Signals involved in thymocyte positive and negative selection

The early proximal αβ TCR signalosome specifies thymic selection outcome through a quantitative protein interaction network

During αβ T cell development, T cell antigen receptor (TCR) engagement transduces biochemical signals through a protein-protein interaction (PPI) network that dictates dichotomous cell fate decisions. It remains unclear how signal specificity is communicated, instructing either positive selection to advance cell differentiation or death by negative selection. Early signal discrimination might occur by PPI signatures differing qualitatively (customized, unique PPI combinations for each signal), quantitatively (graded amounts of a single PPI series), or kinetically (speed of PPI pathway progression). Using a novel PPI network analysis, we found that early TCR-proximal signals distinguishing positive from negative selection appeared to be primarily quantitative in nature. Furthermore, the signal intensity of this PPI network was used to find an antigen dose that caused a classic negative selection ligand to induce positive selection of conventional αβ T cells, suggesting that the quantity of TCR triggering was sufficient to program selection outcome. Because previous work had suggested that positive selection might involve a qualitatively unique signal through CD3δ, we reexamined the block in positive selection observed in CD3δ⁰ mice. We found that CD3δ⁰ thymocytes were inhibited but capable of signaling positive selection, generating low numbers of MHC-dependent αβ T cells that expressed diverse TCR repertoires and participated in immune responses against infection. We conclude that the major role for CD3δ in positive selection is to quantitatively boost the signal for maximal generation of αβ T cells. Together, these data indicate that a quantitative network signaling mechanism through the early proximal TCR signalosome determines thymic selection outcome.

TCR ITAM multiplicity is required for the generation of follicular helper T-cells

The T-cell antigen receptor (TCR) complex contains 10 copies of a di-tyrosine Immunoreceptor-Tyrosine-based-Activation-Motif (ITAM) that initiates TCR signalling by recruiting protein tyrosine kinases. ITAM multiplicity amplifies TCR signals, but the importance of this capability for T-cell responses remains undefined. Most TCR ITAMs (6 of 10) are contributed by the CD3ζ subunits. We generated 'knock-in' mice that express non-signalling CD3ζ chains in lieu of wild-type CD3ζ. Here we demonstrate that ITAM multiplicity is important for the development of innate-like T-cells and follicular helper T-cells, events that are known to require strong/sustained TCR-ligand interactions, but is not essential for 'general' T-cell responses including proliferation and cytokine production or for the generation of a diverse antigen-reactive TCR repertoire.

Strict Major Histocompatibility Complex Molecule Class-Specific Binding by Co-Receptors Enforces MHC-Restricted αβ TCR Recognition during T Lineage Subset Commitment

Since the discovery of co-receptor dependent αβTCR recognition, considerable effort has been spent on elucidating the basis of CD4 and CD8 lineage commitment in the thymus. The latter is responsible for generating mature CD4 helper and CD8αβ cytotoxic T cell subsets. Although CD4⁺ and CD8⁺ T cell recognition of peptide antigens is known to be MHC class II- and MHC class I-restricted, respectively, the mechanism of single positive (SP) thymocyte lineage commitment from bipotential double-positive (DP) progenitors is not fully elucidated. Classical models to explain thymic CD4 vs. CD8 fate determination have included a stochastic selection model or instructional models. The latter are based either on strength of signal or duration of signal impacting fate. More recently, differential co-receptor gene imprinting has been shown to be involved in expression of transcription factors impacting cytotoxic T cell development. Here, we address commitment from a structural perspective, focusing on the nature of co-receptor binding to MHC molecules. By surveying 58 MHC class II and 224 MHC class I crystal structures in the Protein Data Bank, it becomes clear that CD4 cannot bind to MHC I molecules, nor can CD8αβ or CD8αα bind to MHC II molecules. Given that the co-receptor delivers Lck to phosphorylate exposed CD3 ITAMs within a peptide/MHC (pMHC)-ligated TCR complex to initiate cell signaling, this strict co-receptor recognition fosters MHC class-restricted SP thymocyte lineage commitment at the DP stage even though both co-receptors are expressed on a single cell. In short, the binding preference of an αβTCR for a peptide complexed with an MHC molecule dictates which co-receptor subsequently binds, thereby supporting development of that subset lineage. How function within the lineage is linked further to biopotential fate determination is discussed.

A conserved hydrophobic patch on Vβ domains revealed by TCRβ chain crystal structures: Implications for pre-TCR dimerization

The αβ T cell receptor (TCR) is a multimeric complex whose β chain plays a crucial role in thymocyte development as well as antigen recognition by mature T lymphocytes. We report here crystal structures of individual β subunits, termed N15β (Vβ5.2Dβ2Jβ2.6Cβ2) and N30β (Vβ13Dβ1Jβ1.1Cβ2), derived from two αβ TCRs specific for the immunodominant vesicular stomatitis virus octapeptide (VSV-8) bound to the murine H-2K^b MHC class I molecule. The crystal packing of the N15β structure reveals a homodimer formed through two Vβ domains. The Vβ/Vβ module is topologically very similar to the Vα/Vβ module in the N15αβ heterodimer. By contrast, in the N30β structure, the Vβ domain’s external hydrophobic CFG face is covered by the neighboring molecule’s Cβ domain. In conjunction with systematic investigation of previously published TCR single-subunit structures, we identified several conserved residues forming a concave hydrophobic patch at the center of the CFG outer face of the Vβ and other V-type Ig-like domains. This hydrophobic patch is shielded from solvent exposure in the crystal packing, implying that it is unlikely to be thermodynamically stable if exposed on the thymocyte surface. Accordingly, we propose a dimeric pre-TCR model distinct from those suggested previously by others and discuss its functional and structural implications.

Kinase suppressor of Ras 1 is required for full ERK activation in thymocytes but not for thymocyte selection

The scaffold protein kinase suppressor of Ras 1 (KSR1) is critical for efficient activation of ERK in a number of cell types. Consistent with this, we observed a defect in ERK activation in thymocytes that lack KSR1. Interestingly, we found that the defect was much greater after PMA stimulation than by CD3 activation. Since ERK activation is believed to be important for thymocyte development, we analyzed thymocyte selection in KSR1-deficient (KSR1(-/-) ) mice. We found that positive selection in two different TCR transgenic models, HY and AND, was normal. On the other hand, negative selection in the HY model was slightly impaired in KSR1(-/-) mice. However, a defect in negative selection was not apparent in the AND TCR model system or in an endogenous superantigen-mediated model of negative selection. These results suggest that, despite a requirement for KSR1 for full ERK activation in thymocytes, full and efficient ERK activation is not essential for the majority of thymocyte selection events.

The essential role of LAT in thymocyte development during transition from the double-positive to single-positive stage

The linker for activation of T cells (LAT) is an adaptor protein that couples TCR engagement to downstream signaling cascades. LAT is important in early thymocyte development as LAT-deficient mice have a complete block at the double-negative (DN) 3 stage. To study the role of LAT beyond the DN3 stage, we generated mice in which the lat gene could be deleted by the Cre recombinase. Analysis of these mice showed that deletion of LAT after the DN3 stage allowed thymocytes to develop past the DN3 to DN4 checkpoint and to generate double-positive thymocytes. However, LAT-deficient DP thymocytes were severely defective in responding to stimulation via the TCR and failed to differentiate into single-positive thymocytes efficiently. Consequently, few LAT-deficient mature T cells could be found in the periphery. These T cells had undergone extensive homeostatic proliferation and expressed low levels of the TCR on their surface. Collectively, our data indicate that in addition to its role in pre-TCR signaling, LAT also plays an essential role in thymocyte development during transition from the double-positive to single-positive stage.

The RhoA transcriptional program in pre-T cells

The GTPase RhoA is essential for the development of pre-T cells in the thymus. To investigate the mechanisms used by RhoA to control thymocyte development we have used Affymetrix gene profiling to identify RhoA regulated genes in T cell progenitors. The data show that RhoA plays a specific and essential role in pre-T cells because it is required for the expression of transcription factors of the Egr-1 and AP-1 families that have critical functions in thymocyte development. Loss of RhoA function in T cell progenitors causes a developmental block that pheno-copies the consequence of losing pre-TCR expression in Recombinase gene 2 (Rag2) null mice. Transcriptional profiling reveals both common and unique gene targets for RhoA and the pre-TCR indicating that RhoA participates in the pre-TCR induced transcriptional program but also mediates pre-TCR independent gene transcription.

Adhesion- and degranulation-promoting adapter protein is required for efficient thymocyte development and selection

Adhesion- and degranulation-promoting adapter protein (ADAP) is required in TCR-induced activation and proliferation of peripheral T cells. Loss of ADAP also impairs TCR-initiated inside-out activation of the integrin LFA-1 (CD11a/CD18, alphaLbeta2). In this study, we demonstrate that ADAP-deficient CD4/CD8 double-positive (DP) cells have a diminished ability to proliferate, and that these DP thymocytes up-regulate CD69 poorly in vivo. Moreover, in both MHC class I- and class II-restricted TCR transgenic models, loss of ADAP interferes with both positive and negative selection. ADAP deficiency also impairs the ability of transgene-bearing DP thymocytes to form conjugates with Ag-loaded presenting cells. These findings suggest that ADAP is critical for thymocyte development and selection.

Immunity by ubiquitylation: a reversible process of modification

Conjugation of ubiquitin to a protein substrate provides a tag that either marks the labelled protein for degradation or modulates its function.

The process of ubiquitylation, which is catalysed by coordinated enzymatic reactions that require enzymes known as E1, E2 and E3, has an important role in the modulation of immune responses.

Immune tolerance is induced in the thymus and the periphery through diverse mechanisms, and E3 ligases are involved in thymic antigen presentation, T-cell anergy and follicular B helper T-cell development.

The immunological defect in mice with a disrupted itchy (Itch) locus results from a defect in degradation of the transcription factor JUNB. This process is tightly regulated by upstream protein kinases that modulate the activity of the E3 ligase ITCH rather than directly affect JUNB, as commonly thought.

Nuclear factor-κB (NF-κB) signalling is crucial for both innate and adaptive immunity and is regulated by K48 (Lys48)-linked polyubiquitylation (which targets inhibitor of NF-κB (IκB) for proteasomal-dependent degradation), K63-linked polyubiquitylation (which activates IκB kinase, IKK) and A20-mediated de-ubiquitylation.

E3 ligases also regulate other cytokine-induced cellular responses, such as transforming-growth-factor-β-mediated signalling and interferon (IFN)-triggered gene expression.

The ubiquitin-like molecule ISG15 (IFN-stimulated protein of 15 kDa) participates in IFN-mediated signalling, and defects in de-ISGylation result in resistance to viral infection.

The conjugation of ubiquitin, a 76-amino-acid peptide, to a protein substrate provides a tag that either marks the labelled protein for degradation or modulates its function. The process of protein ubiquitylation — which is catalysed by coordinated enzymatic reactions that are mediated by enzymes known as E1, E2 and E3 — has an important role in the modulation of immune responses. Importantly, protein ubiquitylation is a reversible process, and removal of ubiquitin molecules is mediated by de-ubiquitylating enzymes: for example, A20, which has been implicated in the regulation of immune responses. In addition, the conjugation of ubiquitin-like molecules, such as ISG15 (interferon-stimulated protein of 15 kDa), to proteins is also involved in immune regulation. This Review covers recent progress in our understanding of protein ubiquitylation in the immune system.

Premature Expression of Chemokine Receptor CCR9 Impairs T Cell Development

During thymocyte development, CCR9 is expressed on late CD4-CD8- (double-negative (DN)) and CD4+CD8+ (double-positive) cells, but is subsequently down-regulated as cells transition to the mature CD4+ or CD8+ (single-positive (SP)) stage. This pattern of expression has led to speculation that CCR9 may regulate thymocyte trafficking and/or export. In this study, we generated transgenic mice in which CCR9 surface expression was maintained throughout T cell development. Significantly, forced expression of CCR9 on mature SP thymocytes did not inhibit their export from the thymus, indicating that CCR9 down-regulation is not essential for thymocyte emigration. CCR9 was also expressed prematurely on immature DN thymocytes in CCR9 transgenic mice. Early expression of CCR9 resulted in a partial block of development at the DN stage and a marked reduction in the numbers of double-positive and SP thymocytes. Moreover, in CCR9-transgenic mice, CD25high DN cells were scattered throughout the cortex rather than confined to the subcapsular region of the thymus. Together, these results suggest that regulated expression of CCR9 is critical for normal development of immature thymocytes, but that down-regulation of CCR9 is not a prerequisite for thymocyte emigration.

Rescuing developing thymocytes from death by neglect

The major function of the thymus is to eliminate developing thymocytes that are potentially useless or autoreactive, and select only those that bear functional T cell antigen receptors (TCRs) through fastidious screening. It is believed that glucocorticoids (GCs) are at least in part responsible for cell death during death by neglect. In this review, we will mainly cover the topic of the GC-induced apoptosis of developing thymocytes. We will also discuss how thymocytes that are fated to die by GCs can be rescued from GC-induced apoptosis in response to a variety of signals with antagonizing properties for GC receptor (GR) signaling. Currently, a lot of evidence supports the notion that the decision is made as a result of the integration of the multiple signal transduction networks that are triggered by GR, TCR, and Notch. A few candidate molecules at the converging point of these multiple signaling pathyways will be discussed. We will particularly describe the role of the SRG3 protein as a potent modulator of GC-induced apoptosis in the crosstalk.

Mutation of the phospholipase C-gamma1-binding site of LAT affects both positive and negative thymocyte selection

Linker for activation of T cells (LAT) is a scaffolding adaptor protein that is critical for T cell development and function. A mutation of LAT (Y136F) that disrupts phospholipase C-γ1 activation and subsequent calcium influx causes a partial block in T cell development and leads to a severe lymphoproliferative disease in homozygous knock-in mice. One possible contribution to the fatal disease of LAT Y136F knock-in mice could be from autoreactive T cells generated in these mice because of altered thymocyte selection. To examine the impact of the LAT Y136F mutation on thymocyte positive and negative selection, we bred this mutation onto the HY T cell receptor (TCR) transgenic, recombination activating gene-2 knockout background. Female mice with this genotype showed a severe defect in positive selection, whereas male mice exhibited a phenotype resembling positive selection (i.e., development and survival of CD8^hi HY TCR-specific T cells) instead of negative selection. These results support the hypothesis that in non-TCR transgenic, LAT Y136F knock-in mice, altered thymocyte selection leads to the survival and proliferation of autoreactive T cells that would otherwise be negatively selected in the thymus.

Ligation of retinoic acid receptor alpha regulates negative selection of thymocytes by inhibiting both DNA binding of nur77 and synthesis of bim

Negative selection refers to the selective deletion of autoreactive thymocytes. Its molecular mechanisms have not been well defined. Previous studies in our laboratory have demonstrated that retinoic acids, physiological ligands for the nuclear retinoid receptors, selectively inhibit TCR-mediated death under in vitro conditions, and the inhibition is mediated via the retinoic acid receptor (RAR) alpha. The present studies were undertaken to investigate whether ligation of RARalpha leads to inhibition of TCR-mediated death in vivo and to identify the molecular mechanisms involved. Three models of TCR-mediated death were studied: anti-CD3-mediated death of thymocytes in wild-type mice, and Ag- and bacterial superantigen-driven thymocyte death in TCR-transgenic mice expressing a receptor specific for a fragment of pigeon cytochrome c in the context of the E(k) (class II MHC) molecule. Our data demonstrate that the molecular program of both anti-CD3- and Ag-driven, but not that of superantigen-mediated apoptosis involves up-regulation of nur77, an orphan nuclear receptor, and bim, a BH3-only member of the proapoptotic bcl-2 protein family, proteins previously implicated to participate in the negative selection. Ligation of RARalpha by the synthetic agonist CD336 inhibited apoptosis, DNA binding of nur77, and synthesis of bim induced by anti-CD3 or the specific Ag, but had no effect on the superantigen-driven cell death. Our data imply that retinoids are able to inhibit negative selection in vivo as well, and they interfere with multiple steps of the T cell selection signal pathway.

A role for the alpha-chain connecting peptide motif in mediating TCR-CD8 cooperation

To generate peripheral T cells that are both self-MHC restricted and self-MHC tolerant, thymocytes are subjected to positive and negative selection. How the TCR discriminates between positive and negative selection ligands is not well understood, although there is substantial evidence that the CD4 and CD8 coreceptors play an important role in this cell fate decision. We have previously identified an evolutionarily conserved motif in the TCR, the alpha-chain connecting peptide motif (alpha-CPM), which allows the TCR to deliver positive selection signals. Thymocytes expressing alpha-CPM-deficient receptors do not undergo positive selection, whereas their negative selection is not impaired. In this work we studied the ligand binding and receptor function of alpha-CPM-deficient TCRs by generating T cell hybridomas expressing wild-type or alpha-CPM-deficient forms of the T1 TCR. This K(d)-restricted TCR is specific for a photoreactive derivative of the Plasmodium berghei circumsporozoite peptide(252-260) IASA-YIPSAEK(ABA)I and is therefore amenable to TCR photoaffinity labeling. The experiments presented in this work show that alpha-CPM-deficient TCRs fail to cooperate with CD8 to enhance ligand binding and functional responses.

Diacylglycerol kinase alpha activity promotes survival of CD4+ 8+ double positive cells during thymocyte development

The diacylglycerol kinases (DGK) form a family of isoenzymes that catalyse the conversion of diacylglycerol (DAG) to phosphatidic acid (PA), both powerful second messengers in the cell. DGKalpha is expressed in brain, peripheral T cells and thymocytes and has been shown to translocate to the nuclear matrix upon T-cell receptor (TCR) engagement. Here, we show that high level expression of DGKalpha is induced following a signal transmitted through the pre-TCR and the protein tyrosine kinase, lck. Activity of DGKalpha contributes to survival in CD4+ 8+ (DP) thymocytes as pharmacological inhibition of DGK activity results in death of this cell population both in cell suspension and thymic explants. DGKalpha promotes survival in these thymocytes through a Bcl-regulated pathway. A consequence of inhibition of DGKalpha is the specific down-regulation of Bcl-xl, whereas in transgenic mice that over-express Bcl-2, death induced by the inhibitor is partially blocked. Thus we report a novel activity of DGKalpha in survival of thymocytes immediately after entry into the DP stage in development.

Requirement for Ca2+/calmodulin-dependent kinase type IV/Gr in setting the thymocyte selection threshold

The outcome of thymocyte selection is influenced by the nature of Ca2+ signals transduced by the TCR. Robust Ca2+ responses characterize high-affinity, negatively selecting peptide/TCR interactions, while modest responses typify lower-affinity, positively selecting interactions. To elucidate mechanisms by which thymocytes decode distinct Ca2+ signals, we examined selection events in mice lacking Ca2+/calmodulin-dependent protein kinase type IV/Gr (CaMKIV/Gr), which is enriched in thymocytes. CaMKIV/Gr-deficient thymocytes exhibited impaired positive selection and defective Ca2+-dependent gene transcription. Significantly, CaMKIV/Gr deficiency raised the selection threshold of peptide/TCR interactions such that a peptide that normally induced weak negative selection instead promoted positive selection. These results demonstrate an important role for CaMKIV/Gr in sensitizing thymocytes to selection by low-affinity peptides.

Tumor necrosis factor-related apoptosis-inducing ligand in T cell development: sensitivity of human thymocytes

TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) is a recently identified member of the tumor necrosis factor cytokine superfamily. TRAIL has been shown to induce apoptosis in various tumor cell lines, whereas most primary cells seem to be resistant. These observations have raised considerable interest in the use of TRAIL in tumor therapy. Yet little is known about the physiological function of TRAIL. This is particularly the case in the immune system, where TRAIL has been suggested by some to be involved in target cell killing and lymphocyte death. We have developed a panel of mAbs and soluble proteins to address the role of TRAIL in lymphocyte development. These studies demonstrate activation-induced sensitization of thymocytes to TRAIL-mediated apoptosis and expression of the apoptosis-inducing TRAIL receptors. However, with the use of several model systems, our subsequent experiments rule out the possibility that TRAIL plays a major role in antigen-induced deletion of thymocytes. In contrast to thymocytes, there is no up-regulation of TRAIL receptors in peripheral T cells on activation, which remain resistant to TRAIL. Thus, susceptibility to TRAIL-induced apoptosis is controlled differently by central and peripheral T cells.

Glucocorticoids attenuate T cell receptor signaling

Glucocorticoids (GCs) affect peripheral immune responses by inhibiting T cell immunity at several stages of the activation cascade, causing impaired cytokine production and effector function. The recent demonstration that the thymic epithelium and possibly thymocytes themselves produce steroids suggests that endogenous GCs also play a role in the control of T cell development. As both peripheral responsiveness and thymic differentiation appear to be regulated by the quantity and quality of intracellular signals issued by antigen-major histocompatibility complex-engaged T cell receptor (TCR) complexes, we investigated the effects of GCs on the signaling properties of T cells stimulated by anti-CD3 monoclonal antibodies or agonist peptides. We demonstrate in this work that dexamethasone, a synthetic GC, inhibits the early signaling events initiated upon TCR ligation, such as tyrosine phosphorylation of several TCR-associated substrates including the zeta chain, the ZAP70 kinase, and the transmembrane adapter molecule linker for activation of T cells. Hypophosphorylation was not a consequence of reduced kinase activity of src protein tyrosine kinases, but was correlated with an altered- membrane compartmentalization of these molecules. These observations indicate that in addition to their well-described ability to interfere with the transcription of molecules involved in peripheral responses, GCs inhibit T cell activation by affecting the early phosphorylating events induced after TCR ligation.

The lack of NF-kappa B transactivation and PKC epsilon expression in CD4(+)CD8(+) thymocytes correlates with negative selection

Deletion of autoreactive thymocytes at the DP stage is the basis for tolerance to thymus-expressed self antigens. In this study we investigated whether distinct signalling pathways are induced in DP thymocytes as compared to mature T cells upon stimulation with antigen. Using triple transgenic mice expressing a TCR transgene, dominant negative ras/Mek proteins and a reporter gene construct with AP-1 or NF-kappa B binding sites, we showed a complete lack of transcriptional activity of NF-kappa B but not AP-1 in DP thymocytes, whereas both were transcriptionally active in mature T cells after antigenic stimulation. Lack of NF-kappa B induction correlated with increased death in response to antigen. AP-1 induction was dependent on the integrity of the ras/Mek pathway indicating that this pathway was activated in the DP thymocytes. In contrast, we found a complete lack of constitutive expression of the epsilon isoform of Protein Kinase C (PKC) in DP thymocytes, although it was present in mature thymocytes and peripheral T cells. Taken together the results suggest that the lack of PKC epsilon in DP thymocytes could lead to the absence of NF-kappa B activity after antigenic stimulation contributing to negative selection. Cell Death and Differentiation (2000) 7, 1253 - 1262.

Immature CD4+CD8+ thymocytes do not polarize lipid rafts in response to TCR-mediated signals

TCR-mediated stimulation induces activation and proliferation of mature T cells. When accompanied by signals through the costimulatory receptor CD28, TCR signals also result in the recruitment of cholesterol- and glycosphingolipid-rich membrane microdomains (lipid rafts), which are known to contain several molecules important for T cell signaling. Interestingly, immature CD4(+)CD8(+) thymocytes respond to TCR/CD28 costimulation not by proliferating, but by dying. In this study, we report that, although CD4(+)CD8(+) thymocytes polarize their actin cytoskeleton, they fail to recruit lipid rafts to the site of TCR/CD28 costimulation. We show that coupling of lipid raft mobilization to cytoskeletal reorganization can be mediated by phosphoinositide 3-kinase, and discuss the relevance of these findings to the interpretation of TCR signals by immature vs mature T cells.

Competitive displacement of pT alpha by TCR-alpha during TCR assembly prevents surface coexpression of pre-TCR and alpha beta TCR

During alphabeta T cell development, CD4(-)CD8(-) thymocytes first express pre-TCR (pTalpha/TCR-beta) before their differentiation to the CD4(+)CD8(+) stage. Positive selection of self-tolerant T cells is then determined by the alphabeta TCR expressed on CD4(+)CD8(+) thymocytes. Conceivably, an overlap in surface expression of these two receptors would interfere with the delicate balance of thymic selection. Therefore, a mechanism ensuring the sequential expression of pre-TCR and TCR must function during thymocyte development. In support of this notion, we demonstrate that expression of TCR-alpha by immature thymocytes terminates the surface expression of pre-TCR. Our results reveal that expression of TCR-alpha precludes the formation of pTalpha/TCR-beta dimers within the endoplasmic reticulum, leading to the displacement of pre-TCR from the cell surface. These findings illustrate a novel posttranslational mechanism for the regulation of pre-TCR expression, which may ensure that alphabeta TCR expression on thymocytes undergoing selection is not compromised by the expression of pre-TCR.

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.

TCR-independent T cell development mediated by gain-of-oncogene function or loss-of-tumor-suppressor gene function

The mechanisms that govern differentiation of T cell precursors during intrathymic development bridge an interdisciplinary research field of immunology, oncology and developmental biology. Critical checkpoints controlling early thymic T cell development and homeostasis are set by the proper signaling function of the IL-7 receptor, c-Kit receptor, and the pre-T cell antigen receptor (pre-TCR). Given the intimate link between cell cycle control and differentiation in T cell development, proto-oncogenes and tumor suppressors participate as physiological effectors downstream of these receptors not only to influence the cell cycle but also to determine differentiation and survival. Gain- or loss-of-function mutations of these downstream effectors uncouples partially or completely T cell precursors from these checkpoints, providing a selective advantage and enabling aberrant development. These effectors can be identified by provirus tagging in normal mice and more readily by complementation tagging in mice with a predefined block in T cell differentiation.

Thymic selection generates T cells expressing self-reactive TCRs in the absence of CD45

The CD45 protein tyrosine phosphatase regulates Ag receptor signaling in T and B cells. In the absence of CD45, TCR coupling to downstream signaling cascades is profoundly reduced. Moreover, in CD45-null mice, the maturation of CD4+CD8+ thymocytes into CD4+CD8- or CD4-CD8+ thymocytes is severely impaired. These findings suggest that thymic selection may not proceed normally in CD45-null mice, and may be biased in favor of thymocytes expressing TCRs with strong reactivity toward self-MHC-peptide ligands to compensate for debilitated TCR signaling. To test this possibility, we purified peripheral T cells from CD45-null mice and fused them with the BWalpha-beta- thymoma to generate hybridomas expressing normal levels of TCR and CD45. The reactivity of these hybridomas to self or foreign MHC-peptide complexes was assessed by measuring the amount of IL-2 secreted upon stimulation with syngeneic or allogeneic splenocytes. A very high proportion (55%) of the hybridomas tested reacted against syngeneic APCs, indicating that the majority of T cells in CD45-null mice express TCRs with high avidity for self-MHC-peptide ligands, and are thus potentially autoreactive. Furthermore, a large proportion of TCRs selected in CD45-null mice (H-2b) were also shown to display reactivity toward closely related MHC-peptide complexes, such as H-2bm12. These results support the notion that modulating the strength of TCR-mediated signals can alter the outcome of thymic selection, and demonstrate that CD45, by molding the window of affinity/avidity for positive and negative selection, directly participates in the shaping of the T cell repertoire.

A motif in the alphabeta T-cell receptor controls positive selection by modulating ERK activity

Positive selection allows thymocytes that recognize an individual's own major histocompatibility complex (self-MHC) molecules to survive and differentiate, whereas negative selection removes overtly self-reactive thymocytes. Although both forms of thymic selection are mediated by the alphabeta T-cell receptor (TCR) and require self-MHC recognition, an important question is whether they are controlled by distinct signalling cascades. We have shown that mutation of an essential motif within the TCR alpha-chain-connecting peptide domain (alpha-CPM) profoundly affects positive but not negative selection. Using transgenic mice expressing a mutant alpha-CPM TCR we examined the contribution of several mitogen-activated protein kinase (MAPK) cascades to thymic selection. Here we show that in thymocytes expressing a mutant alpha-CPM receptor, a positively selecting peptide failed to activate the extracellular signal-regulated kinase (ERK), although other MAPK cascades were induced normally. The defect in ERK activation was associated with impaired recruitment of the activated tyrosine kinases Lck and ZAP-70, phosphorylated forms of the TCR component CD3zeta and the adaptor protein LAT to detergent-insoluble glycolipid-enriched microdomains (DIGs). Therefore, an intact DIG-associated signalosome is essential for sustained ERK activation, which leads to positive selection.

Function of PI3Kgamma in thymocyte development, T cell activation, and neutrophil migration

Phosphoinositide 3-kinases (PI3Ks) regulate fundamental cellular responses such as proliferation, apoptosis, cell motility, and adhesion. Viable gene-targeted mice lacking the p110 catalytic subunit of PI3Kgamma were generated. We show that PI3Kgamma controls thymocyte survival and activation of mature T cells but has no role in the development or function of B cells. PI3Kgamma-deficient neutrophils exhibited severe defects in migration and respiratory burst in response to heterotrimeric GTP-binding protein (G protein)-coupled receptor (GPCR) agonists and chemotactic agents. PI3Kgamma links GPCR stimulation to the formation of phosphatidylinositol 3,4,5-triphosphate and the activation of protein kinase B, ribosomal protein S6 kinase, and extracellular signal-regulated kinases 1 and 2. Thus, PI3Kgamma regulates thymocyte development, T cell activation, neutrophil migration, and the oxidative burst.