Proliferation kinetics associated with T cell receptor-beta chain selection of fetal murine thymocytes

Mechanism study of ubiquitination in T cell development and autoimmune disease

T cells play critical role in multiple immune processes including antigen response, tumor immunity, inflammation, self-tolerance maintenance and autoimmune diseases et. Fetal liver or bone marrow-derived thymus-seeding progenitors (TSPs) settle in thymus and undergo T cell-lineage commitment, proliferation, T cell receptor (TCR) rearrangement, and thymic selections driven by microenvironment composed of thymic epithelial cells (TEC), dendritic cells (DC), macrophage and B cells, thus generating T cells with diverse TCR repertoire immunocompetent but not self-reactive. Additionally, some self-reactive thymocytes give rise to Treg with the help of TEC and DC, serving for immune tolerance. The sequential proliferation, cell fate decision, and selection during T cell development and self-tolerance establishment are tightly regulated to ensure the proper immune response without autoimmune reaction. There are remarkable progresses in understanding of the regulatory mechanisms regarding ubiquitination in T cell development and the establishment of self-tolerance in the past few years, which holds great potential for further therapeutic interventions in immune-related diseases.

Identifying T Cell Receptors from High-Throughput Sequencing: Dealing with Promiscuity in TCRα and TCRβ Pairing

Characterisation of the T cell receptors (TCR) involved in immune responses is important for the design of vaccines and immunotherapies for cancer and autoimmune disease. The specificity of the interaction between the TCR heterodimer and its peptide-MHC ligand derives largely from the juxtaposed hypervariable CDR3 regions on the TCRα and TCRβ chains, and obtaining the paired sequences of these regions is a standard for functionally defining the TCR. A brute force approach to identifying the TCRs in a population of T cells is to use high-throughput single-cell sequencing, but currently this process remains costly and risks missing small clones. Alternatively, CDR3α and CDR3β sequences can be associated using their frequency of co-occurrence in independent samples, but this approach can be confounded by the sharing of CDR3α and CDR3β across clones, commonly observed within epitope-specific T cell populations. The accurate, exhaustive, and economical recovery of TCR sequences from such populations therefore remains a challenging problem. Here we describe an algorithm for performing frequency-based pairing (alphabetr) that accommodates CDR3α- and CDR3β-sharing, cells expressing two TCRα chains, and multiple forms of sequencing error. The algorithm also yields accurate estimates of clonal frequencies.

Hematopoiesis and T-cell specification as a model developmental system

The pathway to generate T cells from hematopoietic stem cells guides progenitors through a succession of fate choices while balancing differentiation progression against proliferation, stage to stage. Many elements of the regulatory system that controls this process are known, but the requirement for multiple, functionally distinct transcription factors needs clarification in terms of gene network architecture. Here, we compare the features of the T-cell specification system with the rule sets underlying two other influential types of gene network models: first, the combinatorial, hierarchical regulatory systems that generate the orderly, synchronized increases in complexity in most invertebrate embryos; second, the dueling 'master regulator' systems that are commonly used to explain bistability in microbial systems and in many fate choices in terminal differentiation. The T-cell specification process shares certain features with each of these prevalent models but differs from both of them in central respects. The T-cell system is highly combinatorial but also highly dose-sensitive in its use of crucial regulatory factors. The roles of these factors are not always T-lineage-specific, but they balance and modulate each other's activities long before any mutually exclusive silencing occurs. T-cell specification may provide a new hybrid model for gene networks in vertebrate developmental systems.

Deoxycytidine kinase augments ATM-Mediated DNA repair and contributes to radiation resistance

Efficient and adequate generation of deoxyribonucleotides is critical to successful DNA repair. We show that ataxia telangiectasia mutated (ATM) integrates the DNA damage response with DNA metabolism by regulating the salvage of deoxyribonucleosides. Specifically, ATM phosphorylates and activates deoxycytidine kinase (dCK) at serine 74 in response to ionizing radiation (IR). Activation of dCK shifts its substrate specificity toward deoxycytidine, increases intracellular dCTP pools post IR, and enhances the rate of DNA repair. Mutation of a single serine 74 residue has profound effects on murine T and B lymphocyte development, suggesting that post-translational regulation of dCK may be important in maintaining genomic stability during hematopoiesis. Using [(18)F]-FAC, a dCK-specific positron emission tomography (PET) probe, we visualized and quantified dCK activation in tumor xenografts after IR, indicating that dCK activation could serve as a biomarker for ATM function and DNA damage response in vivo. In addition, dCK-deficient leukemia cell lines and murine embryonic fibroblasts exhibited increased sensitivity to IR, indicating that pharmacologic inhibition of dCK may be an effective radiosensitization strategy.

Pharmacological inhibition of glycogen synthase kinase 3 regulates T cell development in vitro

The development of functional T cells requires receptor-mediated transition through multiple checkpoints in the thymus. Double negative 3 (DN3) thymocytes are selected for the presence of a rearranged TCR beta chain in a process termed β-selection which requires signalling via the pre-TCR, Notch1 and CXCL12. Signal integration by these receptors converges on core pathways including the Phosphatidylinositol-3-kinase (PI3K) pathway. Glycogen Synthase Kinase 3 (GSK3) is generally thought to be negatively regulated by the PI3K pathway but its role in β-selection has not been characterised. Here we show that developmental progression of DN3 thymocytes is promoted following inhibition of GSK3 by the synthetic compound CHIR99021. CHIR99021 allows differentiation in the absence of pre-TCR-, Notch1- or CXCL12-mediated signalling. It antagonizes IL-7-mediated inhibition of DP thymocyte differentiation and increases IL-7-promoted cell recovery. These data indicate a potentially important role for inactivation of GSK3 during β-selection. They might help to establish an in vitro stromal cell-free culture system of thymocyte development and offer a new platform for screening regulators of proliferation, differentiation and apoptosis.

Interaction of Ras with p110γ is required for thymic β-selection in the mouse

Thymocytes are tested for productive rearrangement of the tcrb locus by expression of a pre-TCR in a process termed β-selection, which requires both Notch1 and CXCR4 signaling. It has been shown that activation of the GTPase Ras allows thymocytes to proliferate and differentiate in the absence of a Pre-TCR; the direct targets of Ras at this checkpoint have not been identified, however. Mice with a mutant allele of p110γ unable to bind active Ras revealed that CXCR4-mediated PI3K activation is Ras dependent. The Ras-p110γ interaction was necessary for efficient β-selection-promoted proliferation but was dispensable for the survival or differentiation of thymocytes. Uncoupling Ras from p110γ provides unambiguous identification of a Ras interaction required for thymic β-selection.

Molecular and cellular basis of T cell lineage commitment

The thymus forms as an alymphoid thymic primordium with T cell differentiation requiring the seeding of this anlage. This review will focus on the characteristics of the hematopoietic progenitors which colonize the thymus and their subsequent commitment/differentiation, both in mice and men. Within the thymus, the interplay between Notch1 and IL-7 signals is crucial for the orchestration of T cell development, but the precise requirements for these factors in murine and human thympoeisis are not synonymous. Recent advances in our understanding of the mechanisms regulating precursor entry and their maintenance in the thymus will also be presented.

Requirement for deoxycytidine kinase in T and B lymphocyte development

Deoxycytidine kinase (dCK) is a rate-limiting enzyme in deoxyribonucleoside salvage, a metabolic pathway that recycles products of DNA degradation. dCK phosphorylates and therefore activates nucleoside analog prodrugs frequently used in cancer, autoimmunity, and viral infections. In contrast to its well established therapeutic relevance, the biological function of dCK remains enigmatic. Highest levels of dCK expression are found in thymus and bone marrow, indicating a possible role in lymphopoiesis. To test this hypothesis we generated and analyzed dCK knockout (KO) mice. dCK inactivation selectively and profoundly affected T and B cell development. A 90-fold decrease in thymic cellularity was observed in the dCK KO mice relative to wild-type littermates. Lymphocyte numbers in the dCK KO mice were 5- to 13-fold below normal values. The severe impact of dCK inactivation on lymphopoiesis was unexpected given that nucleoside salvage has been thought to play a limited, "fine-tuning" role in regulating deoxyribonucleotide triphosphate pools produced by the de novo pathway. The dCK KO phenotype challenges this view and indicates that, in contrast to the great majority of other somatic cells, normal lymphocyte development critically requires the deoxyribonucleoside salvage pathway.

Beta-selection: abundance of TCRbeta-/gammadelta- CD44- CD25- (DN4) cells in the foetal thymus

Expression of TCRβ and pre-TCR signalling are essential for differentiation of CD4^–CD8^– double negative (DN) thymocytes to the CD4⁺CD8⁺ double-positive (DP) stage. Thymocyte development in adult Rag1, Rag2 or TCRβδ-deficient mice is arrested at the DN3 stage leading to the assumption that pre-TCR signalling and β-selection occur at, and are obligatory for, the transition from DN3 to DN4. We show that the majority of DN3 and DN4 cells that differentiate during early embryogenesis in wild-type mice do not express intracellular (ic) TCRβ/γδ. These foetal icTCRβ⁻/γδ⁻ DN4 cells were T lineage as determined by expression of Thy1 and icCD3 and TCRβ DJ rearrangement. In addition, in the foetal Rag1^–/– thymus, a normal percentage of DN4 cells were present. In wild-type mice after hydrocortisone-induced synchronisation of differentiation, the majority of DN4 cells that first emerged did not express icTCRβ/γδ, showing that adult thymocytes can also differentiate to the DN4 stage independently of pre-TCR signalling. Pre-TCR signalling induced expansion in the DN4 population, but lack of TCRβ/γδ expression did not immediately induce apoptosis. Our data demonstrate in vivo differentiation from DN3 to DN4 cell in the absence of TCRβ/γδ expression in the foetal thymus, and after hydrocortisone treatment of adult mice.

Metalloproteinase-dependent control of thymocyte differentiation and proliferation

The development of T cells in the thymus is dependent on interactions between thymocytes and thymic stromal cells, on stimulation by growth factors, and on the binding to and migration along extracellular matrix (ECM) components. As metalloproteinases (MP) are involved in processes such as growth factor release and ECM modelling, we assessed the effect of MP inhibitors on T-cell development using fetal thymic organ culture systems. MP inhibitors significantly reduced the numbers of CD4/CD8 double-positive (DP) and mature single-positive thymocytes generated, correlated with a reduced number of cell cycles between the double-negative (DN)3 and DP stages. The progression of early thymocyte progenitors through the DN1-4 stages of development was also severely affected, including incomplete upregulation of CD25, decreased DN3 cell numbers, reduced rearrangement of the T-cell receptor (TCR)-beta locus and expression of intracellular TCR-beta by fewer DN3 cells. When purified DN1 cells were utilized as donor cells in reaggregate thymic organ cultures, essentially no DP thymocytes were produced in the presence of MP inhibitors. The results suggest that MP inhibitors affect the differentiation of developing thymocytes before, and reduce proliferation after, pre-TCR-mediated selection.

Control of Genotypic Allelic Inclusion through TCR Surface Expression

To gain insight into the molecular causes and functional consequences of allelic inclusion of TCR alpha-chains, we develop a computational model for thymocyte selection in which the signal that determines cell fate depends on surface expression. Analysis of receptor pairs on selected dual TCR cells reveals that allelic inclusion permits both autoreactive TCR and receptors not in the single TCR cell repertoire to be selected. However, in comparison with earlier theoretical studies, relatively few dual TCR cells display receptors with high avidity for thymic ligands because their alpha-chains compete aggressively for the beta-chain, which hinders rescue from clonal deletion. This feature of the model makes clear that allelic inclusion does not in itself compromise central tolerance. A specific experiment based on modulation of TCR surface expression levels is proposed to test the model.

Glucose transporter 1 expression identifies a population of cycling CD4+ CD8+ human thymocytes with high CXCR4-induced chemotaxis

GLUT1, the major glucose transporter in peripheral T lymphocytes, is induced upon T cell receptor activation. However, the role of GLUT1 during human thymocyte differentiation remains to be evaluated. Our identification of GLUT1 as the human T lymphotrophic virus (HTLV) receptor has enabled us to use tagged HTLV-receptor-binding domain fusion proteins to specifically monitor surface GLUT1 expression. Here, we identify a unique subset of CD4+ CD8+ double-positive (DP) thymocytes, based on their GLUT1 surface expression. Whereas these cells express variable levels of CD8, they express uniformly high levels of CD4. Glucose uptake was 7-fold higher in CD4(hi) DP thymocytes than in CD4(lo) DP thymocytes (P = 0.0002). Further analyses indicated that these GLUT1+ thymocytes are early post-beta-selection, as demonstrated by low levels of T cell receptor (TCR)alphabeta and CD3. This population of immature GLUT1+ DP cells is rapidly cycling and can be further distinguished by specific expression of the transferrin receptor. Importantly, the CXCR4 chemokine receptor is expressed at 15-fold higher levels on GLUT1+ DP thymocytes, as compared with the DP GLUT1- subset, and the former cells show enhanced chemotaxis to the CXCR4 ligand CXCL12. Thus, during human thymopoiesis, GLUT1 is up-regulated after beta-selection, and these immature DP cells constitute a population with distinct metabolic and chemotactic properties.

Polycomb group gene mel-18 regulates early T progenitor expansion by maintaining the expression of Hes-1, a target of the Notch pathway

Polycomb group (PcG) proteins play a role in the maintenance of cellular identity throughout many rounds of cell division through the regulation of gene expression. In this report we demonstrate that the loss of the PcG gene mel-18 impairs the expansion of the most immature T progenitor cells at a stage before the rearrangement of the TCR beta-chain gene in vivo and in vitro. This impairment of these T progenitors appears to be associated with increased susceptibility to cell death. We also show that the expression of Hes-1, one of the target genes of the Notch signaling pathway, is drastically down-regulated in early T progenitors isolated from mel-18(-/-) mice. In addition, mel-18(-/-) T precursors could not maintain the Hes-1 expression induced by Delta-like-1 in monolayer culture. Collectively, these data indicate that mel-18 contributes to the maintenance of the active state of the Hes-1 gene as a cellular memory system, thereby supporting the expansion of early T progenitors.

Early Growth Response Gene 3 Regulates Thymocyte Proliferation during the Transition from CD4−CD8− to CD4+CD8+1

In thymocytes developing in the alphabeta lineage, the transition from CD4, CD8 double negative (DN) to CD4, CD8 double positive (DP) is associated with several rounds of cell division and changes in the expression of multiple genes. This transition is induced by the formation of a pre-TCR that includes a rearranged TCR beta-chain and the pre-TCR alpha-chain. The mechanism by which the pre-TCR influences both gene expression and proliferation has not been defined. We have evaluated the role played by early growth response gene 3 (Egr3) in translating pre-TCR signals into differentiation and proliferation. Egr3 is a transcriptional regulator that contains a zinc-finger DNA binding domain. We find that Egr3-deficient mice have a reduced number of thymocytes compared with wild-type mice, and that this is due to poor proliferation during the DN to DP transition. Treatment of both Egr3(+/+) and Egr3(-/-) mice on the Rag1(-/-) background with anti-CD3epsilon Ab in vivo results in similar differentiation events, but reduced cell recovery in the Egr3(-/-) mice. We have also generated transgenic mice that express high levels of Egr3 constitutively, and when these mice are bred onto a Rag1(-/-) background they exhibit increased proliferation in the absence of stimulation and have pre-TCR alpha-chain and CD25 down-regulation, as well as increased Calpha expression. The results show that Egr3 is an important regulator of proliferation in response to pre-TCR signals, and that it also may regulate some specific aspects of differentiation.

Regulation of thymocyte differentiation: pre-TCR signals and beta-selection

The specificity of the adaptive immune response is, in part, dependent on the clonal expression of the mature T cell receptor (TCR) on T lymphocytes. One mechanism regulating the clonality of the TCR occurs at the level of TCR-beta gene rearrangements during lymphocyte development. Expression of a nascent TCR-beta chain together with pre-Talpha (pTalpha) and CD3 molecules to form the pre-TCR complex, represents a critical checkpoint in T cell differentiation known as beta-selection. Indeed, failure to generate a functionally rearranged TCR-beta chain at this stage of development results in apoptosis. Signals derived from the pre-TCR complex trigger a maturation program within developing thymocytes that includes: rescue from apoptosis; inhibition of further DNA recombination at the TCR-beta gene locus (allowing for the clonality of antigen receptor expression; allelic exclusion); and induction of proliferation and differentiation. The signaling mechanisms that control this developmental program remain largely undefined. Here, we discuss recent evidence investigating the molecular mechanisms that regulate thymocyte differentiation downstream of pre-TCR formation.

Stage-specific changes in fetal thymocyte proliferation during the CD4-8- to CD4+8+ transition in wild type, Rag1-/-, and Hoxa3,Pax1 mutant mice

BACKGROUND

The function of the thymic microenvironment is to promote thymocyte maturation, in part via regulation of thymocyte proliferation and cell death. Defects in fetal thymic epithelial cell (TEC) development and function, and therefore in the formation of a functional microenvironment, can be caused either directly by TEC differentiation defects or indirectly by defective thymocyte maturation. In this paper we studied fetal thymocyte proliferation during the early transition from the CD3-4-8- (triple negative, TN) to CD4+8+ (double positive, DP) stages. We compared wild type mice with Rag1-/- mice and with Hoxa3+/-Pax1-/- compound mutant mice, which have blocks at different stages of thymocyte development.

RESULTS

Wild type fetal and adult thymus showed stage-specific differences in the proliferation profiles of developing thymocytes, with fetal stages showing generally higher levels of proliferation. The proliferation profile of fetal thymocytes from Rag1-/- mutants also had stage-specific increases in proliferation compared to wild type fetal thymocytes, in contrast to the lower proliferation previously reported for thymocytes from adult Rag1-/- mutants. We have previously shown that Hoxa3+/-Pax1-/- mice have abnormal fetal TEC development, resulting in increased apoptosis at the TN to DP transition and decreased DP cell numbers. Fetal thymocytes from Hoxa3+/-Pax1-/- compound mutants had increased proliferation, but fewer proliferating cells, at the DP stage. We also observed a decrease in the level of the cytokines IL-7 and SCF produced by Hoxa3+/-Pax1-/-TECs.

CONCLUSION

Our results indicate complex and stage-specific effects of abnormal TEC development on thymocyte proliferation.

Enforced expression of Runx2 perturbs T cell development at a stage coincident with beta-selection

The development of T cells in the thymus is regulated by a series of stage-specific transcription factors. Deregulated expression of these factors can lead to alterations in thymocyte development with the production of aberrant cell subsets and predispose to tumor formation. The three genes of the Runx family are multilineage regulators of differentiation that have been reported to be expressed in the T cell lineage. However, their roles in thymocyte development and T cell function are largely unknown. While the Runx2/Cbfa1/AML3/Pebp2alphaa gene plays a primary role in osteogenesis and regulates a number of key bone regulatory genes, we show here that Runx2 is also expressed during the earliest phase of thymic development, in the double-negative subset. Furthermore, enforced expression of Runx2 in transgenic mice under the CD2 promoter was found to affect T cell development at a stage coincident with beta-selection, resulting in an expansion of double-negative CD4 and CD8 immature single-positive cells. Unlike wild-type controls this preselection population (CD4-CD8+heat-stable Ag+TCR-) is in a nonproliferative state, but appears to be primed for further transformation events. Overall the data suggest that Runx2 accelerates development to the CD8 immature single-positive stage, but retards subsequent differentiation to the double-positive stage. Thus, Runx2 joins a small group of transcription factors that can interfere with early T cell development, cause an expansion of a specific subset, and predispose to lymphoma.

Immature thymocytes that fail to express TCRbeta and/or TCRgamma delta proteins die by apoptotic cell death in the CD44(-)CD25(-) (DN4) subset

Pre-TCR/CD3 signals are essential for survival and maturation of (CD44(-)25(+)) DN3 thymocytes via the (CD44(-)25(-)) DN4 stage to CD4(+)CD8(+) (DP) cells, a process termed beta-selection. The exact developmental stages of apoptosis resulting from lack of pre-TCR/CD3 signals have so far not been determined. Here we analyzed apoptotic cell death in relation to expression of clonotypic TCR polypeptides and to cell cycle status in immature thymocyte subpopulations of wild type (wt) mice and of several strains of mice with compromised pre-TCR/CD3 signaling complexes. In wt mice or pre-TCR/CD3-deficient mice, apoptotic cells could not be detected among DN3 cells but accumulated in a subset of DN4 expressing CD69. Apoptotic CD69(+)DN4 cells were rare in wt mice and were found among DN4 cells that were negative or low for intracellular TCRbeta and negative for TCRgamma delta polypeptide chains. Apoptotic CD69(+)DN4 cells were abundant in pre-TCR/CD3 signaling-deficient mice in which most DN4 cells failed to express clonotypic TCR polypeptides. Survival of DN4 cells, but not maturation of DN3 cells to DN4, was found to depend on the expression of clonotypic TCR polypeptides in the same cell. The results suggest that thymocytes unsuccessful in alpha beta or in gamma delta lineage development die by apoptosis in the DN4 subset.

Developmentally regulated expression of the transmembrane adaptor protein trim in fetal and adult T cells

TRIM is a recently identified transmembrane adaptor protein which is exclusively expressed in T cells and natural killer (NK) cells. In peripheral blood T cells TRIM has been reported to coprecipitate, comodulate, and cocap with the T-cell receptor (TCR), suggesting that it is an integral component of the TCR/CD3/zeta complex. Here we investigate the expression of TRIM mRNAs and proteins in developing thymocytes. Two splicing isoforms with open reading frames are observed, namely a full length (TRIM) and a truncated version (DeltaTM-TRIM). The latter lacks the extracellular and transmembrane domains as well as the first 10 cytoplasmic aminoacids and is significantly expressed only as mRNA in early fetal thymocytes. TRIM mRNA is detected in all mainstream thymocyte subsets in adult mice. TRIM protein, in contrast, first appears in the DN2 (CD44+ CD25+) subset of adult double negative (DN) cells. In fetal thymocyte development, TRIM mRNA is seen from dg 14.5 onwards whereas TRIM protein appears first on dg 16.5. In contrast to the adult, the TRIM protein was seen in a subset of fetal DN1 cells. In fetal and adult thymocytes, TRIM protein expression was highest in DN2, DN3 (CD44-25+) and in DP cells, compatible with a functional role at or around phases of thymic selection.

Biased V beta usage in immature thymocytes is independent of DJ beta proximity and pT alpha pairing

During thymus development, the TCR beta locus rearranges before the TCR alpha locus. Pairing of productively rearranged TCR beta-chains with an invariant pT alpha chain leads to the formation of a pre-TCR and subsequent expansion of immature pre-T cells. Essentially nothing is known about the TCR V beta repertoire in pre-T cells before or after the expression of a pre-TCR. Using intracellular staining, we show here that the TCR V beta repertoire is significantly biased at the earliest developmental stage in which VDJ beta rearrangement has occurred. Moreover (and in contrast to the V(H) repertoire in immature B cells), V beta repertoire biases in immature T cells do not reflect proximity of V beta gene segments to the DJ beta cluster, nor do they depend upon preferential V beta pairing with the pT alpha chain. We conclude that V gene repertoires in developing T and B cells are controlled by partially distinct mechanisms.

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.

Extracellular signal-regulated kinase (ERK) activation by the pre-T cell receptor in developing thymocytes in vivo

The first checkpoint in T cell development occurs between the CD4(-)CD8(-) and CD4(+)CD8(+) stages and is associated with formation of the pre-T cell receptor (TCR). The signaling mechanisms that drive this progression remain largely unknown. Here, we show that extracellular signal-regulated kinases (ERKs)-1/2 are activated upon engagement of the pre-TCR. Using a novel experimental system, we demonstrate that expression of the pre-TCR by developing thymocytes induces ERK-1/2 activation within the thymus. In addition, the activation of this pre-TCR signaling cascade is mediated through Lck. These findings directly link pre-TCR complex formation with specific downstream signaling components in vivo.

Ontogeny of synonymous T cell populations with specificity for a self MHC epitope mimicked by a bacterial homologoue: an antigen-specific T cell analysis in a non-transgenic system

By means of a novel technique for identification and isolation of MHC class II-restricted antigen-specific T cells, we describe here in non-transgenic BALB / c mice physiological positive selection of an oligoclonal population of T cells which recognizes both a self MHC-derived peptide (Ialpha52) and a bacterial homologoue (Hi15). The results support a model for self peptide-mediated generation of T cells which have specificity for microbial antigens through molecular mimicry. This mechanism may be a model for the ontogeny of a physiological T cell response to infectious agents. Loss of control of these circuits may be part of the inciting factors of autoimmunity.

TCR α-Chain Repertoire in pTα-Deficient Mice Is Diverse and Developmentally Regulated: Implications for Pre-TCR Functions and TCRA Gene Rearrangement

Pre-TCR expression on developing thymocytes allows cells with productive TCRB gene rearrangements to further differentiate. In wild-type mice, most TCRA gene rearrangements are initiated after pre-TCR expression. However, in pTα-deficient mice, a substantial number of αβ+ thymocytes are still produced, in part because early TCR α-chain expression can rescue immature thymocytes from cell death. In this study, the nature of these TCR α-chains, produced and expressed in the absence of pre-TCR expression, have been analyzed. We show, by FACS analysis and sequencing of rearranged transcripts, that the TCRA repertoire is diverse in pTα−/− mice and that the developmental regulation of AJ segment use is maintained, yet slightly delayed around birth when compared with wild-type mice. We also found that T cell differentiation is more affected by pTα inactivation during late gestation than later in life. These data suggest that the pre-TCR is not functionally required for the initiation and regulation of TCRA gene rearrangement and that fetal thymocytes are more dependent than adult cells on pTα-derived signals for their differentiation.

Unexpectedly late expression of intracellular CD3epsilon and TCR gammadelta proteins during adult thymus development

During adult thymus development immature CD4(-)CD8(-) [double-negative (DN)] precursor cells pass through four phenotypically distinct stages defined by expression of CD44 and CD25: CD44(hi)CD25(-) (DN1), CD44(hi)CD25(+) (DN2), CD44(lo)CD25(+) (DN3) and CD44(lo)CD25(-) (DN4). Although it is well established that the TCR beta, gamma and delta genes are rearranged and expressed in association with the CD3 components in DN thymocytes, the precise timing of expression of the TCR and CD3 proteins has not been determined. In this report we have utilized a sensitive intracellular (ic) staining technique to analyze the expression of ic CD3epsilon, TCR beta and TCR gammadelta proteins in immature DN subsets. As expected from previous studies of TCR beta rearrangement and mRNA expression, icTCR beta(+) cells were first detected in the DN3 subset and their proportion increased thereafter. Surprisingly, however, both icCD3epsilon(+) and icTCR gammadelta(+) cells were detected at later stages of development than was predicted by molecular studies. In particular icCD3epsilon protein expression coincided with the transition from the DN2 to DN3 stage of development, whereas icTCR gammadelta protein expression was only detected in a minor subset of DN4 cells. The implications of these findings for alphabeta lineage divergence will be discussed.

Models for Antigen Receptor Gene Rearrangement. II. Multiple Rearrangement in the TCR: Allelic Exclusion or Inclusion?

This series of papers addresses the effects of continuous Ag receptor gene rearrangement in lymphocytes on allelic exclusion. The previous paper discussed light chain gene rearrangement and receptor editing in B cells, and showed that these processes are ordered on three different levels. This order, combined with the constraints imposed by a strong negative selection, was shown to lead to effective allelic exclusion. In the present paper, we discuss rearrangement of TCR genes. In the TCR α-chain, allelic inclusion may be the rule rather than the exception. Several previous models, which attempted to explain experimental observations, such as the fractions of cells containing two productive TCRα rearrangements, did not sufficiently account for TCR gene organization, which limits secondary rearrangement, and for the effects of subsequent thymic selection. We present here a detailed, comprehensive computer simulation of TCR gene rearrangement, incorporating the interaction of this process with other aspects of lymphocyte development, including cell division, selection, cell death, and maturation. Our model shows how the observed fraction of T cells containing productive TCRα rearrangements on both alleles can be explained by the parameters of thymic selection imposed over a random rearrangement process.

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

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

Distinct roles of prostaglandin H synthases 1 and 2 in T-cell development

Prostaglandin G and H synthases, or cyclooxygenases (COXs), catalyze the formation of prostaglandins (PGs). Whereas COX-1 is diffusely expressed in lymphoid cells in embryonic day 15.5 thymus, COX-2 expression is sparse, apparently limited to stromal cells. By contrast, COX-2 is predominant in a subset of medullary stromal cells in three- to five-week-old mice. The isozymes also differ in their contributions to lymphocyte development. Thus, experiments with selective COX-1 inhibitors in thymic lobes from normal and recombinase-activating gene-1 knockout mice support a role for this isoform in the transition from CD4(-)CD8(-) double-negative (DN) to CD4(+)CD8(+) double-positive (DP). Concordant data were obtained in COX-1 knockouts. Pharmacological inhibition and genetic deletion of COX-2, by contrast, support its role during early thymocyte proliferation and differentiation and, later, during maturation of the CD4 helper T-cell lineage. PGE2, but not other PGs, can rescue the effects of inhibition of either isoform, although it acts through distinct EP receptor subtypes. COX-dependent PG generation may represent a mechanism of thymic stromal support for T-cell development.

Reduced Generation but Efficient TCRβ-Chain Selection of CD4+8+ Double-Positive Thymocytes in Mice with Compromised CD3 Complex Signaling

Maturation to the CD4+8+ double-positive (DP) stage of thymocyte development is restricted to cells that have passed TCRβ selection, an important checkpoint at which immature CD4−8− double-negative (DN) cells that express TCRβ polypeptide chains are selected for further maturation. The generation of DP thymocytes following TCRβ selection is dependent on cellular survival, differentiation, and proliferation, and the entire process appears to be mediated by the pre-TCR/CD3 complex. In this study, we investigate the signaling requirements for TCRβ selection using mice single deficient and double deficient for CD3ζ/η and/or p56lck. While the numbers of DP cells are strongly reduced in the single-deficient mice, a further drastic reduction in the generation of DP thymocytes is seen in the double-deficient mice. The poor generation of DP cells in the mutant mice is primarily due to an impaired ability of CD25+ DN thymocytes to proliferate following expression of a TCRβ-chain. Nevertheless, the residual DP cells in all mutant mice are strictly selected for expression of TCRβ polypeptide chains. DN thymocytes of mutant mice expressed TCRβ and CD3ε at the cell surface and contained mRNA for pre-Tα, but not for clonotypic TCRα-chains, together suggesting that TCRβ selection is mediated by pre-TCR signaling in all cases. The data suggest differential requirements of pre-TCR signaling for cell survival on the one hand, and for the proliferative burst associated with TCRβ selection on the other.

Requirement of CD3 complex-associated signaling functions for expression of rearranged T cell receptor beta VDJ genes in early thymic development

During alpha beta thymocyte development, the clonotypic alpha beta-T cell receptor (TCR) is preceded by sequentially expressed immature versions of the TCR-CD3 complex: the pre-TCR, containing a clonotypic TCR-beta chain and invariant pre-Talpha, is expressed on pre-T cells before rearrangement of the TCR-alpha locus. Moreover, clonotype-independent CD3 complexes (CIC) appear on pro-T cells before VDJ rearrangements of TCR-beta genes. The pre-TCR is known to mediate TCR-beta selection, the prerequisite for maturation of CD4(-)8(-) double negative (DN) thymocytes to the CD4(+)8(+) double positive stage. A developmental function of CIC has so far not been delineated. In mice single deficient and double deficient for CD3zeta/eta and/or p56(lck), we observe a pronounced reduction in the proportions of CD25(+) DN thymocytes that express intracellular TCR-beta chains. TCR-beta transcripts are reduced in parallel with TCR-beta polypeptide chains whereas no reduction in TCR-beta locus rearrangements could be detected. Wild-type levels of TCR-beta transcripts and of cells expressing TCR-beta polypeptide chains are induced by treatment with anti-CD3epsilon mAb. The data suggest that the initial expression of rearranged TCR-beta VDJ genes in pro-T cell to pre-T cell progression is dependent on CD3 complex signaling, and thus define a putative developmental function for CIC.

Schlafen, a new family of growth regulatory genes that affect thymocyte development

The Schlafen (Slfn) family of genes are differentially regulated during thymocyte maturation and are preferentially expressed in the lymphoid tissues. Ectopic expression of the prototype member Slfn1 early in the T lineage profoundly alters cell growth and development. In these mice, the DP thymocytes fail to complete maturation, and, depending on the transgene dosage, the number of thymocytes is reduced to 1%-30% of normal. Furthermore, expression of the Schlafen family members in fibroblasts and thymoma cells either retards or ablates cell growth. The conceptual protein sequences deduced for each of the family members have no similarity to characterized proteins and must therefore participate in a heretofore unknown regulatory mechanism guiding both cell growth and T cell development.

Identification of a late stage of small noncycling pTalpha- pre-T cells as immediate precursors of T cell receptor alpha/beta+ thymocytes

During thymocyte development, progression from T cell receptor (TCR)beta to TCRalpha rearrangement is mediated by a CD3-associated pre-TCR composed of the TCRbeta chain paired with pre-TCRalpha (pTalpha). A major issue is how surface expression of the pre-TCR is regulated during normal thymocyte development to control transition through this checkpoint. Here, we show that developmental expression of pTalpha is time- and stage-specific, and is confined in vivo to a limited subset of large cycling human pre-T cells that coexpress low density CD3. This restricted expression pattern allowed the identification of a novel subset of small CD3(-) thymocytes lacking surface pTalpha, but expressing cytoplasmic TCRbeta, that represent late noncycling pre-T cells in which recombination activating gene reexpression and downregulation of T early alpha transcription are coincident events associated with cell cycle arrest, and immediately preceding TCRalpha gene expression. Importantly, thymocytes at this late pre-T cell stage are shown to be functional intermediates between large pTalpha+ pre-T cells and TCRalpha/beta+ thymocytes. The results support a developmental model in which pre-TCR-expressing pre-T cells are brought into cycle, rapidly downregulate surface pre-TCR, and finally become small resting pre-T cells, before the onset of TCRalpha gene expression.

Requirement for the thymus in alphabeta T lymphocyte lineage commitment

We recently identified a fetal thymic developmental stage (NK1.1+/CD117(lo)) that characterizes committed T/NK progenitors. We now report the existence of phenotypically and functionally identical T/NK progenitors in mouse fetal blood and spleen but not in fetal liver. These precursors are indistinguishable from previously characterized fetal blood "prothymocytes" (CD90+/CD117(lo)), with the exception that they express NK1.1, lack markers associated with T lineage commitment, maintain a germline TCRbeta locus, and can give rise to both T and NK cells. Moreover, NK1.1+/CD90+/CD117(lo) fetal blood precursors are present in athymic nude mice. These results suggest that the T/NK lineage commitment pathway is thymus-independent. In contrast, full commitment to the alphabeta T lineage does not precede thymus colonization.

Antigen-receptor junctional diversity in growth-factor-receptor mutant mice

Precursor lymphocytes undergo expansion prior to immunoglobulin (Ig) or T cell receptor (TCR) rearrangements. Development of thymocytes, but not B cells, is entirely blocked in mice lacking both the receptor-tyrosine-kinase c-kit and the common cytokine receptor gamma chain (gamma c). In c-kit-gamma c-mice, TCR beta rearrangements are limited to mono- or oligoclonal DJ junctions. Here, effects of lack of c-kit or gamma c, or both, on the junctional diversity of TCR gamma and delta, and Ig VH(DH)JH loci were analyzed. All rearrangements were present in wildtype and mutant mice. However, sequencing of the junctions revealed monoclonal TCR gamma (V gamma 2 J gamma 1) and TCR delta (V delta 1(D delta)J delta 2) joints in c-kit-gamma c-, but not c-kit+ gamma c- or wildtype thymocytes. In contrast to TCR beta, gamma and delta loci, VHDHJH junctions were more diverse in c-kit-gamma c-mice. Thus, the two analyzed growth factor receptors mediate signaling pathways required for progenitor expansion and generation of junctional diversity at TCR loci, but have less influence on the diversity of IgH junctions.