Double-positive T cell receptor(high) thymocytes are resistant to peptide/major histocompatibility complex ligand-induced negative selection

Stochastic optical active rheology

We demonstrate a stochastic based method for performing active rheology using optical tweezers. By monitoring the displacement of an embedded particle in response to stochastic optical forces, a rapid estimate of the frequency dependent shear moduli of a sample is achieved in the range of 10(-1)-10(3) Hz. We utilize the method to probe linear viscoelastic properties of hydrogels at varied cross-linker concentrations. Combined with fluorescence imaging, our method demonstrates non-linear changes of bond strength between T cell receptors and an antigenic peptide due to force-induced cell activation.

The alphabeta T cell receptor is an anisotropic mechanosensor

Thymus-derived lymphocytes protect mammalian hosts against virus- or cancer-related cellular alterations through immune surveillance, eliminating diseased cells. In this process, T cell receptors (TCRs) mediate both recognition and T cell activation via their dimeric alphabeta, CD3 epsilon gamma, CD3 epsilon delta, and CD3 zeta zeta subunits using an unknown structural mechanism. Here, site-specific binding topology of anti-CD3 monoclonal antibodies (mAbs) and dynamic TCR quaternary change provide key clues. Agonist mAbs footprint to the membrane distal CD3 epsilon lobe that they approach diagonally, adjacent to the lever-like C beta FG loop that facilitates antigen (pMHC)-triggered activation. In contrast, a non-agonist mAb binds to the cleft between CD3 epsilon and CD3 gamma in a perpendicular mode and is stimulatory only subsequent to an external tangential but not a normal force ( approximately 50 piconewtons) applied via optical tweezers. Specific pMHC but not irrelevant pMHC activates a T cell upon application of a similar force. These findings suggest that the TCR is an anisotropic mechanosensor, converting mechanical energy into a biochemical signal upon specific pMHC ligation during immune surveillance. Activating anti-CD3 mAbs mimic this force via their intrinsic binding mode. A common TCR quaternary change rather than conformational alterations can better facilitate structural signal initiation, given the vast array of TCRs and their specific pMHC ligands.

PlexinD1 glycoprotein controls migration of positively selected thymocytes into the medulla

Precise intrathymic cell migration is important for thymocyte maturation and organ architecture. The orchestration of thymocyte trafficking, however, is not well understood at a molecular level. Here, we described highly regulated plexinD1 expression on CD4+CD8+ double positive (DP) thymocytes. PlexinD1 expression was further affected by the engagement of T cell receptor complex. Activation of plexinD1 via the ligand, semaphorin 3E, repressed CCL25 chemokine signaling via its receptor CCR9 in CD69+ thymocytes. In the absence of plexinD1, CD69+ thymocytes remained in the cortex, maturing to form ectopic single positive (SP) thymocyte clusters in Plxnd1-deficient fetal liver cell-transplanted mice. As a consequence, the boundary between DP and SP thymocytes at corticomedullary junctions was disrupted and medullary structures formed under the thymic capsule. These results demonstrate the importance of plexinD1 in directing migration of maturing thymocytes via modulation of biological responses to chemokine gradients.

Structural and functional evidence that Nck interaction with CD3epsilon regulates T-cell receptor activity

Recruitment of signaling molecules to the cytoplasmic domains of the CD3 subunits of the T-cell receptor (TCR) is crucial for early T-cell activation. These transient associations either do or do not require tyrosine phosphorylation of CD3 immune tyrosine activation motifs (ITAMs). Here we show that the non-ITAM-requiring adaptor protein Nck forms a complex with an atypical PxxDY motif of the CD3epsilon tail, which encompasses Tyr166 within the ITAM and a TCR endocytosis signal. As suggested by the structure of the complex, we find that Nck binding inhibits phosphorylation of the CD3epsilon ITAM by Fyn and Lck kinases in vitro. Moreover, the CD3epsilon-Nck interaction downregulates TCR surface expression upon physiological stimulation in mouse primary lymph node cells. This indicates that Nck performs an important regulatory function in T lymphocytes by inhibiting ITAM phosphorylation and/or removing cell surface TCR via CD3epsilon interaction.

Functional Role for IκBNS in T Cell Cytokine Regulation As Revealed by Targeted Gene Disruption

Triggering of the TCR by cognate peptide/MHC ligands induces expression of I kappa BNS, a member of the I kappa B family of NF-kappaB inhibitors whose expression is associated with apoptosis of immature thymocytes. To understand the role of I kappa BNS in TCR triggering, we created a targeted disruption of the I kappa BNS gene. Surprisingly, mice lacking I kappa BNS show normal thymic progression but both thymocytes and T cells manifest reduced TCR-stimulated proliferation. Moreover, I kappa BNS knockout thymocytes and T cells produce significantly less IL-2 and IFN-gamma than wild-type cells. Transfection analysis demonstrates that I kappa BNS and c-Rel individually increase IL-2 promoter activity. The effect of I kappa BNS on the IL-2 promoter, unlike c-Rel, is dependent on the NF-kappaB rather than the CD28RE site; mutation of the NF-kappaB site extinguishes the induction of transcription by I kappa BNS in transfectants and prevents association of I kappa BNS with IL-2 promoter DNA. Microarray analyses confirm the reduction in IL-2 production and some IFN-gamma-linked transcripts in I kappa BNS knockout T cells. Collectively, our findings demonstrate that I kappa BNS regulates production of IL-2 and other cytokines induced via "strong" TCR ligation.

Structurally distinct ligand-binding or ligand-independent Notch1 mutants are leukemogenic but affect thymocyte development, apoptosis, and metastasis differently

We previously found that provirus insertion in T cell tumors of mouse mammary tumor virus/c-myc transgenic (Tg) mice induced two forms of Notch1 mutations. Type I mutations generated two truncated molecules, one intracellular (IC) (Notch1(IC)) and one extracellular (Notch1(EC)), while in type II mutations Notch1 was deleted of its C terminus (Notch1(DeltaCT)). We expressed these mutants in Tg mice using the CD4 promoter. Both Notch1(IC) and Notch1(DeltaCT), but not Notch1(EC), Tg mice developed double-positive (DP) thymomas. These disseminated more frequently in Notch1(DeltaCT) Tg mice. Double (Notch1(IC) x myc) or (Notch1(DeltaCT) x myc) Tg mice developed thymoma with a much shorter latency than single Tg mice, providing genetic evidence of a collaboration between these two oncogenes. FACS analysis of preleukemic thymocytes did not reveal major T cell differentiation anomalies, except for a higher number of DP cells and an accumulation of TCR(high)CD2(high)CD25(high) DP cells in Notch1(IC), and less so in Notch1(DeltaCT) Tg mice. This was associated with enhanced in vivo thymocyte proliferation. However, Notch1(IC), but not Notch1(DeltaCT), DP thymocytes were protected against apoptosis induced in vivo by dexamethasone and anti-CD3 and in vitro by anti-CD3/CD28 Abs. This indicates that the C terminus of Notch1 and/or the conserved regulation by its ligands have a significant impact on the induced T cell phenotype. Therefore, Notch1(IC) and Notch1(DeltaCT) behave as oncogenes for T cells. Because these two Notch1 mutations are very similar to those described in some forms of human T cell leukemia, these Tg mice may represent relevant models of these human leukemias.

The TCR C beta FG loop regulates alpha beta T cell development

The TCRbeta chain constant domain contains an unusually elongated, solvent-exposed FG loop. This structural element forms one component of an alphabeta TCR cavity against which CD3epsilongamma may abut to facilitate Ag-specific signaling. Consistent with this notion, in the present study we show that N15alphabeta TCR transfectants expressing a FG loop-deleted chain (betaDeltaFG) stimulate less tyrosine protein phosphorylation than those bearing a wild-type beta-chain (betawt) upon TCR cross-linking. Furthermore, coimmunoprecipitation studies suggest a weakened association between the CD3epsilongamma heterodimer and the beta-chain in TCR complexes containing the betaDeltaFG variant. To further investigate the biologic role of the Cbeta FG loop in development, we competitively reconstituted the thymus of Ly5 congenic or RAG-2-/- mice using bone marrow cells from betawt or betaDeltaFG transgenic C57BL/6 (B6) mice. Both betawt and betaDeltaFG precursor cells generate thymocytes representative of all maturational stages. However, betaDeltaFG-expressing thymocytes dominate during subsequent development, resulting in an excess of betaDeltaFG-expressing peripheral T cells with reduced proliferative and cytokine production abilities upon TCR stimulation. Collectively, our results show that the unique Cbeta FG loop appendage primarily controls alphabeta T cell development through selection processes.

A General Survey of Thymocyte Differentiation by Transcriptional Analysis of Knockout Mouse Models

The thymus is the primary site of T cell lymphopoiesis. To undergo proper differentiation, developing T cells follow a well-ordered genetic program that strictly depends on the heterogeneous and highly specialized thymic microenvironment. In this study, we used microarray technology to extensively describe transcriptional events regulating alphabeta T cell fate. To get an integrated view of these processes, both whole thymi from genetically engineered mice together with purified thymocytes were analyzed. Using mice exhibiting various transcriptional perturbations and developmental blockades, we performed a transcriptional microdissection of the organ. Multiple signatures covering both cortical and medullary stroma as well as various thymocyte maturation intermediates were clearly defined. Beyond the definition of histological and functional signatures (proliferation, rearrangement), we provide the first evidence that such an approach may also highlight the complex cross-talk events that occur between maturing T cells and stroma. Our data constitute a useful integrated resource describing the main gene networks set up during thymocyte development and a first step toward a more systematic transcriptional analysis of genetically modified mice.

Peptide variants of viral CTL epitopes mediate positive selection and emigration of Ag-specific thymocytes in vivo

During development, thymocytes carrying TCRs mediating low-affinity interactions with MHC-bound self-peptides are positively selected for export into the mature peripheral T lymphocyte pool. Thus, exogenous administration of certain altered peptide ligands (APL) with reduced TCR affinity relative to cognate Ags may provide a tool to elicit maturation of desired TCR specificities. To test this "thymic vaccination" concept, we designed APL of the viral CTL epitopes gp33-41 and vesicular stomatitis virus nucleoprotein octapeptide N52-59 relevant for the lymphocytic choriomeningitis virus-specific P14- and vesicular stomatitis virus-specific N15-TCRs, respectively, and examined their effects on thymocytes in vivo using irradiation chimeras. Injection of APL into irradiated congenic (Ly-5.1) mice, reconstituted with T cell progenitors from the bone marrow of P14 RAG2(-/-) (Ly-5.2) or N15 RAG2(-/-) (Ly-5.2) transgenic mice, resulted in positive selection of T cells expressing the relevant specificity. Moreover, the variants led to export of virus-specific T cells to lymph nodes, but without inducing T cell proliferation. These findings show that the mature T cell repertoire can be altered by in vivo peptide administration through manipulation of thymic selection.

Involvement of the TCR Cbeta FG loop in thymic selection and T cell function

The asymmetric disposition of T cell receptor (TCR) Cbeta and Calpha ectodomains creates a cavity with a side-wall formed by the rigid Cbeta FG loop. To investigate the significance of this conserved structure, we generated loop deletion (betaDeltaFG) and betawt transgenic (tg) mice using the TCR beta subunit of the N15 CTL. N15betawt and N15betaDeltaFG H-2(b) animals have comparable numbers of thymocytes in S phase and manifest developmental progression through the CD4(-)CD8(-) double-negative (DN) compartment. N15betaDeltaFG facilitates transition from DN to CD4(+)8(+) double-positive (DP) thymocytes in recombinase activating gene (RAG)-2(-/-) mice, showing that pre-TCR function remains. N15betaDeltaFG animals possess approximately twofold more CD8(+) single-positive (SP) thymocytes and lymph node T cells, consistent with enhanced positive selection. As an altered Valpha repertoire observed in N15betaDeltaFG mice may confound the deletion's effect, we crossed N15alphabeta TCR tg RAG-2(-/-) with N15betaDeltaFG tg RAG-2(-/-) H-2(b) mice to generate N15alphabeta RAG-2(-/-) and N15alphabeta.betaDeltaFG RAG-2(-/-) littermates. N15alphabeta.betaDeltaFG RAG-2(-/-) mice show an 8-10-fold increase in DP thymocytes due to reduced negative selection, as evidenced by diminished constitutive and cognate peptide-induced apoptosis. Compared with N15alphabeta, N15alphabeta.betaDeltaFG T cells respond poorly to cognate antigens and weak agonists. Thus, the Cbeta FG loop facilitates negative selection of thymocytes and activation of T cells.

Induction of thymocyte positive selection does not convey immediate resistance to negative selection

The acquisition of functional competence represents a critical phase during intrathymic development of T cells. Thymocytes reaching this stage represent cells which have been positively selected on the basis of major histocompatibility complex reactivity, but which have also been purged of potentially autoreactive T-cell receptor specificities by negative selection. While the developmental window in which thymocytes are subjected to positive selection is now well defined, the precise developmental timing of negative selection, in relation to positive selection events, is less clear. Moreover, the underlying mechanism allowing single-positive thymocytes to respond to T-cell receptor ligation by activation rather than death, remains controversial. Here we have analysed the developmental timing of negative selection in relation to positive selection, using measurement of thymocyte susceptibility to dendritic cell presentation of the superantigen staphylococcal enterotoxin B (SEB). We show that thymocytes which have received initial positive selection signals, namely CD4+ CD8+ CD69+ thymocytes, like their CD4+ CD8+ CD69minus sign precursors, are susceptible to negative selection, indicating that induction of positive selection does not convey immediate resistance to negative selection. In contrast, newly generated CD4+ CD8minus sign CD69+ cells are not only resistant to deletion by SEB, but respond to SEB-mediated T-cell receptor-ligation by activation, indicating that the acquisition of functional competence occurs at the newly generated CD4+ CD8minus sign CD69+ stage. Finally, by using direct retroviral infection of primary CD4+ CD8+ thymocytes, we also show that Notch-1 activation in CD4+ CD8+ thymocytes does not correlate with, nor convey resistance to superantigen-mediated negative selection. Thus, our data suggest that although Notch-1 has been implicated in resistance to thymocyte apoptosis, the acquisition of resistance to negative selection occurs independently of Notch-1 signalling.

Developmentally regulated glycosylation of the CD8alphabeta coreceptor stalk modulates ligand binding

The functional consequences of glycan structural changes associated with cellular differentiation are ill defined. Herein, we investigate the role of glycan adducts to the O-glycosylated polypeptide stalk tethering the CD8alphabeta coreceptor to the thymocyte surface. We show that immature CD4(+)CD8(+) double-positive thymocytes bind MHCI tetramers more avidly than mature CD8 single-positive thymocytes, and that this differential binding is governed by developmentally programmed O-glycan modification controlled by the ST3Gal-I sialyltransferase. ST3Gal-I induction and attendant core 1 sialic acid addition to CD8beta on mature thymocytes decreases CD8alphabeta-MHCI avidity by altering CD8alphabeta domain-domain association and/or orientation. Hence, glycans on the CD8beta stalk appear to modulate the ability of the distal binding surface of the dimeric CD8 globular head domains to clamp MHCI.

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.

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.

Positive selection of thymocytes: the long and winding road

Positive selection is a crucial stage in T-cell development because it is here that CD4+CD8+ cells bearing T-cell receptors that interact with self-major histocompatibility complex molecules are rescued from cell death, resulting in the generation of mature T cells. Here, Graham Anderson and colleagues review recent studies indicating that positive selection is a multistage process involving interactions with thymic epithelium.

Expression, purification, and functional analysis of murine ectodomain fragments of CD8alphaalpha and CD8alphabeta dimers

Soluble mouse CD8alphaalpha and CD8alphabeta dimers corresponding to the paired ectodomains (CD8(f)) or their respective component Ig-like domains (CD8) were expressed in Chinese hamster ovary cells or the glycosylation variant Lec3.2.8.1 cells as secreted proteins using a leucine zipper strategy. The affinity of CD8alphaalpha(f) for H-2K(b) as measured by BIAcore revealed a approximately 65 microM K(d), similar to that of CD8alphabeta(f). Consistent with this result, CD8alphaalpha(f) as well as CD8alphabeta(f) blocked the effector function of N15 T cell receptor transgenic cytolytic T cells in a comparable, dose-dependent fashion. Furthermore, both Lec3.2.8.1-produced and Chinese hamster ovary-produced CD8 homodimers and heterodimers were active in the inhibition assay. These results suggest that the Ig-like domains of CD8 molecules are themselves sufficient to block the requisite transmembrane CD8-pMHC interaction between cytolytic T lymphocytes and target cells. Moreover, given the similarities in co-receptor affinities for pMHC, the findings suggest that the greater efficiency of CD8alphabeta versus CD8alphaalpha co-receptor function on T cells is linked to differences within their membrane-bound stalk regions and/or intracellular segments. As recently shown for sCD8alphaalpha, the yield, purity and homogeneity of the deglycosylated protein resulting from this expression system is sufficient for crystallization and x-ray diffraction at atomic resolution.

Selection of the T cell repertoire

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

Differential roles of ERK and p38 MAP kinase pathways in positive and negative selection of T lymphocytes

Clonal selection of T lymphocytes is essential for establishing self/non-self discrimination of immune recognition. It is known that cell surface signals such as avidity and valency of TCR-ligand interactions influence the fate of individual thymocytes, founding a primary repertoire of T cells. However, intracellular signals that govern positive and negative selection in the thymus have been unclear. The present study using the retroviral gene transfer technique shows that MKK1 activation in developing T cells is sufficient for providing positive selection signals. We also show that the MKK6-p38 signaling pathway is critically involved in inducing negative selection of thymocytes. These results suggest that intracellular signals through different MAP kinase cascades selectively guide positive and negative selection of T lymphocytes.

Differential thymic selection outcomes stimulated by focal structural alteration in peptide/major histocompatibility complex ligands

The T lineage repertoire is shaped by T cell receptor (TCR)-dependent positive and negative thymic selection processes. Using TCR-transgenic (N15tg) beta2-microglobulin-deficient (beta2m-/-) RAG-2(-/-) H-2(b) mice specific for the VSV8 (RGYVYQGL) octapeptide bound to Kb, we identified a single weak agonist peptide variant V4L (L4) inducing phenotypic and functional T cell maturation. The cognate VSV8 peptide, in contrast, triggers negative selection. The crystal structure of L4/Kb was determined and refined to 2.1 A for comparison with the VSV8/Kb structure at similar resolution. Aside from changes on the p4 side chain of L4 and the resulting alteration of the exposed Kb Lys-66 side chain, these two structures are essentially identical. Hence, a given TCR recognizes subtle distinctions between highly related ligands, resulting in dramatically different selection outcomes. Based on these finding and the recent structural elucidation of the N15-VSV8/Kb complex, moreover, it appears that the germ-line Valpha repertoire contributes in a significant way to positive selection.