Another view of the selective model of thymocyte selection

Single-cell multiomic analysis of thymocyte development reveals drivers of CD4+ T cell and CD8+ T cell lineage commitment

The development of CD4+ T cells and CD8+ T cells in the thymus is critical to adaptive immunity and is widely studied as a model of lineage commitment. Recognition of self-peptide major histocompatibility complex (MHC) class I or II by the T cell antigen receptor (TCR) determines the CD8+ or CD4+ T cell lineage choice, respectively, but how distinct TCR signals drive transcriptional programs of lineage commitment remains largely unknown. Here we applied CITE-seq to measure RNA and surface proteins in thymocytes from wild-type and T cell lineage-restricted mice to generate a comprehensive timeline of cell states for each T cell lineage. These analyses identified a sequential process whereby all thymocytes initiate CD4+ T cell lineage differentiation during a first wave of TCR signaling, followed by a second TCR signaling wave that coincides with CD8+ T cell lineage specification. CITE-seq and pharmaceutical inhibition experiments implicated a TCR-calcineurin-NFAT-GATA3 axis in driving the CD4+ T cell fate. Our data provide a resource for understanding cell fate decisions and implicate a sequential selection process in guiding lineage choice.

The order and logic of CD4 versus CD8 lineage choice and differentiation in mouse thymus

CD4 and CD8 mark helper and cytotoxic T cell lineages, respectively, and serve as coreceptors for MHC-restricted TCR recognition. How coreceptor expression is matched with TCR specificity is central to understanding CD4/CD8 lineage choice, but visualising coreceptor gene activity in individual selection intermediates has been technically challenging. It therefore remains unclear whether the sequence of coreceptor gene expression in selection intermediates follows a stereotypic pattern, or is responsive to signaling. Here we use single cell RNA sequencing (scRNA-seq) to classify mouse thymocyte selection intermediates by coreceptor gene expression. In the unperturbed thymus, Cd4+Cd8a- selection intermediates appear before Cd4-Cd8a+ selection intermediates, but the timing of these subsets is flexible according to the strength of TCR signals. Our data show that selection intermediates discriminate MHC class prior to the loss of coreceptor expression and suggest a model where signal strength informs the timing of coreceptor gene activity and ultimately CD4/CD8 lineage choice.

CD4 Helper and CD8 Cytotoxic T Cell Differentiation

A fundamental question in developmental immunology is how bipotential thymocyte precursors generate both CD4⁺ helper and CD8⁺ cytotoxic T cell lineages. The MHC specificity of αβ T cell receptors (TCRs) on precursors is closely correlated with cell fate-determining processes, prompting studies to characterize how variations in TCR signaling are linked with genetic programs establishing lineage-specific gene expression signatures, such as exclusive CD4 or CD8 expression. The key transcription factors ThPOK and Runx3 have been identified as mediating development of helper and cytotoxic T cell lineages, respectively. Together with increasing knowledge of epigenetic regulators, these findings have advanced our understanding of the transcription factor network regulating the CD4/CD8 dichotomy. It has also become apparent that CD4⁺ T cells retain developmental plasticity, allowing them to acquire cytotoxic activity in the periphery. Despite such advances, further studies are necessary to identify the molecular links between TCR signaling and the nuclear machinery regulating expression of ThPOK and Runx3.

T Lymphocyte Development and Activation in Humanized Mouse Model

Humanized mice, containing engrafted human cells and tissues, are emerging as an important in vivo platform for studying human diseases. Since the development of Nod scid gamma (NSG) mice bearing mutations in the IL-2 receptor gamma chain, many investigators have used NSG mice engrafted with human hematopoietic stem cells (HSCs) to generate functional human immune systems in vivo, results in high efficacy of human cell engraftment. The development of NSG mice has allowed significant advances to be made in studies on several human diseases, including cancer and graft-versus-host-disease (GVHD), and in regenerative medicine. Based on the human HSC transplantation, organ transplantation including thymus and liver in the renal capsule has been performed. Also, immune reconstruction of cells, of the lymphoid as well as myeloid lineages, has been partly accomplished. However, crosstalk between pluripotent stem cell derived therapeutic cells with human leukocyte antigen (HLA) mis/matched types and immune CD3 T cells have not been fully addressed. To overcome this hurdle, human major histocompatibility complex (MHC) molecules, not mouse MHC molecules, are required to generate functional T cells in a humanized mouse model. Here, we briefly summarize characteristics of the humanized mouse model, focusing on development of CD3 T cells with MHC molecules. We also highlight the necessity of the humanized mouse model for the treatment of various human diseases.

Cernunnos/Xlf Deficiency Results in Suboptimal V(D)J Recombination and Impaired Lymphoid Development in Mice

Xlf/Cernunnos is unique among the core factors of the non-homologous end joining (NHEJ) DNA double strand breaks (DSBs) repair pathway, in the sense that it is not essential for V(D)J recombination in vivo and in vitro. Unlike other NHEJ deficient mice showing a SCID phenotype, Xlf^−/− mice present a unique immune phenotype with a moderate B- and T-cell lymphopenia, a decreased cellularity in the thymus, and a characteristic TCRα repertoire bias associated with the P53-dependent apoptosis of CD4+CD8+ DP thymocytes. Here, we thoroughly analyzed Xlf^−/− mice immune phenotype and showed that it is specifically related to the DP stage but independent of the MHC-driven antigen presentation and T-cell activation during positive selection. Instead, we show that V(D)J recombination is subefficient in Xlf^−/− mice in vivo, exemplified by the presence of unrepaired DSBs in the thymus. This results in a moderate developmental delay of both B- and T-lymphocytes at key V(D)J recombination dependent stages. Furthermore, subefficient V(D)J recombination waves are accumulating during TCRα rearrangement, causing the typical TCRα repertoire bias with loss of distal Vα and Jα rearrangements.

αβ T cell receptor germline CDR regions moderate contact with MHC ligands and regulate peptide cross-reactivity

αβ T cells respond to peptide epitopes presented by major histocompatibility complex (MHC) molecules. The role of T cell receptor (TCR) germline complementarity determining regions (CDR1 and 2) in MHC restriction is not well understood. Here, we examine T cell development, MHC restriction and antigen recognition where germline CDR loop structure has been modified by multiple glycine/alanine substitutions. Surprisingly, loss of germline structure increases TCR engagement with MHC ligands leading to excessive loss of immature thymocytes. MHC restriction is, however, strictly maintained. The peripheral T cell repertoire is affected similarly, exhibiting elevated cross-reactivity to foreign peptides. Our findings are consistent with germline TCR structure optimising T cell cross-reactivity and immunity by moderating engagement with MHC ligands. This strategy may operate alongside co-receptor imposed MHC restriction, freeing germline TCR structure to adopt this novel role in the TCR-MHC interface.

Regulation of CD4 and CD8 coreceptor expression and CD4 versus CD8 lineage decisions

During blood cell development, hematopoietic stem cells generate diverse mature populations via several rounds of binary fate decisions. At each bifurcation, precursors adopt one fate and inactivate the alternative fate either stochastically or in response to extrinsic stimuli and stably maintain the selected fates. Studying of these processes would contribute to better understanding of etiology of immunodeficiency and leukemia, which are caused by abnormal gene regulation during the development of hematopoietic cells. The CD4(+) helper versus CD8(+) cytotoxic T-cell fate decision serves as an excellent model to study binary fate decision processes. These two cell types are derived from common precursors in the thymus. Positive selection of their TCRs by self-peptide presented on either MHC class I or class II triggers their fate decisions along with mutually exclusive retention and silencing of two coreceptors, CD4 and CD8. In the past few decades, extensive effort has been made to understand the T-cell fate decision processes by studying regulation of genes encoding the coreceptors and selection processes. These studies have identified several key transcription factors and gene regulatory networks. In this chapter, I will discuss recent advances in our understanding of the binary cell fate decision processes of T cells.

Expanding roles for ThPOK in thymic development

The role of the zinc finger transcription factor ThPOK (T-helper-inducing POZ-Kruppel-like factor) in promoting commitment of αβ T cells to the CD4 lineage is now well established. New results indicate that ThPOK is also important for the development and/or acquisition of effector functions by other T cell subsets, including several not marked by CD4 expression, i.e. double-negative invariant natural killer T (iNKT) cells, γδ cells, and even memory CD8(+) T cells. There is compelling evidence that ThPOK expression in most or all of these cases is dependent on T-cell receptor signaling and that differences in relative TCR signal strength/length may induce different levels of ThPOK expression. The developmental consequences of ThPOK expression vary according to cell type, which may partly reflect differences in ThPOK levels and/or in transcriptional networks between cell types.

Interaction between the mouse homologue of CD99 and its ligand PILR as a mechanism of T cell receptor-independent thymocyte apoptosis

Here, we show that the interaction between two membrane proteins, the mouse homologue of CD99 (designated D4) and its ligand, paired immunoglobulin-like type 2 receptor (PILR), is one of the major mechanisms of thymocyte apoptosis. Using the polymeric fusion protein of PILR and IgG1 (PILR-Ig), we demonstrated that D4 ligation in the absence of T cell receptor (TCR) engagement leads to the induction of apoptosis, mainly at the double-positive stage of thymocytes. This was further confirmed by a blocking study in which blocking the interaction between D4 and PILR by soluble D4 protein led to reduced apoptosis in the fetal thymic organ culture with wild type and TCRalpha(-/-) mice. Furthermore, the dissection of intracellular signaling pathway demonstrated that D4 cross-linking led to caspase activation without any change in mitochondrial membrane potential. Based on these data, we propose a mechanism for thymocyte depletion in which the interaction between D4 and PILR delivers an active signal.

The role of ThPOK in control of CD4/CD8 lineage commitment

During alphabeta T cell development, cells diverge into alternate CD4 helper and CD8(+) cytotoxic T cell lineages. The precise correlation between a T cell's CD8 and CD4 choice and its TCR specificity to class I or class II MHC was noted more than 20 years ago, and establishing the underlying mechanism has remained a focus of intense study since then. This review deals with three formerly discrete topics that are gradually becoming interconnected: the role of TCR signaling in lineage commitment, the regulation of expression of the CD4 and CD8 genes, and transcriptional regulation of lineage commitment. It is widely accepted that TCR signaling exerts a decisive influence on lineage choice, although the underlying mechanism remains intensely debated. Current evidence suggests that both duration and intensity of TCR signaling may control lineage choice, as proposed by the kinetic signaling and quantitative instructive models, respectively. Alternate expression of the CD4 and CD8 genes is the most visible manifestation of lineage choice, and much progress has been made in defining the responsible cis elements and transcription factors. Finally, important clues to the molecular basis of lineage commitment have been provided by the recent identification of the transcription factor ThPOK as a key regulator of lineage choice. ThPOK is selectively expressed in class II-restricted cells at the CD4(+)8(lo) stage and is necessary and sufficient for development to the CD4 lineage. Given the central role of ThPOK in lineage commitment, understanding its upstream regulation and downstream gene targets is expected to reveal further important aspects of the molecular machinery underlying lineage commitment.

ALTERATIONS IN THE THYMOPOIESIS IN EXPERIMENTAL AUTOIMMUNE MYASTHENIA GRAVIS

Experimental autoimmune myasthenia gravis (EAGM) was induced in female AO rats, by a single immunization with Torpedo acetylcholine receptor (AChR). Animals injected with saline served as controls. The study showed substantial changes in EAMG rats in the thymopoiesis, causing an increase in the relative proportion of mature CD8+ and, particularly, CD4+ (possibly autoreactive) single positive (SP) cells expressing TCRalphabeta at high level (TCRalphabeta(high)), as well as in that of mature double negative (DN) TCRalphabeta(high) cells, which are believed to be the immunoregulatory cells that augment autoantibody (autoAb) production. These results indicate that an augmented production of autoreactive CD4+ cells, on one side, and an increase of the immunoregulatory T cells that augment autoAb production, on the other side (reflecting, most likely, an increased entry of activated autoreactive CD4+ T cells from the periphery into the thymus), may have a significant role in the sustention of immune response in EAMG, and may suggest a putative mechanism underlying the sustention of autoimmune response in acquired MG.

Targeting CD4 coreceptor expression to postselection thymocytes reveals that CD4/CD8 lineage choice is neither error-prone nor stochastic

The mechanism by which CD4/CD8 lineage choice is coordinated with TCR specificity during positive selection remains an unresolved problem in immunology. The stochastic/selection model proposes that CD4/CD8 lineage choice in TCR-signaled CD4(+)CD8(+) thymocytes occurs randomly and therefore is highly error-prone. This perspective is strongly supported by "coreceptor rescue" experiments in which transgenic CD4 coreceptors were ectopically expressed on thymocytes throughout their development and caused significant numbers of cells bearing MHC-II-specific TCR to differentiate into mature, CD8 lineage T cells. However, it is not known if forced coreceptor expression actually rescued positively selected thymocytes making an incorrect lineage choice or if it influenced developing thymocytes into making an incorrect lineage choice. We have now reassessed coreceptor rescue and the concept that lineage choice is highly error-prone with a novel CD4 transgene (referred to as E8(I)-CD4) that targets expression of transgenic CD4 coreceptors specifically to thymocytes that have already undergone positive selection and adopted a CD8 lineage fate. Unlike previous CD4 transgenes, the E8(I)-CD4 transgene has no effect on early thymocyte development and cannot itself influence CD4/CD8 lineage choice. We report that the E8(I)-CD4 transgene did in fact induce expression of functional CD4 coreceptor proteins on newly arising CD8 lineage thymocytes precisely at the point in thymic development that transgenic CD4 coreceptors would putatively rescue MHC-II-specific thymocytes that incorrectly adopted the CD8 lineage. However, the E8(I)-CD4 transgene did not reveal any MHC-II-selected thymocytes that adopted the CD8 lineage fate. These results demonstrate that CD4/CD8 lineage choice is neither error-prone nor stochastic.

Lineage fate and intense debate: myths, models and mechanisms of CD4- versus CD8-lineage choice

Following successful gene rearrangement at alphabeta T-cell receptor (TCR) loci, developing thymocytes express both CD4 and CD8 co-receptors and undergo a life-or-death selection event, which is known as positive selection, to identify cells that express TCRs with potentially useful ligand specificities. Positively selected thymocytes must then differentiate into either CD4(+) helper T cells or CD8(+) cytotoxic T cells, a crucial decision known as CD4/CD8-lineage choice. In this Review, we summarize recent advances in our understanding of the cellular and molecular events involved in lineage-fate decision and discuss them in the context of the major models of CD4/CD8-lineage choice.

CD4-CD8 lineage commitment is regulated by a silencer element at the ThPOK transcription-factor locus

The transcription factor ThPOK is necessary and sufficient to trigger adoption of the CD4 lymphocyte fate. Here we investigate the regulation of ThPOK expression and its subsequent control of CD4+ T cell commitment. Treatment of immature thymocytes with anti-TCR (T cell receptor) showed that TCR signals were important in ThPOK induction and that the CD4+8lo stage was the likely target of the inductive TCR signal. We identified at the ThPOK locus a key distal regulatory element (DRE) that mediated its differential expression in class I- versus II-restricted CD4+8lo thymocytes. The DRE was both necessary for suppression of ThPOK expression in class I-restricted thymocytes and sufficient for its induction in class II-restricted thymocytes. Mutagenesis analysis defined an essential 80bp core DRE sequence and its potential regulatory motifs. We propose a silencer-dependent model of lineage choice, whereby inactivation of the DRE silencer by a strong TCR signal leads to CD4 commitment, whereas continued silencer activity leads to CD8 commitment.

MHC class II antigen presentation and immunological abnormalities due to deficiency of MHC class II and its associated genes

Antigen presentation by Major Histocompatibility Complex (MHC) class II molecules plays an important role in controlling immunity and autoimmunity. Multiple co-factors including the invariant chain (Ii), HLA-DM and HLA-DO are involved in this process. While the role for Ii and DM has been well defined, the biological function of DO remains obscure. Our data indicate that DO inhibits presentation of endogenous self-antigens and that developmentally-regulated DO expression enables antigen presenting cells to preferentially present different sources of peptide antigens at different stages of development. Disruption of this regulatory mechanism can result in not only immunodeficiency but also autoimmunity. Despite the fact that deletion of each of the three genes in experimental animals is associated with profound immunological abnormalities, no corresponding human diseases have been reported. This discrepancy suggests the possibility that primary immunodeficiencies due to a genetic defect of Ii, DM and DO in humans are under diagnosed or diagnosed as "common variable immunodeficiency", a category of immunodeficiency of heterogeneous or undefined etiology. Clinical tests for any of these potential genetic defects are not yet available. We propose the use of multi-color flow cytometry in conjunction with intracellular staining to detect expression of Ii, DM, DO in peripheral blood B cells as a convenient reliable screening test to identify individuals with defects in antigen presentation.

Vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) provide co-stimulation in positive selection along with survival of selected thymocytes

T-cell differentiation in the thymus depends on positive selection of CD4+CD8+ double positive (DP) thymocytes by thymic major histocompatibility complex (MHC) molecules. Positive selection allows maturation of only those thymocytes that are capable of self-peptide-MHC recognition. Thymocytes that fail to bind self-peptide-MHC die by apoptosis. An important question in thymocyte differentiation is whether co-stimulation is required for positive selection and on which cells co-stimulatory molecules may be expressed in the thymus. The vascular cell adhesion molecule (VCAM-1) and the intercellular cell adhesion molecule (ICAM-1) are known to be potent co-stimulatory molecules in activation of peripheral T-cells by interacting with the integrins VLA-4 and LFA-1, respectively. We were prompted to investigate whether VCAM-1 and ICAM-1 may also act as co-stimulators during selection of thymocytes. By using recombinant proteins of murine VCAM-1 and ICAM-1 fused to the Fc region of human IgG1 (rVCAM-1, rICAM-1) we examined the capacity of VCAM-1 and ICAM-1 to act as co-stimulatory molecules in positive selection in vitro. Triggering the CD3/TCR complex together with co-stimulation applied by rVCAM-1 or rICAM-1 induced the generation of CD4+ single positive (SP) thymocytes from CD4+CD8+ DP thymocytes whereas either signal alone did not result in generation of CD4+ SP thymocytes. VCAM-1 and ICAM-1 act therefore as co-stimulatory molecules in thymocyte positive selection in vitro. The generation of CD4+ SP cells is accompanied by cell survival both when it was co-stimulated with rVCAM-1 and with rICAM-1. Importantly we show here that VCAM-1 expression in the murine thymus is restricted to cortical F4/80 positive hematopoietic antigen presenting cells (hAPC) present exclusively in the cortex whereas expression of ICAM-1 has been reported on the epithelium both in cortex and medulla. This suggests that not only the cortical epithelium may use the co-stimulatory molecule ICAM-1 to mediate positive selection, but also cortical hAPCs may contribute to positive selection of thymocytes by using the co-stimulator VCAM-1.

Decision‐making by the immune response

Decisions by uncommitted cells to differentiate down one lineage pathway or another is fundamental to developmental biology. In the immune system, lymphocyte precursors commit to T- or B-cell lineages and T-cell precursors to CD4 or CD8 independently of foreign antigen. T and B cells must also decide whether or not to respond to antigen and when a response is initiated, what sort of response to make such as the type of antibody, CD4 or CD8, and CD4 Th1 or Th2. The two basic mechanisms for these decision-making processes are selection and instruction. Selection depends on prior stochastic production of precommitted cells, which are then selected to respond by an appropriate signal; for example, CD8 and CD4 responses selected by peptide presented in association with major histocompatibility complex class I or II. In contrast, instruction occurs when an uncommitted precursor embarks upon a differentiation pathway in response to a particular set of signals; for example, Th1 and Th2 lineage commitment. In this paper, the signals that determine Th1 and Th2 differentiation are examined with a mathematical model and shown to act as a bistable switch permitting either Tbet or Gata3 to be expressed in an individual cell but not both. The model is used to show how the Tbet Gata3 network within an individual cell interacts with cytokine signals between cells and suggests how Th1 and Th2 lineage commitment can become irreversible. These considerations provide an example of how mathematical models can be used to gain a better understanding of lymphocyte differentiation in an immune response.

Agonist ligands expressed by thymic epithelium enhance positive selection of regulatory T lymphocytes from precursors with a normally diverse TCR repertoire

CD4+CD25+ regulatory T lymphocytes play a crucial role in inhibition of autoimmune pathology. In accordance with this physiological role, it is now well established that the repertoire of these lymphocytes is strongly enriched in autospecific cells. However, despite extensive investigation, the thymic mechanisms involved in development of regulatory T cells remain incompletely defined. To address the issue of selection of regulatory T cell precursors in mice with a naturally diverse TCR repertoire, we have analyzed development of superantigen-specific regulatory T cells in hemopoietic chimeras in which endogenous super-antigens are exclusively presented by thymic epithelial cells. Our results demonstrate that recognition of agonist ligands expressed by thymic epithelium does not lead to deletion but substantially enhances development of mature regulatory T cells. Interestingly, also development of a small subpopulation of CD25-expressing T cells lacking expression of the transcription factor Foxp3, thought to be autospecific, is enhanced by expression of the agonist ligand on thymic epithelium. Based on quantitative arguments, we propose that commitment to the regulatory T cell lineage is not dictated by the specificity of precursors, but that recognition of the agonist ligand expressed by thymic epithelium substantially enhances their positive selection.

Self-recognition is crucial for maintaining the peripheral CD4+ T-cell pool in a nonlymphopenic environment

The role of self-recognition in the maintenance of the peripheral CD4+ T-cell pool has been extensively studied, but no clear answer has so far emerged. Indeed, in studies of the role of self-major histocompatibility complex (MHC) molecules in CD4+ T-cell survival, several parameters must be taken into account when interpreting the results: (1) in a lymphopenic environment, observations are biased by concomitant proliferation of T cells arising in MHC-expressing mice; (2) the peripheral T-cell compartment is qualitatively and quantitatively different in nonlymphopenic, normal, and MHC class II-deficient mice; and (3) in C57BL/6 Aβ-/- mice (traditionally considered MHC class II-deficient), the Aα chain and the Eβ chain associate to form a hybrid AαEβ MHC class II molecule. In light of these considerations, we revisited the role of interactions with MHC class II molecules in the survival of peripheral CD4+ T cells. We found that the answer to the question “is self-recognition required for CD4+ T cells to survive?” is not a simple yes or no. Indeed, although long-term survival of CD4+ T cells does not depend on self-recognition in lymphopenic mice, interactions with MHC class II molecules are required for maintaining the peripheral CD4+ T-cell pool in a nonlymphopenic environment. (Blood. 2006;108:270-277)

Age-associated changes in CD90 expression on thymocytes and in TCR-dependent stages of thymocyte maturation in male rats

To elucidate the effects of ageing on T-cell-maturation, in 3- and 18-month-old rats, we analysed the expression of: (i) CD4/CD8/TCRalphabeta and (ii) Thy-1, which is supposed to be a regulator of TCRalphabeta signalling, and thereby the thymocyte selection thresholds. Since an essential role for TCRalphabeta signalling in the development of CD4+25+T(reg)-cells was suggested, the frequency of these cells was also quantified. We demonstrated that, as for mice, early thymocyte differentiational steps within the CD4-8- double negative (DN) developmental stage are age-sensitive. Furthermore, we revealed that TCRalphabeta-dependent stages of T-cell development are affected by ageing, most likely due to an impaired expression of Thy-1 on TCRalphabeta(low) thymocytes entering selection processes. The diminished frequency of the post-selection CD4+8+ double positive (DP) cells in aged rats, together with an overrepresentation of mature single positive (SP) cells, most probably suggests more efficient differentiational transition from the DP TCRalphabeta(high) to the SP TCRalphabeta(high) developmental stage, which is followed by an increase in pre-migration proliferation of the mature SP cells. Moreover, the study indicated impaired intrathymic generation of CD4+25+T(reg)-cells in aged rats, thus providing a possible explanation for the increased frequency of autoimmune diseases in ageing.

Characterization of thymocyte phenotypic alterations induced by long-lasting beta-adrenoceptor blockade in vivo and its effects on thymocyte proliferation and apoptosis

Adult male Wistar rats were subjected to propranolol (P, 0.40 mg/100 g/day) or saline (S) administration (controls) over 14 days. The expression of major differentiation molecules on thymocytes and Thy-1 (CD90) molecules, which are shown to adjust thymocyte sensitivity to TCRalphabeta signaling, was studied. In addition, the sensitivity of thymocytes to induction of apoptosis and concanavalin A (Con A) signaling was estimated. The thymocytes from P-treated (PT) rats exhibited an increased sensitivity to induction of apoptosis, as well as to Con A stimulation. Furthermore, P treatment produced changes in the distribution of thymocyte subsets suggesting that more cells passed positive selection and further differentiated into mature CD4+ or CD8+ single positive (SP) TCRalphabeta(high) cells. These changes may, at least partly, be related to the markedly increased density of Thy-1 surface expression on TCRalphabeta(low) thymocytes from these rats. The increased frequency of cells expressing the CD4+25+ phenotype, which has been shown to be characteristic for regulatory cells in the thymus, may also indicate alterations in thymocyte selection following P treatment. Inasmuch as positive and negative selections play an important role in continuously reshaping the T-cell repertoire and maintaining tolerance, the hereby presented study suggests that pharmacological manipulations with beta-AR signaling, or chemically evoked alterations in catecholamine release, may interfere with the regulation of thymocyte selection, and consequently with the immune response.

Commitment issues: linking positive selection signals and lineage diversification in the thymus

The thymus is responsible for the production of CD4+ helper and CD8+ cytotoxic T cells, which constitute the cellular arm of the immune system. These cell types derive from common precursors that interact with thymic stroma in a T-cell receptor (TCR)-specific fashion, generating intracellular signals that are translated into function-specific changes in gene expression. This overall process is termed positive selection, but it encompasses a number of temporally distinct and possibly mechanistically distinct cellular changes, including rescue from apoptosis, initiation of cell differentiation, and commitment to the CD4+ or CD8+ T-cell lineage. One of the puzzling features of positive selection is how specificity of the TCR controls lineage commitment, as both helper and cytolytic T cells utilize the same antigen-receptor components, with the exception of the CD4 or CD8 coreceptors themselves. In this review, we focus on the signals required for positive selection, particularly as they relate to lineage commitment. Identification of genes encoding transcriptional regulators that play a role in T-cell development has led to significant recent advances in the field. We also provide an overview of nuclear factors in this context and, where known, how their regulation is linked to the same TCR signals that have been implicated in initiating and regulating positive selection.

Role of the transcription factor Th-POK in CD4:CD8 lineage commitment

The molecular basis of CD4:CD8 lineage commitment, in particular the mechanism by which the precise correlation between lineage choice and T-cell receptor (TCR) specificity toward class I or II major histocompatibility complex is achieved, remains controversial. Both stochastic/selective and instructive models in various forms have been proposed to explain this correlation. The two main experimental approaches previously employed to elucidate this process have focused on the beginning and end of the process, i.e. the influence of TCR signaling and the alternate transcriptional control of the CD4 and CD8 loci during commitment. The recent finding that the transcription factor Th-POK is necessary and sufficient for CD4 commitment has now provided a direct entry point for studying the intracellular pathways that govern lineage commitment. Here, we review data leading to the identification and characterization of this factor and discuss the implications of these studies in the context of current models of lineage commitment.

Molecular signature of recent thymic selection events on effector and regulatory CD4+ T lymphocytes

Natural CD4+CD25+ regulatory T lymphocytes (Treg) are key protagonists in the induction and maintenance of peripheral T cell tolerance. Their thymic origin and biased repertoire continue to raise important questions about the signals that mediate their development. We validated analysis of MHC class II capture by developing thymocytes from thymic stroma as a tool to study quantitative and qualitative aspects of the cellular interactions involved in thymic T cell development and used it to analyze Treg differentiation in wild-type mice. Our data indicate that APCs of bone marrow origin, but, surprisingly and importantly, not thymic epithelial cells, induce significant negative selection among the very autoreactive Treg precursors. This fundamental difference between thymic development of regulatory and effector T lymphocytes leads to the development of a Treg repertoire enriched in cells specific for a selected subpopulation of self-Ags, i.e., those specifically expressed by thymic epithelial cells.

Analyzing expression of perforin, Runx3, and Thpok genes during positive selection reveals activation of CD8-differentiation programs by MHC II-signaled thymocytes

Intrathymic positive selection matches CD4-CD8 lineage differentiation to MHC specificity. However, it is unclear whether MHC signals induce lineage choice or simply select thymocytes of the appropriate lineage. To investigate this issue, we assessed thymocytes undergoing positive selection for expression of the CD8 lineage markers perforin and Runx3. Using both population-based and single-cell RT-PCR analyses, we found large subsets of MHC class II (MHC-II)-signaled thymocytes expressing these genes within the CD4+ 8+ and CD4+ 8(int), but not the CD4+ 8- populations of signaling competent mice. This indicates that MHC-II signals normally fail to impose CD4 differentiation and further implies that the number of mature CD8 single-positive (SP) thymocytes greatly underestimates CD8 lineage choice. We next examined whether MHC-II-restricted CD4+ 8- thymocytes remain competent to initiate CD8 lineage gene expression. In mice in which expression of the tyrosine kinase Zap70 and thereby TCR signaling were impaired selectively in SP thymocytes, MHC-II-signaled CD4+ 8- thymocytes expressed perforin and Runx3 and failed to up-regulate the CD4 marker Thpok. This indicated that impairing TCR signals at the CD4 SP stage switched gene expression patterns from CD4- to CD8-lineage specific. We conclude from these findings that MHC-II-signaled thymocytes remain competent to initiate CD8-specific gene expression even after CD8 down-regulation and that CD4 lineage differentiation is not fixed before the CD4 SP stage.

CD4-CD8 lineage commitment: an inside view

The mechanism of CD4-CD8 lineage commitment, which ensures the correlation between T cell receptor specificity and adoption of the T killer or T helper phenotype, has long been the subject of intense debate. Various approaches are slowly elucidating the underlying molecular pathways. Analysis of the function of T cell receptor signaling (the 'top-down' approach) supports the view that differences in signal strength and/or duration 'instruct' alternative commitment. Analysis of the transcriptional regulation of the genes encoding CD4 and CD8 (the 'bottom-up' approach) has identified critical cis-acting elements and their interacting factors. Finally, identification of the transcription factor Th-POK as a central component of the CD4 lineage-determining pathway has provided a new starting point from which to unravel this intriguing process 'from the inside out'.

Fisiopatologia da dermatite de contato alérgica: papel das células T CD8 efetoras e das células T CD4 regulatórias

A dermatite de contato alérgica (DCA), também conhecida como hipersensibilidade de contato (HSC) é uma das dermatoses inflamatórias mais freqüentes, sendo caracterizada por eritema, pápulas e vesículas, seguidas de ressecamento e descamação. A DCA é induzida pelo contato da pele com substâncias químicas não protéicas denominadas haptenos, e corresponde a uma reação de hipersensibilidade cutânea do tipo tardio, mediada por células T hapteno-específicas. Durante a fase de sensibilização, tanto os precursores de células T CD4+ quanto os de CD8+ são ativados nos linfonodos de drenagem através da apresentação de peptídeos conjugados a haptenos pelas células dendríticas (CD) da pele. A subseqüente exposição de pele ao hapteno em um local a distância induz o recrutamento e ativação de células T específicas no local de provocação, levando à apoptose dos queratinócitos, recrutamento de células inflamatórias e desenvolvimento de sintomas clínicos. Estudos experimentais dos últimos 10 anos demonstraram que, em respostas normais de HSC a haptenos fortes, as células T CD8+ do tipo 1 são efetoras da HSC através de citotoxicidade e produção de IFNgama, enquanto que as células T CD4+ são dotadas de funções de regulação negativa. Estas últimas podem corresponder à população de células T regulatórias CD4+ CD25+ recentemente descritas. Entretanto, em algumas situações, especialmente naquelas em que há um pool deficiente de células T CD8, as células T CD4+ podem ser efetoras da HSC. Estudos em andamento deverão confirmar que a fisiopatologia da DCA em humanos é semelhante à HSC em camundongos, e que a resposta de HSC a haptenos fracos comuns, mais freqüentemente envolvidos na DCA em humanos, é semelhante à descrita para haptenos fortes.

Mixed-haplotype MHC class II molecules select functional CD4+ T cells

MHC class II molecules are formed from polymorphic alpha and beta chains. While pairing of chains is most efficient within class II isotypes and haplotypes, limited pairing and surface expression of mixed-haplotype and -isotype class II molecules is common. The function of such molecules in antigen presentation has been established by the unique restriction of responses in F1 mice. However, it has not been established whether mixed class II molecules are able to mediate selection of functional T cells and how the reduced avidity of the TCR/MHC interaction influences the repertoire. In this report we have addressed these issues through the production of mice expressing solely mixed-haplotype class II molecules. The mixed class II molecules promote selection of a small CD4+ T cell repertoire with modified TCR use. The selected CD4+ T cells are functional in vivo and in vitro.

Modulation of Coreceptor Transcription during Positive Selection Dictates Lineage Fate Independently of TCR/Coreceptor Specificity

For developing T cells, coreceptor choice is matched to T cell antigen receptor (TCR) MHC specificity during positive selection in the thymus, but the mechanism remains uncertain. Here, we document that TCR-mediated positive selection signals inactivate the immature CD8(III) enhancer in double positive (DP) thymocytes, explaining in part the cessation of CD8 coreceptor transcription that occurs during positive selection. More importantly, by placing CD4 protein expression under the control of CD8 transcriptional regulatory elements, we demonstrate that cessation of CD4 coreceptor transcription during positive selection results in precisely the same lineage fate as cessation of CD8 coreceptor transcription. That is, MHC-II-signaled DP thymocytes differentiated into CD8-lineage cytotoxic T cells, despite the MHC-II specificity and CD4 dependence of their TCRs. This study demonstrates that termination of coreceptor transcription during positive selection promotes CD8-lineage fate, regardless of TCR specificity or coreceptor protein identity.

An antagonist peptide mediates positive selection and CD4 lineage commitment of MHC class II-restricted T cells in the absence of CD4

The CD4 coreceptor works together with the T cell receptor (TCR) to deliver signals to the developing thymocyte, yet its specific contribution to positive selection and CD4 lineage commitment remains unclear. To resolve this, we used N3.L2 TCR transgenic, RAG-, and CD4-deficient mice, which are severely impaired in positive selection, and asked whether altered peptide ligands can replace CD4 function in vivo. Remarkably, in the presence of antagonist ligands that normally deleted CD4⁺ T cells in wild-type mice, we induced positive selection of functional CD4 lineage T cells in mice deficient in CD4. We show that the kinetic threshold for positive and negative selection was lowered in the absence of CD4, with no evident skewing toward the CD8 lineage with weaker ligands. These results suggest that CD4 is dispensable as long as the affinity threshold for positive selection is sustained, and strongly argue that CD4 does not deliver a unique instructional signal for lineage commitment.

Changes in thymopoiesis in myasthenia gravis

This study was undertaken to investigate T-cell maturation in hyperplastic thymi of patients suffering from myasthenia gravis (MG). For this purpose, the expression of the major differentiational molecules (CD4, CD8, and CD3/TCRalphabeta) and that of the regulatory and activation molecules on thymocytes from MG patients and control subjects were estimated by flow cytometric analysis. In the MG patients the increase in relative proportion of immature (CD4-8- TCRalphabeta-) and the most mature (CD4+8- TCRalphabetahigh and CD4-8- TCRhigh encompassing immunoregulatory NKT) thymocytes followed by a decrease in that of CD4+8+CD3-/TCRalphabeta- cells was found. Furthermore, in these patients the relative proportion of CD4+HLA-DR+ and CD4+71+ cells was increased, whereas that of CD4+25+ cells was slightly, but significantly, decreased (reflecting, most likely, decreased contribution of T reg cells bearing this phenotype). Moreover, in MG thymi the percentage of CD45RA+ cells was reduced indicating changes in the selection processes. In keeping with this finding the reduced thymocyte apoptotic index and percentage of cells bearing apoptosing (CD4-8- TCRalphabetalow) phenotype were detected. In conclusion, the study demonstrates substantial changes in intrathymic differentiation of T cells in hyperplastic MG thymi and suggests alterations in selection events providing an increased escape of potentially autoreactive T-cell clones, on one side, and an altered maturation and/or selection of immunoregulatory cells (NKT and CD4+8-25+ T reg cells) keeping these cell clones under control, on the other side.

Interferon-gamma mediates antigen trafficking to MHC class II-positive late endosomes of enterocytes

MHC class II-positive late endosomes of enterocytes are thought to be involved in antigen presentation to CD4(+) T cells. In contrast to enterocytes of BALB/c mice, severe combined immunodeficiency (SCID) enterocytes lack MHC class II expression and fail to transport internalized ovalbumin (OVA) into late endosomes. IFN-gamma is known to induce MHC class II in enterocytes and antigen targeting to late endosomes in macrophages. In this study, we investigated the influence of IFN-gamma and MHC class II on the processes of antigen traffic in enterocytes. Subcellular targeting of OVA and MHC class II expression within enterocytes were examined in SCID, IFN-gamma-treated SCID, BALB/c and C57BL/6 MHC class II knockout (KO) mice after a single feed with OVA. Sorting of OVA into late endosomes was found in enterocytes from BALB/c, C57BL/6 KO and IFN-gamma-stimulated SCID mice, but not from untreated SCID mice. MHC class II expression was restricted to enterocytes of IFN-gamma-treated SCID and BALB/c mice, present at basolateral membranes and within endosomal compartments. These enterocytes further revealed colocalization of class II antigens and OVA in endosomes. We suggest that antigen trafficking into late endosomes of enterocytes is mediated by IFN-gamma and occurs in the absence of MHC class II.

A novel pathway of alloantigen presentation by dendritic cells

In the context of transplantation, dendritic cells (DCs) can sensitize alloreactive T cells via two pathways. The direct pathway is initiated by donor DCs presenting intact donor MHC molecules. The indirect pathway results from recipient DCs processing and presenting donor MHC as peptide. This simple dichotomy suggests that T cells with direct and indirect allospecificity cannot cross-regulate each other because distinct APCs are involved. In this study we describe a third, semidirect pathway of MHC alloantigen presentation by DCs that challenges this conclusion. Mouse DCs, when cocultured with allogeneic DCs or endothelial cells, acquired substantial levels of class I and class II MHC:peptide complexes in a temperature- and energy-dependent manner. Most importantly, DCs acquired allogeneic MHC in vivo upon migration to regional lymph nodes. The acquired MHC molecules were detected by Ab staining and induced proliferation of Ag-specific T cells in vitro. These data suggest that recipient DCs, due to acquisition of donor MHC molecules, may link T cells with direct and indirect allospecificity.

Detailed analysis of gene expression during development of T cell lineages in the thymus

The genetic mechanisms that promote lineage commitment and eliminate autoreactive cells in the thymus are not well understood. To better understand this process, we have identified and quantitated transcripts in the two major thymocyte lineages by using serial analysis of gene expression. Approximately 25 genes displayed almost complete segregation to one or the other T cell lineage. Commitment to the CD4 lineage was marked by up-regulation of genes associated with increased survival and chaperone function followed by expression of genes that regulate nucleosome remodeling and T cell receptor signaling. Differentiation within the CD8 lineage, on the other hand, was marked by up-regulation of genes that regulate lymphocyte homing, followed by quenching of genes that inhibit apoptosis. Definition of differential gene expression during development of the two major thymocyte lineages will allow insight into mechanisms of T cell development after positive and negative selection.

The Novel Murine CD4+CD8+ Thymocyte Cell Line Exhibits Lineage Commitment into Both CD4+ and CD8+ T Cells by Altering the Intensity and the Duration of Anti-CD3 Stimulation In Vitro

A CD4(+)CD8(+) double-positive thymocyte cell line, 257-20-109 was established from BALB/c mice thymocytes and used to analyze the requirements to induce CD4 or CD8 single-positive (SP) T cells. CD4SP cells were induced from 257-20-109 cells by anti-CD3 stimulation in the presence of the FcR-positive macrophage cell line, P388D1. During stimulation, maturation events, such as the down-regulation of CD24 and the up-regulation of CD69, H-2D(d), CD5, and Bcl-2, were recognized. Furthermore, these CD4SP cells appeared to be functional because the cells produced IL-2 and IL-4 when activated with phorbol ester and calcium ionophore. In contrast, CD8SP cells could be induced by stimulation with fixed anti-CD3 after removal of stimulation. To investigate the extent of signals required for CD4SP and CD8SP, the cells stimulated under either condition for 2 days were sorted and transferred to different culture conditions. These results suggested that the fate of lineage commitment was determined within 2 days, and that CD4 lineage commitment required longer activation. Furthermore, the experiments with subclones of 257-20-109 demonstrated that the lower density of CD3 did not shift the cells from CD4SP to CD8SP, but only reduced the amount of CD4SP cells. In contrast, when the 257-20-109 cells were stimulated by the combination of fixed anti-CD3 and anti-CD28, the majority of the cells shifted to CD4SP, with an enhancement of extracellular signal-regulated kinase 1 phosphorylation. Our results indicate that the signals via TCR/CD3 alone shifted the double-positive cells to CD8SP cells, but the reinforced signals via TCR/CD3 and costimulator could commit the cells to CD4SP.

Unraveling a revealing paradox: Why major histocompatibility complex I-signaled thymocytes "paradoxically" appear as CD4+8lo transitional cells during positive selection of CD8+ T cells

The mechanism by which T cell receptor specificity determines the outcome of the CD4/CD8 lineage decision in the thymus is not known. An important clue is the fact that major histocompatibility complex (MHC)-I-signaled thymocytes paradoxically appear as CD4+8lo transitional cells during their differentiation into CD8+ T cells. Lineage commitment is generally thought to occur at the CD4+8+ (double positive) stage of differentiation and to result in silencing of the opposite coreceptor gene. From this perspective, the appearance of MHC-I-signaled thymocytes as CD4+8lo cells would be due to effects on CD8 surface protein expression, not CD8 gene expression. But contrary to this perspective, this study demonstrates that MHC-I-signaled thymocytes appear as CD4+8lo cells because of transient down-regulation of CD8 gene expression, not because of changes in CD8 surface protein expression or distribution. This study also demonstrates that initial cessation of CD8 gene expression in MHC-I-signaled thymocytes is not necessarily indicative of commitment to the CD4+ T cell lineage, as such thymocytes retain the potential to differentiate into CD8+ T cells. These results challenge classical concepts of lineage commitment but fulfill predictions of the kinetic signaling model.

Extrathymic T cell lymphopoiesis: ontogeny and contribution to gut intraepithelial lymphocytes in athymic and euthymic mice

In the absence of thymopoiesis, T lymphocytes are nevertheless present, mainly in the gut epithelium. Ontogeny of the extrathymic pathway and the extent of its involvement in euthymic mice are controversial. These questions have been addressed by assessing the expression of recombinase activating gene (RAG) through the use of green fluorescent protein RAG2 transgenic mouse models. In athymic mice, T lymphopoiesis occurs mainly in the mesenteric lymph node and less in the Peyer's patches. Ontogenic steps of this lymphopoiesis resemble those of thymopoiesis, but with an apparent bias toward gamma delta T cell production and with a paucity of oligoclonal alpha beta T cells possibly resulting from a deficit in positive selection. Whether in athymic or euthymic mice, neither T intraepithelial lymphocytes (IEL) nor cryptopatch cells (reported to contain precursors of IEL) displayed fluorescence indicating recent RAG protein synthesis. Newly made T cells migrate from the mesenteric node into the thoracic duct lymph to reach the gut mucosa. In euthymic mice, this extrathymic pathway is totally repressed, except in conditions of severe lymphocytic depletion. Thus, in normal animals, all gut T IEL, including CD8 alpha alpha(+) cells, are of thymic origin, CD8 alpha alpha(+) TCR alpha beta(+) IEL being the likely progeny of double negative NK1-1(-) thymocytes, which show polyclonal V alpha and V beta repertoires.

In vivo maintenance of T-lymphocyte unresponsiveness induced by thymic medullary epithelium requires antigen presentation by radioresistant cells

The T-cell repertoire developing in the thymus is rid of autospecific cells by the process of thymic negative selection. Recognition of major histocompatibility complex (MHC)/self-peptide complexes expressed by thymic antigen-presenting cells (APC) of bone marrow origin leads to induction of apoptotic death of autospecific thymocytes. Induction of tolerance to self-antigens not presented by thymic APC is mediated by medullary thymic epithelial cells (mTEC) which express a very wide range of proteins, e.g. inducible and tissue-specific proteins. The main type of tolerance induced by mTEC is non-deletional and the issue of how it is maintained outside the thymus is therefore of crucial interest. We have previously shown that the non-T-cell receptor (TCR) -transgenic T-cell repertoire developing in conditions in which tolerance to self-MHC/peptide ligands is exclusively induced by mTEC is tolerant to syngeneic targets in vivo but lyses such targets in vitro. Here we report that this non-deletional in vivo self-tolerance is not due to active tolerance assured by known naturally occurring regulatory or immune-modulating T lymphocytes. Importantly, we show that in vivo maintenance of this therefore probably anergic state requires continued interaction of autospecific T cells with self-MHC/peptide ligands expressed by radioresistant cells while APC are incapable of maintaining the tolerant state. Therefore, maintenance of non-deletional T-lymphocyte tolerance to the wide range of self-antigens expressed by mTEC depends on continued interaction with radioresistant cells that very probably express a much more limited repertoire of antigens. Our data may therefore have important consequences for tolerance to tissue-specific and inducible self-antigens.

Expression profiling of a transformed thymocyte cell line undergoing maturation in vitro identifies multiple genes involved in positive selection

Biochemical and genetic studies of thymocyte maturation would be facilitated by the development of cultured cell lines that reflect stages of positive selection. We have derived a CD4(+)CD8(+)TCR(+) T-lymphoid cell line (M20) from a murine thymic tumor induced by a retrovirus carrying the v-myc oncogene (M-MuLV(myc)). M20 subclones undergo several aspects of positive selection in response to co-culture with a thymic stromal cell line (St3), including down-regulation of CD4 and CD8, and up-regulation of CD5 and TCR. M20 possesses a functional TCR complex, and ligation of this complex produces changes similar to co-culture with St3 stroma. Expression profiling of M20 cells in this system identified 23 genes previously shown to be important in thymocyte maturation, as well as several novel candidate genes. This system provides a new model to elucidate the molecular mechanisms of thymocyte maturation and TCR-mediated cell signaling in double-positive thymocytes.

Soluble factor-mediated differentiation of CD4+CD8+ thymocytes to single positives in vitro

Although the thymic microenvironment provides the necessary elements for T-cell differentiation, the precise role of individual components remains to be determined. In this paper, attempts were made to address the possibility that CD4 or CD8 single-positive (SP) thymocytes could be developed from immature CD4+CD8+ (double-positive; DP) thymocytes in a suspension culture in the presence of soluble factors. We observed that IL-4 and IFN-gamma weakly induced DP cells to differentiate to CD4 cells, but not to CD8. In contrast, IL-2 weakly induced differentiation to CD8. Interestingly, Con A sup strongly induced differentiation to CD8 SP from the purified DP thymocytes prepared from C57BL/6 or LCMV TCRtg mice. In particular, it was found that thymocyte culture with Con A sup generated CD69+DP cells, and the CD69+DP differentiated to CD8 SP under the suspension culture with soluble factors. Thus, Con A sup or combinations of IL-2, IL-4 and IL-7 strongly induced differentiation of CD69+DP to CD8 SP, whereas individual cytokines did not. These results suggest that soluble factors like cytokines play an important role in the generation of SP thymocytes in the absence of thymic stromal cells, at least from a distinctive subpopulation like CD69+DP thymocytes, and perhaps from those of broader range when in conjunction with TCR/MHC interaction.

Rat peripheral CD4+CD8+ T lymphocytes are partially immunocompetent thymus-derived cells that undergo post-thymic maturation to become functionally mature CD4+ T lymphocytes

CD4+CD8+ double-positive (DP) T cells represent a minor subpopulation of T lymphocytes found in the periphery of adult rats. In this study, we show that peripheral DP T cells appear among the first T cells that colonize the peripheral lymphoid organs during fetal life, and represent approximately 40% of peripheral T cells during the perinatal period. Later their proportion decreases to reach the low values seen in adulthood. Most DP T cells are small size lymphocytes that do not exhibit an activated phenotype, and their proliferative rate is similar to that of the other peripheral T cell subpopulations. Only 30-40% of DP T cells expresses CD8beta chain, the remaining cells expressing CD8alphaalpha homodimers. However, both DP T cell subsets have an intrathymic origin since they appear in the recent thymic emigrant population after injection of FITC intrathymically. Functionally, although DP T cells are resistant to undergo apoptosis in response to glucocorticoids, they show poor proliferative responses upon CD3/TCR stimulation due to their inability to produce IL-2. A fraction of DP T cells are not actively synthesizing the CD8 coreceptor, and they gradually differentiate to the CD4 cell lineage in reaggregation cultures. Transfer of DP T lymphocytes into thymectomized SCID mice demonstrates that these cells undergo post-thymic maturation in the peripheral lymphoid organs and that their CD4 cell progeny is fully immunocompetent, as judged by its ability to survive and expand in peripheral lymphoid organs, to proliferate in response to CD3 ligation, and to produce IL-2 upon stimulation.

T-cell development and the CD4-CD8 lineage decision

Cell-fate decisions are controlled typically by conserved receptors that interact with co-evolved ligands. Therefore, the lineage-specific differentiation of immature CD4+ CD8+ T cells into CD4+ or CD8+ mature T cells is unusual in that it is regulated by clonally expressed, somatically generated T-cell receptors (TCRs) of unpredictable fine specificity. Yet, each mature T cell generally retains expression of the co-receptor molecule (CD4 or CD8) that has an MHC-binding property that matches that of its TCR. Two models were proposed initially to explain this remarkable outcome--'instruction' of lineage choice by initial signalling events or 'selection' after a stochastic fate decision that limits further development to cells with coordinated TCR and co-receptor specificities. Aspects of both models now appear to be correct; mistake-prone instruction of lineage choice precedes a subsequent selection step that filters out most incorrect decisions.

Naive T cells proliferate strongly in neonatal mice in response to self-peptide/self-MHC complexes

Adult naive T cells, which are at rest in normal conditions, proliferate strongly when transferred to lymphopenic hosts. In neonates, the first mature thymocytes to migrate to the periphery reach a compartment devoid of preexisting T cells. We have extensively analyzed the proliferation rate and phenotype of peripheral T cells from normal C57BL/6 and T cell antigen receptor transgenic mice as a function of age. We show that, like adult naive T cells transferred to lymphopenic mice, neonatal naive T cells proliferate strongly. By using bone-marrow transfer and thymic-graft models, we demonstrate that the proliferation of the first thymic emigrants reaching the periphery requires T cell antigen receptor-self-peptide/self-MHC interactions and is regulated by the size of the peripheral T cell pool.

New perspectives on a developmental dilemma: the kinetic signaling model and the importance of signal duration for the CD4/CD8 lineage decision

Double-positive thymocytes are short-lived bipotential cells whose developmental fate is determined by the specificity of their TCRs. A relatively small number of double-positive thymocytes undergo positive selection in the thymus and these are signaled to differentiate either into CD4(+) or CD8(+) mature T cells. The mechanism by which double-positive thymocytes determine their appropriate CD4/CD8 fate has been the subject of intense theoretical debate and rigorous experimental analysis. In the last year, 'signal duration' has been offered as a replacement for 'signal strength' as a major determinant of the CD4/CD8 decision, a deceptively minor refinement that requires a major change in our understanding of how signaled double-positive thymocytes differentiate into mature T cells. Indeed, the kinetic signaling model provides a radically new perspective on the mechanism by which the CD4/CD8 lineage decision is made.