor, leading to the recruitment of the ZAP-70 tyrosine kinase, as well as the subsequent phosphorylation and activation of ZAP-70. Molecular and genetic evidence indicates that both the Fyn and Lck members of the Src family can participate in TCR signal transduction; however, it is unclear to what extent they utilize the same signal transduction pathways and activate the same downstream events. We have addressed this issue by examining the ability of Fyn to mediate TCR signal transduction in an Lck-deficient T-cell line (JCaM1). Fyn was able to induce tyrosine phosphorylation of the TCR and recruitment of the ZAP-70 kinase, but the pattern of TCR phosphorylation was altered and activation of ZAP-70 was defective. Despite this, the SLP-76 adapter protein was inducibly tyrosine phosphorylated, and both the Ras-mitogen-activated protein kinase and the phosphatidylinositol 4, 5-biphosphate signaling pathways were activated. TCR stimulation of JCaM1/Fyn cells induced the expression of the CD69 activation marker and inhibited cell growth, but NFAT activation and the production of interleukin-2 were markedly reduced. These results indicate that Fyn mediates an alternative form of TCR signaling which is independent of ZAP-70 activation and generates a distinct cellular phenotype. Furthermore, these findings imply that the outcome of TCR signal transduction may be determined by which Src family kinase is used to initiate signaling.

Restoration of thymic development in an Lck(-/-) thymoma overexpressing ZAP-70

Thymic development is strictly controlled by Src and Syk family protein tyrosine kinases. The major players in this process are Lck and ZAP-70, which regulate critical differentiation steps of thymopoiesis. Notwithstanding the critical role of Lck and ZAP-70 in thymocyte development as compared to the related kinases Fyn and Syk, a partial functional redundancy between members of the same family of protein tyrosine kinases has emerged from studies on genetically manipulated mouse models. Furthermore, a close functional interplay between Lck and ZAP-70 in intracellular signaling has been shown to occur in thymocytes. Here we present the characterization of a thymoma from an Lck(-/-) mouse, where the block in thymocyte development is overcome and the transition between the CD4(-)CD8(-) and CD4(+)CD8(+) stages is fully restored. Determination of the expression levels of Fyn, ZAP-70 and Syk in thymocytes form the Lck(-/-) thymoma revealed high levels of ZAP-70 overexpression and recovery of a specific subset of phosphoproteins as compared to Lck(-/-) thymocytes. Hence ZAP-70 overexpression in thymocytes is associated with recovery from the developmental arrest caused by the absence of Lck, suggesting a role for ZAP-70 downstream of Lck in the maturation of CD4(+)CD8(+) thymocytes.

4-Pyridin-5-yl-2-(3,4,5-trimethoxyphenylamino)pyrimidines: potent and selective inhibitors of ZAP 70

Activation of the tyrosine kinase ZAP 70 has been shown to be crucial to the transduction of the T-cell receptor signalling pathway, which leads ultimately to proliferation, cytokine gene expression and T-cell effector functions. A series of 2-phenylaminopyrimidines have been identified as potent and selective inhibitors of ZAP 70.

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.

Discovery of potent and selective SH2 inhibitors of the tyrosine kinase ZAP-70

A series of 1,2,4-oxadiazole analogues has been shown to be potent and selective SH2 inhibitors of the tyrosine kinase ZAP-70, a potential therapeutic target for immune suppression. These compounds typically are 200-400-fold more potent than the native, monophosphorylated tetrapeptide sequences. When compared with the high-affinity zeta-1-ITAM peptide (Ac-NQL-pYNELNLGRREE-pYDVLD-NH(2), wherein pY refers to phosphotyrosine) some of the best 1,2, 4-oxadiazole analogues are approximately 1 order of magnitude less active. This series of compounds displays an unprecedented level of selectivity over the closely related tyrosine kinase Syk, as well as other SH2-containing proteins such as Src and Grb2. Gel shift studies using a protein construct consisting only of C-terminal ZAP-70 SH2 demonstrate that these compounds can effectively engage this particular SH2 domain.

Involvement of the SHP-1 Tyrosine Phosphatase in Regulation of T Cell Selection

The selection events shaping T cell development in the thymus represent the outcome of TCR-driven intracellular signaling cascades evoked by Ag receptor interaction with cognate ligand. In view of data indicating TCR-evoked thymocyte proliferation to be negatively modulated by the SHP-1 tyrosine phosphatase, a potential role for SHP-1 in regulating selection processes was investigated by analysis of T cell development in H-Y TCR transgenic mice rendered SHP-1 deficient by introduction of the viable motheaten mutation or a dominant negative SHP-1-encoding transgene. Characterization of thymocyte and peripheral T cell populations in H-Y TCR-viable motheaten mice revealed TCR-evoked proliferation as well as the positive and negative selection of H-Y-specific thymocytes to be enhanced in these mice, thus implicating SHP-1 in the negative regulation of each of these processes. T cell selection processes were also augmented in H-Y TCR mice carrying a transgene driving lymphoid-restricted expression of a catalytically inert, dominant-negative form of SHP-1. SHP-1-negative effects on thymocyte TCR signaling were not influenced by co-cross-linking of the CD28 costimulatory and/or CTLA-4 inhibitory receptors and appear, accordingly, to be realized independently of these comodulators. These observations indicate that SHP-1 raises the signaling threshold required for both positive and negative selection and reveal the inhibitory effects of SHP-1 on TCR signaling to be cell autonomous. The demonstrated capacity for SHP-1 to inhibit TCR-evoked proliferation and selection indicate SHP-1 modulatory effects on the magnitude of TCR-generated signal to be a key factor in determining the cellular consequences of TCR-ligand interaction.

Pre-T Cell Receptor Signals Are Responsible for the Down-Regulation of Syk Protein Tyrosine Kinase Expression

Thymocyte development proceeds through two critical checkpoints that involve signaling events through two different receptors, the TCR and the pre-TCR. These receptors employ two families of protein tyrosine kinases to propagate their signals, the Src and Syk families. Genetic and biochemical evidence has shown that the Src family kinases are critical for normal T cell maturation. ZAP-70, a Syk family kinase, has similarly been implicated as a critical component in thymocyte development. Although genetic evidence has suggested that Syk is involved during thymocyte development, a definitive study of Syk expression has not been performed. In this paper we report our reanalysis of Syk expression in subpopulations of murine and human thymocytes by intracellular staining and flow cytometry using anti-Syk mAbs. Syk is expressed at increased levels during the stages in which pre-TCR signaling occurs. Furthermore, Syk is down-regulated after the pre-TCR checkpoint has been passed. Syk may play an important role in thymic development during pre-TCR signal transduction. Finally, incomplete down-regulation of Syk expression was noted in human thymocytes, offering a possible explanation for the distinct phenotypes of mice and humans deficient in ZAP-70.

Redundant Role of the Syk Protein Tyrosine Kinase in Mouse NK Cell Differentiation

Syk and ZAP-70 subserve nonredundant functions in B and T lymphopoiesis. In the absence of Syk, B cell development is blocked, while T cell development is arrested in the absence of ZAP-70. The receptors and the signaling molecules required for differentiation of NK cells are poorly characterized. Here we investigate the role of the Syk protein tyrosine kinase in NK cell differentiation. Hemopoietic chimeras were generated by reconstituting alymphoid (B−, T−, NK−) recombinase-activating gene-2 × common cytokine receptor γ-chain double-mutant mice with Syk−/− fetal liver cells. The phenotypically mature Syk−/− NK cells that developed in this context were fully competent in natural cytotoxicity and in calibrating functional inhibitory receptors for MHC molecules. Syk-deficient NK cells demonstrated reduced levels of Ab-dependent cellular cytotoxicity. Nevertheless, Syk−/− NK cells could signal through NK1.1 and 2B4 activating receptors and expressed ZAP-70 protein. We conclude that the Syk protein tyrosine kinase is not essential for murine NK cell development, and that compensatory signaling pathways (including those mediated through ZAP-70) may sustain most NK cell functions in the absence of Syk.

Reconstitution of killer cell inhibitory receptor-mediated signal transduction machinery in a cell-free model system

Recognition of class I MHC molecules on target cells by killer cell inhibitory receptors (KIRs) blocks natural cytotoxicity and antibody-dependent cell cytotoxicity of NK cells and CD3/TCR dependent cytotoxicity of T cells. The inhibitory effect of KIR ligation requires phosphorylation of the cytoplasmic tail of KIR and subsequent recruitment of an SH2-containing protein tyrosine phosphatase, SHP-1. To better understand the molecular mechanism of the KIR-mediated inhibitory signal transduction, we developed an in vitro assay system using a purified His-tag fusion protein of KIR cytoplasmic tail (His-CytKIR) and Jurkat T cell lysates. We identified a target molecule of SHP-1 by comparing the phosphorylation of major cellular substrates following in vitro phosphorylation of Jurkat cell lysates in the presence and absence of the His-CytKIR in this cell-free model system. The His-CytKIR was tyrosine phosphorylated by Lck in vitro, and the phosphorylated His-CytKIR recruited SHP-1. Interestingly, we observed that among major substrates phosphorylated in vitro, PLC-gamma exhibited a dramatic decrease in phosphorylation when the His-CytKIR was mixed with Jurkat T cell lysates. However, PLC-gamma exhibited no decrease in phosphorylation when SHP-1 or Lck was depleted or deficient in this reaction mixture, suggesting that the SHP-1 recruited by the phosphorylated His-CytKIR directly mediate the dephosphorylation of PLC-gamma. The cell-free model system could be used to reveal the detailed molecular interactions in the KIR-mediated signal transduction.

Control of early thymocyte development by the pre-T cell receptor complex: A receptor without a ligand?

Beta-selection refers to a developmental checkpoint linking thymocyte survival to the outcome of antigen receptor gene rearrangement. Immature thymocytes that productively rear-range the gene segments of the TCRbeta locus undergo proliferative expansion and mature to the CD4(+)CD8(+)stage; those failing to do so die by apoptosis. How are these precursor cells alerted that TCRbeta rearrangement has been productive? While it is clear that this process involves signals transduced by a surrogate form of the TCR termed the pre-TCR, it remains unclear how pre-TCR signals are triggered. In this review, we will discuss the implications of recent experimental attempts to address this issue, as well as how pre-TCR activation is linked to the changes in gene expression that underlie thymocyte development.

Signals involved in thymocyte positive and negative selection

Extensive research has focused upon understanding how thymocytes distinguish between interactions that lead to positive or negative selection. Various intracellular pathways that are activated after TCR engagement are outlined in this review, and their contribution to thymocyte selection is discussed. Although thymocyte fate is generally governed by a quantitative/avidity model, this largely reflects the interactions that occur at the cell surface. Therefore, we outline possible models of how different intercellular interactions are translated into intracellular signals that diverge and lead to thymocyte survival or death.

T cell receptor-mediated signs and signals governing T cell development

The developmental fate of T cells is largely controlled by the nature and success of signals mediated by the pre-T cell receptor (TCR) and TCR complexes. These intracellular signals are regulated by cascades of protein tyrosine phosphorylations initiated following ligand binding to the pre-TCR or TCR complexes. The phosphorylation cascades are primarily orchestrated by two distinct families of protein tyrosine kinases (PTKs), the Src- and the Syk/ZAP-70-families. Germline gene targeting experiments, several human immunodeficiencies, and somatic cell mutants have all contributed to our understanding of how these families of kinases coordinate their actions to promote signaling. Upon activation, the PTKs transmit their signals to a number of newly described adaptor proteins including LAT, SLP-76, and vav, among others. The following review combines results derived from different experimental strategies to examine the contributions of the PTKs and the adaptor molecules to pre-TCR and TCR signaling processes.

Differential CD3ζ Phosphorylation Is Not Required for the Induction of T Cell Antagonism by Altered Peptide Ligands

T cells recognize foreign Ags in the form of short peptides bound to MHC molecules. Ligation of the TCR:CD3 complex gives rise to the generation of two tyrosine-phosphorylated forms of the CD3 ζ-chain, pp21 and pp23. Replacement of residues in MHC-bound peptides that alter its recognition by the TCR can generate altered peptide ligands (APL) that antagonize T cell responses to the original agonist peptide, leading to altered T cell function and anergy. This biological process has been linked to differential CD3ζ phosphorylation and generation of only the pp21 phospho-species. Here, we show that T cells expressing CD3ζ mutants, which cannot be phosphorylated, exhibit a 5-fold reduction in IL-2 production and a 30-fold reduction in sensitivity following stimulation with an agonist peptide. However, these T cells are still strongly antagonized by APL. These data demonstrate that: 1) the threshold required for an APL to block a response is much lower than for an agonist peptide to induce a response, 2) CD3ζ is required for full agonist but not antagonist responses, and 3) differential CD3ζ phosphorylation is not a prerequisite for T cell antagonism.

NK cytokine secretion and cytotoxicity occur independently of the SLP-76 adaptor protein

The adapter protein SLP-76 is required for T cell development and TCR signal transduction. SLP-76 is also expressed in NK cells, yet splenic populations of NK cells develop normally in SLP-76-deficient mice. We examined the effects of SLP-76 deficiency upon cellular activation through studies of NK function in SLP-76(-/-) mice. This study presents evidence that NK populations in both spleen and liver of SLP-76(-/-) mice remain intact. Natural cytotoxic responses of SLP-76(-/-) splenocytes proceed in a manner comparable to those of wild-type control splenocytes. Similar to controls, SLP-76(-/-) splenocytes exhibit enhanced survival and augmented cytotoxic capacity after in vitro culture with IL-2. IL-2-stimulated SLP-76(-/-) splenocytes also retain normal antibody-dependent cellular cytotoxicity and the ability to secrete IFN-gamma in response to IL-12 stimulation. These results indicate that, unlike events stimulated by TCR engagement, signaling cascades engaged by IL-2 and IL-12 receptors, by Fc gammaRIIIA (which mediates antibody-dependent cellular cytotoxicity), and by natural cytotoxicity-associated receptors on murine NK cells can occur independently of SLP-76.

Perturbed Regulation of ZAP-70 and Sustained Tyrosine Phosphorylation of LAT and SLP-76 in c-Cbl-Deficient Thymocytes

Recent studies indicate that c-Cbl and its oncogenic variants can modulate the activity of protein tyrosine kinases. This finding is supported by studies showing that c-Cbl interacts directly with a negative regulatory tyrosine in ZAP-70, and that the levels of tyrosine-phosphorylated ZAP-70 and numerous other proteins are increased in TCR-stimulated thymocytes from c-Cbl-deficient mice. Here, we demonstrate that this enhanced phosphorylation of ZAP-70 and that of two substrates, LAT and SLP-76, is not due to altered protein levels but is the consequence of two separate events. First, we find increased expression of tyrosine-phosphorylated TCRζ chain in c-Cbl-deficient thymocytes, which results in a higher level of ζ-chain-associated ZAP-70 that is initially accessible for activation. Thus, more ZAP-70 is activated and more of its substrates (LAT and SLP-76) become tyrosine-phosphorylated after TCR stimulation. However, an additional mechanism of ZAP-70 regulation is evident at a later time poststimulation. At this time, ZAP-70 from both normal and c-Cbl−/− thymocytes becomes hyperphosphorylated; however, only in normal thymocytes does this correlate with ZAP-70 down-regulation and a diminished ability to phosphorylate LAT and SLP-76. In contrast, c-Cbl-deficient thymocytes display altered phosphorylation kinetics, for which LAT phosphorylation is increased and SLP-76 phosphorylation is sustained. Thus, the ability to down-regulate the phosphorylation of two ZAP-70 substrates is impaired in c-Cbl−/− thymocytes. These findings provide evidence that c-Cbl is involved in the negative regulation of the phosphorylation of LAT and SLP-76 by ZAP-70.

Integration of T cell receptor-dependent signaling pathways by adapter proteins

The initiation of biochemical signal transduction following ligation of surface receptors with intrinsic cytoplasmic tyrosine kinase activity is common for many cell types. T lymphocytes also require activation of tyrosine kinases following T cell receptor (TCR) ligation for maximal stimulation. However, the TCR has no intrinsic tyrosine kinase activity. Instead, the TCR must rely on cytoplasmic tyrosine kinases that localize to the TCR complex and initiate TCR-mediated signaling events. Although much has been learned regarding how these cytosolic tyrosine kinases are activated and recruited to the TCR complex, relatively little is understood about how these initial events are translated into transcriptional activation of genes that regulate cytokine production, cell proliferation, and cell death. Recently, it has become clear that the class of intracellular molecules known collectively as adapter proteins, molecules with modular domains capable of recruiting additional proteins but that exhibit no intrinsic enzymatic activity, serve to couple proximal biochemical events initiated by TCR ligation with more distal signaling pathways.

Selection of the T cell repertoire

Advances in gene technology have allowed the manipulation of molecular interactions that shape the T cell repertoire. Although recognized as fundamental aspects of T lymphocyte development, only recently have the mechanisms governing positive and negative selection been examined at a molecular level. Positive selection refers to the active process of rescuing MHC-restricted thymocytes from programmed cell death. Negative selection refers to the deletion or inactivation of potentially autoreactive thymocytes. This review focuses on interactions during thymocyte maturation that define the T cell repertoire, with an emphasis placed on current literature within this field.