Positive selection of T cells

Cellular and molecular requirements for the selection of in vitro-generated CD8 T cells reveal a role for Notch

Differentiation of CD8 single-positive (SP) T cells is predicated by the ability of lymphocyte progenitors to integrate multiple signaling cues provided by the thymic microenvironment. In the thymus and the OP9-DL1 system for T cell development, Notch signals are required for progenitors to commit to the T cell lineage and necessary for their progression to the CD4(+)CD8(+) double-positive (DP) stage of T cell development. However, it remains unclear whether Notch is a prerequisite for the differentiation of DP cells to the CD8 SP stage of development. In this study, we demonstrate that Notch receptor-ligand interactions allow for efficient differentiation and selection of conventional CD8 T cells from bone marrow-derived hematopoietic stem cells. However, bone marrow-derived hematopoietic stem cells isolated from Itk(-/-)Rlk(-/-) mice gave rise to T cells with decreased IFN-γ production, but gained the ability to produce IL-17. We further reveal that positive and negative selection in vitro are constrained by peptide-MHC class I expressed on OP9 cells. Finally, using an MHC class I-restricted TCR-transgenic model, we show that the commitment of DP precursors to the CD8 T cell lineage is dependent on Notch signaling. Our findings further establish the requirement for Notch receptor-ligand interactions throughout T cell differentiation, including the final step of CD8 SP selection.

Molecular cloning and characterisation of the rock bream, Oplegnathus fasciatus, Fas (CD95/APO-1), and its expression analysis in response to bacterial or viral infection

Fas belongs to the tumour necrosis factor (TNF) receptor superfamily and can transmit a death signal leading to apoptosis. In the present study, we isolated the full-length cDNA for rock bream (Oplegnathus fasciatus) Fas (RbFas). The full-length RbFas cDNA was 1770 bp long and contained an open reading frame of 957 bp that encoded 319 amino acid residues with a predicted molecular mass of 35.1 kDa. The 319 amino-acid predicted RbFas sequence is homologous to other Fas sequences, contains three cysteine-rich domains and a death domain (DD) and two potential N-glycosylation sites. Expression of RbFas mRNA was detected in nine different tissues from healthy rock bream and was the highest in red blood cells. In analyses of mitogen-stimulated RbFas expression in peripheral blood leucocytes, expression of RbFas mRNA was observed between 1 and 36 h after stimulation with LPS, and 1 and 3 h stimulation with poly I:C. In the case of bacterial injection, the RbFas transcript peaked 6 h after injection in both the kidney and the spleen. Otherwise, the RbFas transcript peaked after 1 h in spleen and 6 h in kidney following injection with RSIV.

Impaired thymic selection and abnormal antigen-specific T cell responses in Foxn1(Δ/Δ) mutant mice

Background

Foxn1^Δ/Δ mutant mice have a specific defect in thymic development, characterized by a block in TEC differentiation at an intermediate progenitor stage, and blocks in thymocyte development at both the DN1 and DP cell stages, resulting in the production of abnormally functioning T cells that develop from an atypical progenitor population. In the current study, we tested the effects of these defects on thymic selection.

Methodology/Principal Findings

We used Foxn1^Δ/Δ; DO11 Tg and Foxn1^Δ/Δ; OT1 Tg mice as positive selection and Foxn1^Δ/Δ; MHCII I-E mice as negative selection models. We also used an in vivo system of antigen-specific reactivity to test the function of peripheral T cells. Our data show that the capacity for positive and negative selection of both CD4 and CD8 SP thymocytes was reduced in Foxn1^Δ/Δ mutants compared to Foxn1^+/Δ control mice. These defects were associated with reduction of both MHC Class I and Class II expression, although the resulting peripheral T cells have a broad TCR Vβ repertoire. In this deficient thymic environment, immature CD4 and CD8 SP thymocytes emigrate from the thymus into the periphery. These T cells had an incompletely activated profile under stimulation of the TCR signal in vitro, and were either hypersensitive or hyporesponsive to antigen-specific stimulation in vivo. These cell-autonomous defects were compounded by the hypocellular peripheral environment caused by low thymic output.

Conclusions/Significance

These data show that a primary defect in the thymic microenvironment can cause both direct defects in selection which can in turn cause indirect effects on the periphery, exacerbating functional defects in T cells.

TCF1 and beta-catenin regulate T cell development and function

T cell factor-1 (TCF1) critically regulates T cell development. However, signals that control TCF1 function in developing and mature T cells remain unknown. TCF1 along with beta-catenin activates gene transcription and in cooperation with Groucho family of proteins mediates gene repression. It has been established that the beta-catenin-dependent gene expression is often downstream of the canonical Wnt signaling pathway. We have genetically manipulated the beta-catenin gene and generated mutant mice that have shown an essential role for beta-catenin and TCF1 during pre-T cell receptor (TCR) and TCR-dependent stages of T cell development. We have also demonstrated a function for TCF1 and beta-catenin downstream of TCR signaling in the differentiation of mature CD4 T cells into T helper lineages.

Sustained expression of pre-TCR induced beta-catenin in post-beta-selection thymocytes blocks T cell development

Pre-TCR and IL-7R signals regulate beta-selection of thymocytes and then must be down-regulated for further development. However, the molecular events that control down-regulation remain unknown. We and others have previously shown that beta-catenin in cooperation with TCF regulates beta-selection. In this paper, we demonstrate that beta-catenin expression is stringently regulated by intrathymic signals, it is expressed at the highest levels in the pre-TCR signaled thymocytes, and is down-regulated in post-beta-selection thymocytes. Pre-TCR-induced beta-catenin regulates initial stages of pre-TCR signaling including expression of early growth response (Egr) genes but must be down-regulated to express RORgammat, which is essential for maturation to the CD4+CD8+ double positive (DP) stage. Sustained expression of beta-catenin results in the generation of IL-7R-, Egr-, and TGFbeta-expressing pre-DP thymocytes that are blocked in development. These data are consistent with a model in which post-beta-selection, pre-TCR-induced beta-catenin expression must return to background levels for efficient transition to the DP stage.

Pre-TCR-induced beta-catenin facilitates traversal through beta-selection

Pre-TCR induced signals regulate development of the alphabeta TCR lineage cells at the beta-selection checkpoint. We have previously shown that conditional deletion of beta-catenin, a central mediator of Wnt-beta-catenin-T cell factor signaling pathway, impairs traversal through the beta-selection checkpoint. We now provide a molecular basis for the impairment. We demonstrate that pre-TCR signals specifically stabilize beta-catenin in CD4-CD8- double negative thymocytes during beta-selection. Pre-TCR induced Erk activity was required to stabilize beta-catenin. Enforced expression of stabilized beta-catenin was sufficient to mediate aspects of beta-selection including sustained expression of early growth response (Egr) genes. Consistently, deletion of beta-catenin reduced induction of Egr gene expression by the pre-TCR signal and blocked efficient beta-selection. Thus, we demonstrate that pre-TCR induced beta-catenin sustains expression of Egr genes that facilitate traversal through the beta-selection checkpoint.

Molecular cloning, expression, and functional analysis of caspase-10 from Japanese flounder Paralichthys olivaceus

We isolated and sequenced caspase-10 cDNA and gene from Japanese flounder, Paralichthys olivaceus. The Japanese flounder (JF)-caspase-10 cDNA consisted of 2282 bp and encoded 495 amino acid residues. The characteristic death effector domains (DEDs) of caspases were observed in JF-caspase-10 as well as the three aspartic acid residues (D-186, -382 and -392), which are potential cleavage sites for the large and small subunit structures. The amino acid residue (His-325) and pentapeptide (QACQG), which are involved in catalytic activity, were absolutely conserved in Japanese flounder-caspase-10. JF-caspase-10 gene has a length of 6.6 kb and consists of 11 exons and 10 introns similar to that of human. The strong expression of JF-caspase-10 mRNA was detected in the gills, peripheral blood leukocytes, spleen and posterior kidney, while the weak expression was observed in the head kidney, heart, intestine, skin and stomach. The over-expression analysis of JF-caspase-10 in Japanese flounder cell line HINAE was shown to induce apoptosis 24h post-transfection using TUNEL assay.

Beta-catenin expression enhances IL-7 receptor signaling in thymocytes during positive selection

Differentiation of CD4+CD8+ double-positive thymocytes into CD8+ single-positive (SP) thymocytes is regulated by TCR and cytokine receptor signals. Previously, we have shown that expression of stabilized beta-catenin, the major transcriptional cofactor of T cell factor, results in increase in both CD4SP and CD8SP thymocytes with a preferential effect on CD8SP thymocytes. In this report, using mice expressing stabilized beta-catenin and mice with T cell specific deletion of beta-catenin, we show that beta-catenin expression augments IL-7Ralpha-chain expression and down-regulates suppressor of cytokine signaling-1 expression in thymocytes undergoing positive selection. Consequently, beta-catenin expression augments IL-7R signaling in thymocytes during positive selection and promotes the development of CD8SP thymocytes.

Beta-catenin regulates positive selection of thymocytes but not lineage commitment

Positive selection and lineage commitment to the cytolytic or helper lineage of T cells result in coordinated expression of MHC class I-restricted TCR and CD8 coreceptor or MHC class II-restricted TCR and CD4 molecule. Positive selection signals also regulate the survival and generation of requisite numbers of cytolytic or Th cells. beta-Catenin is the major transcriptional cofactor of T cell factor and plays a role in thymocyte development. In this study, using mice expressing stabilized beta-catenin and mice with T cell-specific deletion of beta-catenin, we show that beta-catenin regulates positive selection, but not lineage commitment of thymocytes. Furthermore, beta-catenin expression accelerates the timing of mature CD8 thymocyte generation such that CD4 and CD8 single-positive thymocytes mature with the same kinetics during development.

Molecular cloning, characterization, expression and functional analysis of Japanese flounder Paralichthys olivaceus Fas ligand

We isolated and sequenced Fas ligand cDNA and its gene from Japanese flounder (JF), Paralichthys olivaceus. The JF-Fas ligand cDNA consisted of 1016 bp and encoded 230 amino acid residues. The identities of the deduced amino acid sequence of the JF-Fas ligand to human Fas ligand, Tumor necrosis factor-alpha and Lymphotoxin-alpha were 26.1%, 24.5% and 23.0%, respectively. A proline-rich domain (PRD) that is important for localization of the protein was found in the N-terminal region, and two cysteine residues, which form a disulfide bond, were conserved. The JF-Fas ligand gene has a length of 1.8 kb and consists of four exons and three introns. The length of the JF-Fas ligand second intron is shorter than that in the human and pig Fas ligand genes. However, the organization of the exons and introns is similar to that of mammals. RT-PCR was conducted for 12 tissues, and expression of JF-Fas ligand mRNA was detected in the kidney, thymus, gills, stomach and spleen. The recombinant JF-Fas ligand prepared in an Escherichia coli protein expression system showed cytotoxic activity against Japanese flounder cell line HINAE and caused the fragmentation of genomic DNA. The cytotoxic activity was measured by MTT assay. These results indicate that fish possess a Fas ligand system.

Cytokine signal transduction is suppressed in preselection double-positive thymocytes and restored by positive selection

Death by neglect requires that CD4⁺8⁺ double-positive (DP) thymocytes avoid cytokine-mediated survival signals, which is presumably why DP thymocytes normally extinguish IL-7R gene expression. We report that DP thymocytes before positive selection (preselection DP thymocytes) fail to transduce IL-7 signals even when they express high levels of transgenic IL-7R on their surface, because IL-7R signal transduction is actively suppressed in preselection DP thymocytes by suppressor of cytokine signaling (SOCS)–1. SOCS-1 is highly expressed in preselection DP thymocytes, but it is down-regulated by T cell receptor–mediated positive selection signals. Interestingly, we found that the uniquely small cell volume of DP thymocytes is largely the result of absent IL-7 signaling in preselection DP thymocytes. We also report that, contrary to current concepts, preselection DP thymocytes express high levels of endogenously encoded IL-4Rs. However, their ability to transduce cytokine signals is similarly suppressed by SOCS-1. Thus, despite high surface expression of transgenic or endogenous cytokine receptors, cytokine signal transduction is actively suppressed in preselection DP thymocytes until it is restored by positive selection.

Characterization and expression analysis of the Spink5 gene, the mouse ortholog of the defective gene in Netherton syndrome

The human SPINK5 gene, encoding the putative 15-domain serine protease inhibitor LEKTI, was identified as the defective gene in the severe autosomal recessive ichthyosiform skin disorder known as Netherton syndrome and as a candidate susceptibility gene for atopic disease. Here we report mapping of the murine Spink5 gene to chromosome 18 and its characterization. We show that, unlike in humans, transcription of the mouse Spink5 gene generates two mRNAs that differ in the 3' untranslated region. The encoded protein, which is detected in differentiated primary cultured keratinocytes and mouse skin as an approximately 130-kDa glycosylated precursor, displays approximately 60% identity with its human counterpart but lacks the human LEKTI domain 6. As in the human, mouse Lekti represents a marker of epithelial differentiation, strongly expressed in the granular layer of the epidermis, in suprabasal layers of stratified epithelia, and in thymic Hassall's bodies. Our data indicate that mouse Spink5/Lekti, like its human counterpart, is involved in the control of epithelial tissue homeostasis, but also highlight specific features of the murine gene and protein.

Homeostatic proliferation of a Qa-1b-restricted T cell: a distinction between the ligands required for positive selection and for proliferation in lymphopenic hosts

Naive T cells proliferate in response to self MHC molecules after transfer into lymphopenic hosts, a process that has been termed homeostatic proliferation (HP). Previous studies have demonstrated that HP is driven by low level signaling induced by interactions with the same MHC molecules responsible for positive selection in the thymus. Little is known about the homeostatic regulation of T cells specific for class Ib molecules, including Qa-1 and H2-M3, though it has been suggested that their capacity to undergo homeostatic expansion may be inherently limited. In this study, we demonstrate that naive 6C5 TCR transgenic T cells with specificity for Qa-1(b) have a capacity similar to conventional T cells to undergo HP after transfer into sublethally irradiated mice. Proliferation was largely dependent on the expression of beta(2)-microglobulin, and experiments with congenic recipients expressing Qa-1(a) instead of Qa-1(b) demonstrated that HP is specifically driven by Qa-1(b) and not through cross-recognition of classical class I molecules. Thus, the same MHC molecule that mediates positive selection of 6C5 T cells is also required for HP. Homeostatic expansion, like positive selection, occurs in the absence of a Qa-1 determinant modifier, the dominant self-peptide bound to Qa-1 molecules. However, experiments with TAP(-/-) recipients demonstrate a clear distinction between the ligand requirements for thymic selection and HP. Positive selection of 6C5 T cells is dependent on TAP function, thus selection is presumably mediated by TAP-dependent peptides. By contrast, HP occurs in TAP(-/-) recipients, providing an example where the ligand requirements for HP are less stringent than for thymic selection.

The Role of Dendritic Cells in Selection of Classical and Nonclassical CD8+T Cells In Vivo

T cell development is determined by positive and negative selection events. An intriguing question is how signals through the TCR can induce thymocyte survival and maturation in some and programmed cell death in other thymocytes. This paradox can be explained by the hypothesis that different thymic cell types expressing self-MHC/peptide ligands mediate either positive or negative selection events. Using transgenic mice that express MHC class I (MHC-I) selectively on DC, we demonstrate a compartmentalization of thymic functions and reveal that DC induce CTL tolerance to MHC-I-positive hemopoietic targets in vivo. However, in normal and bone marrow chimeric mice, MHC-I+ DC are sufficient to positively select neither MHC-Ib (H2-M3)- nor MHC-Ia (H2-K)-restricted CD8+ T cells. Thus, thymic DC are specialized in tolerance induction, but cannot positively select the vast majority of MHC-I-restricted CD8+ T cells.

Qa-1, a nonclassical class I histocompatibility molecule with roles in innate and adaptive immunity

Qa-1, a nonclassical class I histocompatibility molecule expressed in mice, predominantly assembles with a single nonameric peptide, Qdm, derived from the signal sequence of certain class Ia molecules. The Qa-1/Qdm complex is the primary ligand for CD94/NKG2A inhibitory receptors expressed on a major fraction of natural killer (NK) cells. Cells become susceptible to killing by NK cells under conditions where surface expression of the Qa-1/Qdm inhibitory ligand is reduced. The CD94/NKG2 "missing-self" recognition system serves as mechanism for removing cells that have abnormalities in the intracellular machinery required for assembly and expression of class I-peptides complexes, as a consequence of viral infection, for example. Despite its highly focused peptide-binding specificity, Qa-1 also has a capacity to act as an antigen-presentation molecule for CD8+ T cells. It appears that a small subpopulation of these T cells undergoes positive selection by interaction with Qa-1 in the thymus, and they maintain their specificity for Qa-1 after maturation. The role of these unusual T cells in adaptive immune responses remains to be defined.

The influence of the thymic environment on the CD4-versus-CD8 T lineage decision

T cell receptor signaling is an essential factor regulating thymocyte selection, but the function of the thymic environment in this process is not clear. In mice transgenic for major histocompatibility complex class II-restricted T cell receptors, every thymocyte is potentially selectable for maturation in the CD4 lineage. To address whether selection frequency affects positive selection, we created hematopoietic chimeras with mixtures of selectable and nonselectable precursors. With increased proportions of nonselectable thymocytes, positive selection of MHC class II-specific precursors was enhanced, generating not only CD4 but also CD8 thymocytes. These results indicate that the CD4 versus CD8 fate of selectable precursors can be influenced by the selection potential of its neighbors.

MHC class II engagement inhibits CD99-induced apoptosis and up-regulation of T cell receptor and MHC molecules in human thymocytes and T cell line

Major histocompatibility complex (MHC) class II surface levels on thymocytes increase after CD99 ligation. The functional implication of the up-regulated MHC class II was assessed by engaging MHC class II on CD99-ligated cells. MHC class II engagement down-modulated surface levels of T cell receptor and MHC molecules, and inhibited apoptosis of CD99-ligated thymocytes and CEM tumor cells, antagonistic effects on the previously reported CD99 functions. The results were reproducible regardless of the order of ligation of MHC class II and CD99. We suggest that signaling via MHC class II on CD99-engaged cells might be involved in the thymic maturation process by damping CD99 ligation effects.

The Ras/MAPK cascade and the control of positive selection

Immature double positive (DP) thymocytes bearing a T cell receptor (TCR) that interacts with self-major histocompatibility complex (MHC) molecules receive signals that induce either their differentiation (positive selection) or apoptosis (negative selection). Furthermore, those cells that are positively selected develop into two different lineages, CD4 or CD8, depending on whether their TCRs bind to MHC class II or I, respectively. Positive selection therefore involves rescue from the default fate (death), lineage commitment, and progression to the single positive (SP) stage. These are probably temporally distinct events that may require both unique and overlapping signals. Work in the past several years has started to unravel the signaling networks that control these processes. One of the first pathways identified as important for positive selection was Ras and its downstream effector, the Erk mitogen-activated protein kinase (MAPK) cascade. In this review we examine the factors that connect the TCR to the Ras/Erk cascade in DP thymocytes, as well as what we know about the downstream effectors of the Ras/Erk cascade important for positive selection. We also consider the possible role of this cascade in CD4/CD8 lineage development, and the possible interactions of the Ras/Erk cascade with Notch during these cell fate determination processes.

The thymus and negative selection

Maintenance of tolerance to self antigens is presumed to reflect a combination of central and peripheral tolerance. For T cells, central tolerance occurs during early T cell development in the thymus and causes cells with strong reactivity to self antigens to be destroyed in situ (negative selection). Here, we summarize evidence that negative selection can occur in the thymic medulla and affects a population of semimature HSA+ T cells. The influence of costimulatory molecules, Fas and cytokines on negative selection is discussed.

Positive selection of a Qa-1-restricted T cell receptor with specificity for insulin

The phenotype and development of T cells from transgenic mice expressing a T cell receptor with specificity for insulin presented by the MHC class Ib molecule Qa-1(b) was investigated. Peripheral T cells from the transgenic mice express CD8 and, after activation, kill Qa-1(b)-positive lymphoid target cells in the presence of soluble insulin. Thymic selection requires expression of Qa-1(b) but not the dominant Qa-1-associated peptide, Qdm. In contrast to conventional T cells, selection is at least as efficient when the selecting ligand is expressed only on hematopoietic lineage cells as compared to expression on epithelial cells in the thymus. Our findings suggest that there is a dedicated population of Qa-1-restricted T cells that are selected by interaction with Qa-1 and that the cellular requirements for selection may differ from conventional T cells.

CD6: expression during development, apoptosis and selection of human and mouse thymocytes

CD6, a 130-kDa surface glycoprotein, is expressed primarily on cells of T lineage. A co-stimulatory role for CD6 in mature T cells has been shown, but the function of CD6 during thymocyte development is unknown. Since CD6 ligands are expressed on thymic epithelium, their interactions with CD6 could be important in thymic selection. In this report we show that CD6 is developmentally regulated in human and mouse thymocytes, and further demonstrate that increase in the level of CD6 expression correlates with expression of the selection marker CD69. We also show that activation via CD2 induces CD6 expression on mature human thymocytes and on a subset of immature human thymocytes that are resistant to apoptosis. In human and mouse thymocytes that express heterogeneous TCR, CD6 increases occur as double-positive thymocytes are selected to a single-positive stage. In contrast, in thymocytes from TCR transgenic mice, CD6 is barely increased following selection, suggesting that as functional avidity increases, requirements for CD6-dependent co-stimulation decrease. Taken together, these results indicate that during thymic development CD6-dependent signals may contribute both to thymocyte survival, and to the overall functional avidity of selection in both man and mouse.

Structural basis of T cell recognition of peptides bound to MHC molecules

Helper T lymphocytes play a critical role in immune system activation following recognition of MHC class II-bound peptide ligands (pMHCII). These CD4 T cells stimulate B cell antibody production and cytolytic T cell generation. Until recently, the structural basis of coordinate T cell receptor (TCR) and CD4 co-receptor interaction with a given pMHCII was unknown. Here we review current structural data on specific pMHCII recognition by T cells and compare TCR and co-receptor docking to pMHCI versus pMHCII ligands. The implications of these findings for thymic selection, helper versus cytolytic T cell recognition and alloreactivity are discussed.

A lesson for unraveling complex aspects of novel immunodeficiencies from the human equivalent of the nude/SCID phenotype

The human equivalent form of the severe combined congenital immunodeficiency of the nude/SCID mouse has been recently described in 2 siblings originating from a small community in southern Italy. Similar to the mouse, the human phenotype is characterized by a profound T cell ontogenetic defect associated with congenital alopecia and nail dystrophy. The disease is related in either mice and humans to molecular alterations of the gene encoding a forkhead/winged helix transcription factor, which is selectively expressed in thymic epithelia. Remarkably, due to the selectivity of the tissue expressivity of the gene responsible for the syndrome, this is the first example of SCID not primarily related to an hematopoietic cell abnormality, but rather to thymic aplasia. The opportunity to study immunological reconstitution in the absence of a functioning thymus following a bone marrow transplantation from a related HLA-identical sibling gave us insight into the role of the thymus in in vivo T cell ontogeny and maintenance of T cell functionality. This model led us to address a few general issues that may have clinical implications.

A naturally processed mitochondrial self-peptide in complex with thymic MHC molecules functions as a selecting ligand for a viral-specific T cell receptor

Peptide fragments of self-proteins bound to major histocompatibility complex molecules within the thymus are important for positively selecting T cell receptor (TCR)-bearing CD4(+)CD8(+) double positive (DP) thymocytes for further maturation. The relationship between naturally processed thymic self-peptides and TCR-specific cognate peptides is unknown. Here we employ HPLC purification of peptides released from H-2K(b) molecules of the C57BL/6 thymus in conjunction with mass spectrometry (MS) and functional profiling to identify a naturally processed K(b)-bound peptide positively selecting the N15 TCR specific for the vesicular stomatitis virus octapeptide (VSV8) bound to K(b). The selecting peptide was identified in 1 of 80 HPLC fractions and shown by tandem MS (MS/MS) sequencing to correspond to residues 68-75 of the MLRQ subunit of the widely expressed mitochondrial NADH ubiquinone oxidoreductase (NUbO(68-75)). Of note, the peptide differs at six of its eight residues from the cognate peptide VSV8 and functions as a weak agonist for mature CD8 single positive (SP) N15 T cells, with activity 10,000-fold less than VSV8. In N15 transgenic (tg) recombinase activating gene 2(-/)- transporter associated with antigen processing 1(-/)- fetal thymic organ culture, NUbO(68-75) induces phenotypic and functional differentiation of N15 TCR bearing CD8 SP thymocytes. Failure of NUbO(68-75) to support differentiation of a second K(b)-restricted TCR indicates that its inductive effects are not general.

The thymus and central tolerance

T-cell differentiation in the thymus generates a peripheral repertoire of mature T cells that mounts strong responses to foreign antigens but is largely unresponsive to self-antigens. This state of specific immunological tolerance to self-components involves both central and peripheral mechanisms. Here we review the process whereby many T cells with potential reactivity for self-antigens are eliminated in the thymus during early T-cell differentiation. This process of central tolerance (negative selection) reflects apoptosis and is a consequence of immature T cells receiving strong intracellular signalling through T-cell receptor (TCR) recognition of peptides bound to major histocompatibility complex (MHC) molecules. Central tolerance occurs mainly in the medullary region of the thymus and depends upon contact with peptide-MHC complexes expressed on bone-marrow-derived antigen-presenting cells (APCs); whether tolerance also occurs in the cortex is still controversial. Tolerance induction requires a combination of TCR ligation and co-stimulatory signals. Co-stimulation reflects interaction between complementary molecules on T cells and APCs and probably involves multiple molecules acting in consort, which may account for why deletion of individual molecules with known or potential co-stimulatory function has little or no effect on central tolerance. The range of self-antigens that induce central tolerance is considerable and, via low-level expression in the thymus, may also include tissue-specific antigens; central tolerance to these latter antigens, however, is likely to be limited to high-affinity T cells, leaving low-affinity cells to escape. Tolerance to alloantigens and the possibility of using central tolerance to promote acceptance of allografts are discussed.

A critical role for CD2 in both thymic selection events and mature T cell function

To examine the function of CD2 in vivo, N15 TCR transgenic (tg) RAG-2(-/-) H-2(b) mice bearing a single TCR specific for the vesicular stomatitis virus octapeptide bound to the H-2K(b) molecule were compared on a wild-type or CD2(-/-) background. In N15tg RAG-2(-/-) CD2(-/-) mice, thymic dysfunction is evident by 6 wk with a pre-TCR block in the CD4(-)CD8(-) double-negative thymocytes at the CD25(+)CD44(-) stage. Moreover, mature N15tg RAG-2(-/-) CD2(-/-) T cells are approximately 100-fold less responsive to vesicular stomatitis virus octapeptide and unresponsive to weak peptide agonists, as judged by IFN-gamma production. Repertoire analysis shows substantial differences in Valpha usage between non-tg C57BL/6 (B6) and B6 CD2(-/-) mice. Collectively, these findings show that CD2 plays a role in pre-TCR function in double-negative thymocytes, TCR selection events during thymocyte development, and TCR-stimulated cytokine production in mature T cells.

TCR signaling for initiation and completion of thymocyte positive selection has distinct requirements for ligand quality and presenting cell type

Thymocyte selection involves signaling by TCR engaging diverse self-peptide:MHC molecule ligands on various cell types in the cortex and medulla. Here we separately analyze early and late stages of selection to better understand how presenting cell type, ligand quality, and the timing of TCR signaling contribute to intrathymic differentiation. TCR transgenic CD4+CD8+ thymocytes (double positive (DP)) from MHC-deficient mice were stimulated using various presenting cells and ligands. The resulting CD69high cells were isolated and evaluated for maturation in reaggregate cultures with wild-type or MHC molecule-deficient thymic stroma with or without added hemopoietic dendritic cells (DC). Production of CD4+ T cells required TCR signaling in the reaggregates, indicating that transient recognition of self-ligands by DP is inadequate for full differentiation. DC bearing a potent agonist ligand could initiate positive selection, producing activated thymocytes that matured into agonist-responsive T cells in reaggregates lacking the same ligand. DC could also support the TCR signaling necessary for late maturation. These results argue that despite the negative role assigned to DC in past studies, neither the peptide:MHC molecule complexes present on DC nor any other signals provided by these cells stimulate only thymocyte death. These findings also indicate that unique epithelial ligands are not necessary for positive selection. They provide additional insight into the role of ligand quality in selection events and support the concept that following initiation of maturation from the DP state, persistent TCR signaling is characteristic of and perhaps required by T cells.

T cell immune reconstitution after allogeneic bone marrow transplantation in bare lymphocyte syndrome

To study the impact of an MHC class II-negative environment on T cell immune reconstitution, we have analyzed the phenotypical and functional characteristics of FACS-sorted cultured CD4(+) and CD8(+) T cells in two Bare Lymphocyte Syndrome (BLS) patients before and after allo-BMT. A similar analysis was performed in two MHC class II expressing pediatric leukemia patients after treatment with an allo-BMT who were included in our study as control. It was observed that CD4(+) T cells displayed cytolytic alloreactivity in both BLS patients prior to and within the first year after allo-BMT, whereas such cells were absent at a later time-point, in the donors and pediatric leukemia controls. In addition, reduced MHC class II expression was observed in CD8(+) T cells of both recipients early after allo-BMT, irrespective of the T cell chimerism pattern. Lack of endogenous MHC class II expression in BLS patients, therefore, results in aberrant T cell selection within the first year after allo-BMT, analogous to T cell selection before transplantation. These T cell selection processes seem to be normalized at a later time point after allo-BMT probably due to migration and integration of graft-derived MHC class II-positive antigen presenting cells to sites of T cell selection.

A mouse thymic stromal cell line producing macrophage-colony stimulating factor and interleukin-6

A thymic stromal cell line, TFGD, was established from a thymic tumor mass developed spontaneously in p53 knock out mouse, and was found to produce cytokines that could induce bone marrow hematopoietic stem cells (HSCs) to differentiate into macrophages. The cytokines produced by the TFGD line were assessed by immunoassays. High level of macrophage-colony stimulating factor (M-CSF) and interleukin (IL)-6 was detected in the TFGD-culture supernatant, whereas granulocyte/macrophage-colony stimulating factor (GM-CSF), IL-3, IL-4, IL-5, IL-13, or interferon (IFN)-gamma was undetectable. Blocking experiments showed that anti-M-CSF monoclonal antibody could neutralize the differentiation-inducing activity shown by the TFGD-culture supernatant. Dot blot analysis of the total RNA isolated from the cultured fetal thymic stromal cells showed that M-CSF transcripts were expressed in the normal thymus. These observations, together with the earlier finding that M-CSF plus IL-6 is the optimal combination of cytokines for the induction of macrophage differentiation from HSCs in vitro, may indicate that thymic macrophages could be generated within the thymus by cytokines involving M-CSF.

Lck activity controls CD4/CD8 T cell lineage commitment

Thymocytes carrying MHC class I-restricted TCRs differentiate into CD8 T cells, while those recognizing MHC class II become CD4 T cells. The mechanisms underlying how MHC class recognition, coreceptor expression, and effector function are coordinated are not well understood. Since the tyrosine kinase Lck binds with more affinity to CD4 than CD8, it has been proposed as a candidate to mediate this process. By using transgenic mice with altered Lck activity, we show that thymocytes carrying a class II-restricted TCR develop into functional CD8 T cells when Lck activity is reduced. Conversely, thymocytes carrying a class I-restricted TCR develop into functional CD4 T cells when Lck activity is increased. These results directly show that quantitative differences in the Lck signal control the CD4/CD8 lineage decision.

Deriving quantitative constraints on T cell selection from data on the mature T cell repertoire

The T cell repertoire is shaped in the thymus through positive and negative selection. Thus, data about the mature repertoire may be used to infer information on how TCR generation and selection operate. Assuming that T cell selection is affinity driven, we derive the quantitative constraints that the parameters driving these processes must fulfill to account for the experimentally observed levels of alloreactivity, self MHC restriction and the frequency of cells recognizing a given foreign Ag. We find that affinity-driven selection is compatible with experimental estimates of these latter quantities only if 1) TCRs see more peptide residues than MHC polymorphic residues, 2) the majority of positively selected clones are deleted by negative selection, 3) between 1 and 3.6 clonal divisions occur on average in the thymus after completion of TCR rearrangement, and 4) selection is driven by 103-105 self peptides.

Immune-Associated Nucleotide-1 (IAN-1) Is a Thymic Selection Marker and Defines a Novel Gene Family Conserved in Plants

Positive selection of thymocytes is a complex and crucial event in T cell development that is characterized by cell death rescue, commitment toward the helper or cytotoxic lineage, and functional maturation of thymocytes bearing an appropriate TCR. To search for novel genes involved in this process, we compared gene expression patterns in positively selected thymocytes and their immediate progenitors in mice using the differential display technique. This approach lead to the identification of a novel gene, mIAN-1 (murine immune-associated nucleotide-1), that is switched on upon positive selection and predominantly expressed in the lymphoid system. We show that mIAN-1 encodes a 42-kDa protein sharing sequence homology with the pathogen-induced plant protein aig1 and that it defines a novel family of at least three putative GTP-binding proteins. Analysis of protein expression at various stages of thymocyte development links mIAN-1 to CD3-mediated selection events, suggesting that it represents a key player of thymocyte development and that it participates to peripheral specific immune responses. The evolutionary conservation of the IAN family provides a unique example of a plant pathogen response gene conserved in animals.

Expression of the Bcl-2 family member A1 is developmentally regulated in T cells

During T cell development, cells that fail to meet stringent selection criteria undergo programmed cell death. Thymocyte and peripheral T cell susceptibility to apoptosis is influenced by expression of Bcl-2 family members, some of which are expressed in a developmentally patterned manner. We previously showed developmentally regulated expression of A1, an anti-apoptotic Bcl-2 family member, among B cell developmental subsets. Here we show that cells of the T lineage also express A1 in a developmentally regulated manner. Both A1 mRNA and A1 protein are readily detectable in the thymus, and while present among DN cells, A1 mRNA is up-regulated to very high levels among double-positive (DP) thymocytes. It is then down-regulated to moderate levels among single-positive (SP) thymocytes, and finally expressed at approximately 25-fold lower levels among mature SP CD4(+) and CD8(+) lymph node T cells than among DP thymocytes. Furthermore, we find that in vitro TCR ligation up-regulates A1 expression among both DP and SP thymocytes. Together, these data show that A1 expression is developmentally regulated in T lymphocytes and is responsive to TCR signaling, suggesting that A1 may play a role in maintaining the viability of DP thymocytes.

A quantitative theory of affinity-driven T cell repertoire selection

Binding of the T cell antigen receptor (TCR) to peptides presented on molecules encoded by major histocompatibility complex (MHC) genes is the key event driving T cell development and activation. Selection of the T cell repertoire in the thymus involves two steps. First, positive selection promotes the survival of cells binding thymic self-MHC-peptide complexes with sufficient affinity. The resulting repertoire is self-MHC restricted: it recognizes foreign peptides presented on self, but not foreign MHC. Second, negative selection deletes cells which may be potentially harmful because their receptors interact with self-MHC-peptide complexes with too high an affinity. The mature repertoire is also highly alloreactive: a large fraction of T cells respond to tissues harboring foreign MHC. We derive mathematical expressions giving the frequency of alloreactivity, the level of self-MHC restriction, and the fraction of the repertoire activated by a foreign peptide, as a function of the parameters driving the generation and selection of the repertoire: self-MHC and self-peptide diversity, the stringencies of positive and negative selection, and the number of peptide and MHC polymorphic residues that contribute to T cell receptor binding. Although the model is based on a simplified digit string representation of receptors, all the parameters but one relate directly to experimentally determined quantities. The only parameter without a biological counterpart has no effect on the model's behavior besides a trivial and easily preventable discretization effect. We further analyse the role of the MHC and peptide contribution to TCR binding, and find that their relative, rather than absolute value, is important in shaping the mature repertoire. This result makes it possible to adopt different physical interpretations for the digit string formalism. We also find that the alloreactivity level can be inferred directly from data on the stringency of selection, and that, in agreement with recent experiments, it is not affected by thymic selection.

Glucocorticoid-mediated regulation of thymic dendritic cell function

The possible effects of glucocorticoids (GC) on the biology of thymic dendritic cells (DC) have been analyzed. Both DC and GC seem to be involved in intrathymic T cell selection but possible relationships, if any, between them remain currently unknown. For the first time, we have proved the expression of GC receptors in thymic DC. Moreover, our data demonstrate that in vitro dexamethasone (Dex) treatment barely affects the viability of mature thymic DC, which are largely resistant to its apoptotic effect. Dex-treated thymic DC also show a slightly reduced surface expression of some adhesion and co-stimulatory molecules in correlation with diminished allostimulatory properties. Furthermore, the production of both IL-1beta and tumor necrosis factor (TNF)-alpha, but not that of IL-6 and IL-10, diminished in the mixed leukocyte reaction established with Dex-treated thymic DC. However, the addition of recombinant rat IL-1beta and TNF-alpha, alone or in combination, did not recover the allostimulatory capacity. Taken together, these results support certain GC-mediated regulation of the activity of thymic DC which could be relevant for the biology of the thymus gland.

Incomplete T-Cell Immune Reconstitution in Two Major Histocompatibility Complex Class II–Deficiency/Bare Lymphocyte Syndrome Patients After HLA-Identical Sibling Bone Marrow Transplantation

To study the effects of major histocompatibility complex (MHC) class II expression on T-cell development, we have investigated T-cell immune reconstitution in two MHC class II–deficiency patients after allogeneic bone marrow transplantation (allo-BMT). Our study showed that the induction of MHC class II antigen expression on BM graft-derived T cells in these allo-BMT recipients was hampered upon T-cell activation. This reduction was most striking in the CD8+ T-cell subset. Furthermore, the peripheral T-cell receptor (TCR) repertoire in these graft-derived MHC class II–expressing CD4+ and in the CD8+ T-cell fractions was found to be restricted on the basis of TCR complementarity determining region 3 (CDR3) size profiles. Interestingly, the T-cell immune response to tetanus toxoid (TT) was found to be comparable to that of the donor. However, when comparing recipient-derived TT-specific T cells with donor-derived T cells, differences were observed in TCR gene segment usage and in the hydropathicity index of the CDR3 regions. Together, these results reveal the impact of an environment lacking endogenous MHC class II on the development of the T-cell immune repertoire after allo-BMT.

Immunological self/nonself discrimination: integration of self vs nonself during cognate T cell interactions with antigen-presenting cells

The hypothesis is presented that immunological integration of nonefficacious vs efficacious T cell antigen receptor (TCR) signals are foundational for self/nonself discrimination and that multiple integrative mechanisms are intrinsic to the molecular to molar organization of an adaptive immune response. These integrative mechanisms are proposed to adaptively regulate expression of costimulatory signals, such that foreign proteins are associated with the expression of costimulatory signals, whereas self-proteins are associated with the lack of costimulatory signaling. Overall, this model offers several unique contributions to the study of immunology. First, this model postulates that cognate TCR/major histocompatibility complex (MHC) interactions are sufficient to adaptively mediate immunological self/nonself discrimination. This model thereby offers a unique alternative to models that largely rely on innate immunity to prime immune discrimination. Second, the integrative model argues that the immune system can simultaneously reinforce self-tolerance and promote immunity to foreign organisms at the same time and in the same location. Many alternative models presume that pathogenic self-reactive T cells do not exist at the outset of an immune response against foreign agents. Third, the integrative model uniquely predicts relationships between immunodeficiency and autoimmune pathogenesis. Fourth, this model illustrates the regulatory advantages of cognate antigen presenting cell (APC) systems (i.e., T cell or B cell APC) compared to nonspecific APC. Cognate APC systems together with the respective clonotypic responders may comprise a fundamental "network" of lymphoid cells. Such networks would have clone-specific regulatory capabilities and may be central for immunological self/nonself discrimination. Fifth, this model provides an explanation for "infectious" tolerance without creating specialized subsets of "suppressor" or "regulatory" T cells. Each mature T cell retains the potential to reinforce tolerance or mediate immunity, depending on the specific antigenic cues present in the immediate environment.

Differentiation in Culture of Murine Primitive Lymphohematopoietic Progenitors Toward T-Cell Lineage

Earlier, we described a stromal cell-free two-step clonal culture system in which murine primitive lymphohematopoietic progenitors produce myeloid and B-lymphoid lineage cells. In the same culture T-cell potential of the progenitors was maintained. We now report that, in addition to myeloid and B-lymphoid cells, putative T-cell progenitors are also produced in culture. Lineage-negative (Lin−) Ly-6A/E+ c-kit+ bone marrow cells from 5-fluorouracil–treated mice were cultured in methylcellulose in the presence of SF (Steel factor), interleukin (IL)-11, and IL-7, and the resulting primary colonies were picked and pooled. When injected into severe combined immune deficiency (scid) mice, the pooled cells reconstituted the T-cell compartment of the scid mice earlier than freshly prepared primitive marrow cells. This reconstitution activity of the pooled primary colony cells was enriched in the Ly-6A/E+ and FcγRII/III−/low cell fractions. Reverse transcriptase-polymerase chain reaction (RT-PCR) and DNA-PCR analyses showed that some of the primary colony cells are differentiated sufficiently to express messenger RNA (mRNA) of T-cell receptor (TCR) β-chain and pre-TCR alpha (pT) and, although not frequently, to perform Dβ-Jβ rearrangement of the TCR gene. Micromanipulation studies confirmed the clonal origin of myeloid lineage cells and the cells positive for the T-cell–specific transcripts and D-J rearrangement of TCR β-chain. These results suggested that, in the presence of SF, IL-11, and IL-7, primitive lymphohematopoietic progenitors differentiate toward T-cell lineage in addition to myeloid and B-cell lineages.

Differentiation in Culture of Murine Primitive Lymphohematopoietic Progenitors Toward T-Cell Lineage

Earlier, we described a stromal cell-free two-step clonal culture system in which murine primitive lymphohematopoietic progenitors produce myeloid and B-lymphoid lineage cells. In the same culture T-cell potential of the progenitors was maintained. We now report that, in addition to myeloid and B-lymphoid cells, putative T-cell progenitors are also produced in culture. Lineage-negative (Lin−) Ly-6A/E+ c-kit+ bone marrow cells from 5-fluorouracil–treated mice were cultured in methylcellulose in the presence of SF (Steel factor), interleukin (IL)-11, and IL-7, and the resulting primary colonies were picked and pooled. When injected into severe combined immune deficiency (scid) mice, the pooled cells reconstituted the T-cell compartment of the scid mice earlier than freshly prepared primitive marrow cells. This reconstitution activity of the pooled primary colony cells was enriched in the Ly-6A/E+ and FcγRII/III−/low cell fractions. Reverse transcriptase-polymerase chain reaction (RT-PCR) and DNA-PCR analyses showed that some of the primary colony cells are differentiated sufficiently to express messenger RNA (mRNA) of T-cell receptor (TCR) β-chain and pre-TCR alpha (pT) and, although not frequently, to perform Dβ-Jβ rearrangement of the TCR gene. Micromanipulation studies confirmed the clonal origin of myeloid lineage cells and the cells positive for the T-cell–specific transcripts and D-J rearrangement of TCR β-chain. These results suggested that, in the presence of SF, IL-11, and IL-7, primitive lymphohematopoietic progenitors differentiate toward T-cell lineage in addition to myeloid and B-cell lineages.

Requirement for diverse, low-abundance peptides in positive selection of T cells

Whether a single major histocompatibility complex (MHC)-bound peptide can drive the positive selection of large numbers of T cells has been a controversial issue. A diverse population of self peptides was shown to be essential for the in vivo development of CD4 T cells. Mice in which all but 5 percent of MHC class II molecules were bound by a single peptide had wild-type numbers of CD4 T cells. However, when the diversity within this 5 percent was lost, CD4 T cell development was impaired. Blocking the major peptide-MHC complex in thymus organ culture had no effect on T cell development, indicating that positive selection occurred on the diverse peptides present at low levels. This requirement for peptide diversity indicates that the interaction between self peptides and T cell receptors during positive selection is highly specific.

Deletional and mutational analyses of the human CD4 gene promoter: characterization of a minimal tissue-specific promoter

In recent years, considerable interest has arisen in understanding the mechanisms of the CD4 gene transcriptional control which resulted in the identification of a promoter, enhancers and a silencer. While the murine CD4 gene promoter has been well studied, little is known about its human counterpart that we previously identified as a 1.1 kb region. Here, we show that the -170/+20 region represents a minimal tissue-specific promoter with a size compatible with its inclusion in viral vectors for gene therapy. In addition, mutational analyses provided evidence that this fragment contains three regions critical for transcriptional activity: an initiator-like sequence, an Ets consensus site and an ATF consensus site. The latter site is absent in the murine promoter, suggesting that some differences exist between the transcriptional control of the human and murine CD4 genes.

Engagement of CD99 Induces Up-Regulation of TCR and MHC Class I and II Molecules on the Surface of Human Thymocytes

CD99 is a cell surface molecule involved in the aggregation of lymphocytes and apoptosis of immature cortical thymocytes. Despite its high level expression on immature cortical thymocytes, the functional roles of this molecule during thymic selection are only now being elucidated. Examination of the effects of CD99 engagement on the expression kinetics of the TCR and MHC class I and II molecules, which are involved primarily in thymic positive selection, revealed a marked up-regulation of these proteins on the surface of immature thymocytes. This increase was the result of accelerated mobilization of molecules stored in cytosolic compartments to the plasma membrane, rather than increased RNA and protein synthesis. Confocal microscopic analysis revealed the changes in subcellular distribution of these molecules. When CD99 was engaged, TCR and MHC class I and II molecules were concentrated at the plasma membrane, particularly at cell-cell contact sites. The TCRlow subpopulation of immature double positive thymocytes was much more responsive to CD99-mediated up-regulation than was the TCRhigh population. These findings suggest that CD99-dependent up-regulation may have possible implication in positive selection during thymocyte ontogeny.

An alternate pathway for T cell development supported by the bone marrow microenvironment: recapitulation of thymic maturation

In the principal pathway of alpha/beta T cell maturation, T cell precursors from the bone marrow migrate to the thymus and proceed through several well-characterized developmental stages into mature CD4+ and CD8+ T cells. This study demonstrates an alternative pathway in which the bone marrow microenvironment also supports the differentiation of T cell precursors into CD4+ and CD8+ T cells. The marrow pathway recapitulates developmental stages of thymic maturation including a CD4+CD8+ intermediary cell and positive and negative selection, and is strongly inhibited by the presence of mature T cells. The contribution of the marrow pathway in vivo requires further study in mice with normal and deficient thymic or immune function.

Residual MHC class II expression on mature dendritic cells and activated B cells in RFX5-deficient mice

Patients with major histocompatibility complex class II (MHC-II) deficiency are known to carry mutations in either the RFX complex or the trans-activator CIITA. While the pivotal role of CIITA for MHC-II gene transcription is supported by the essential absence of MHC-II molecules in CIITA-deficient mice, we demonstrate here that RFX5-/- mice retain expression of MHC-II in thymic medulla, mature dendritic cells, and activated B cells. Nevertheless, RFX5-/- mice develop a severe immunodeficiency due to the lack of MHC-II in thymic cortex, failure of positive selection of CD4+ T cells, and absence of MHC-II on resting B cells and resident or IFNgamma-activated macrophages. This differential requirement for CIITA and RFX5 in subsets of antigen-presenting cells may be specific for the mouse; it may, however, also exist in humans without having been noticed so far.

Pertussis toxin can replace T cell receptor signals that induce positive selection of CD8 T cells

CD4+ helper T lymphocytes and CD8+ killer T lymphocytes are both generated in the thymus from common precursor cells expressing CD4 and CD8. The development of immature CD4 CD8+ thymocytes into mature 'single-positive' T cells requires T cell antigen-receptor (TCR)-mediated positive selection signals. Although it is known that the recognition specificity of TCR expressed by CD4+ CD8+ thymocytes determines their fate to become either CD4+ or CD8+ T cells, the molecular signals that direct precursor thymocytes to become CD4+ and CD8+ T cells are unclear. By using ZAP-70 mutant thymus organ cultures in which T cell development is arrested at the CD4+ CD8+ thymocyte stage, the present study shows that distinct biochemical treatments can selectively restore the generation of mature CD4+ and CD8+ T cells, bypassing TCR-induced positive selection signals. The combination of phorbol ester and ionomycin selectively restores the generation of CD4+ CD8- TCR(high) cells, consistent with previous results. On the other hand, we find that the generation of CD4- CD8+ TCR(high) cells is selectively induced by pertussis toxin. Interestingly, the signals generated by pertussis toxin, which increase Notch expression, can dominate the signals by phorbol ester and ionomycin, steering thymocyte development to CD8 lineage. These results indicate that distinct biochemical signals replace TCR signals that selectively induce positive selection of CD4+ and CD8+ T cells, and that biochemical treatment can manipulate the development and choice of CD4+ and CD8+ T cells.

An enhancer that directs lineage-specific expression of CD8 in positively selected thymocytes and mature T cells

Positive selection of CD4+CD8+ T cells to the CD4+CD8- helper and CD4- CD8+ cytotoxic lineages is a multistep process that involves complex regulation of coreceptor gene expression. By analyzing expression of a reporter gene in transgenic mice, we have identified a DNA segment, located between the murine CD8beta and CD8alpha genes, that has enhancer activity restricted to CD8 lineage cells. Remarkably, this enhancer functions in thymocytes undergoing positive selection to the CD4-CD8+ phenotype but not in immature double-positive thymocytes. The enhancer also functions in gut intraepithelial lymphocytes that express CD8alpha but not CD8beta, suggesting that it is specific for CD8alpha expression. The tight correlation between activation of this enhancer and the final step in positive selection has important implications for understanding the mechanism of lineage commitment in thymocytes.