Thymocyte development in major histocompatibility complex-deficient mice: evidence for stochastic commitment to the CD4 and CD8 lineages

What's self got to do with it: Sources of heterogeneity among naive T cells

There is a long-standing assumption that naive CD4⁺ and CD8⁺ T cells are largely homogeneous populations despite the extraordinary diversity of their T cell receptors (TCR). The self-immunopeptidome plays a key role in the selection of the naive T cell repertoire in the thymus, and self-peptides are also an important driver of differences between individual naive T cells with regard to their subsequent functional contributions to an immune response. Accumulating evidence suggests that as early as the β-selection stage of T cell development, when only one of the recombined chains of the mature TCR is expressed, signaling thresholds may be established for positive selection of immature thymocytes. Stochastic encounters subsequently made with self-ligands during positive selection in the thymus imprint functional biases that a T cell will carry with it throughout its lifetime, although ongoing interactions with self in the periphery ensure a level of plasticity in the gene expression wiring of naive T cells. Identifying the sources of heterogeneity in the naive T cell population and which functional attributes of T cells can be modulated through post-thymic interventions versus those that are fixed during T cell development, could enable us to better select or generate T cells with particular traits to improve the efficacy of T cell therapies.

NMR: an essential structural tool for integrative studies of T cell development, pMHC ligand recognition and TCR mechanobiology

Early studies of T cell structural biology using X-ray crystallography, surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC) focused on a picture of the αβT cell receptor (αβTCR) component domains and their cognate ligands (peptides bound to MHC molecules, i.e. pMHCs) as static interaction partners. Moving forward requires integrating this corpus of data with dynamic technologies such as NMR, molecular dynamics (MD) simulations and real-time single molecule (SM) studies exemplified by optical tweezers (OT). NMR bridges relevant timescales and provides the potential for an all-atom dynamic description of αβTCR components prior to and during interactions with binding partners. SM techniques have opened up vistas in understanding the non-equilibrium nature of T cell signaling through the introduction of force-mediated binding measurements into the paradigm for T cell function. In this regard, bioforces consequent to T-lineage cell motility are now perceived as placing piconewton (pN)-level loads on single receptor-pMHC bonds to impact structural change and αβT-lineage biology, including peptide discrimination, cellular activation, and developmental progression. We discuss herein essential NMR technologies in illuminating the role of ligand binding in the preT cell receptor (preTCR), the αβTCR developmental precursor, and convergence of NMR, SM and MD data in advancing our comprehension of T cell development. More broadly we review the central hypothesis that the αβTCR is a mechanosensor, fostered by breakthrough NMR-based structural insights. Collectively, elucidating dynamic aspects through the integrative use of NMR, SM, and MD shall advance fundamental appreciation of the mechanism of T cell signaling as well as inform translational efforts in αβTCR and chimeric T cell (CAR-T) immunotherapies and T cell vaccinology.

NMR-directed design of pre-TCRβ and pMHC molecules implies a distinct geometry for pre-TCR relative to αβTCR recognition of pMHC

The pre-T cell receptor (pre-TCR) guides early thymocytes through maturation processes within the thymus via interaction with self-ligands displayed on thymic epithelial cells. The pre-TCR is a disulfide-linked heterodimer composed of an invariant pre-TCR α (pTα) subunit and a variable β subunit, the latter of which is incorporated into the mature TCR in subsequent developmental progression. This interaction of pre-TCR with peptide-major histocompatibility complex (pMHC) molecules has recently been shown to drive robust pre-TCR signaling and thymocyte maturation. Although the native sequences of β are properly folded and suitable for NMR studies in isolation, a tendency to self-associate rendered binding studies with physiological ligands difficult to interpret. Consequently, to structurally define this critical interaction, we have re-engineered the extracellular regions of β, designated as β-c1, for prokaryotic production to be used in NMR spectroscopy. Given the large size of the full extracellular domain of class I MHC molecules such as H-K^b, we produced a truncated form termed K^b-t harboring properties favorable for NMR measurements. This system has enabled robust measurement of a pre-TCR-pMHC interaction directly analogous to that of TCRαβ-pMHC. Binding surface analysis identified a contact surface comparable in size to that of the TCRαβ-pMHC but potentially with a rather distinct binding orientation. A tilting of the pre-TCRβ when bound to the pMHC ligand recognition surface versus the upright orientation of TCRαβ would alter the direction of force application between pre-TCR and TCR mechanosensors, impacting signal initiation.

Pre-T Cell Receptors (Pre-TCRs) Leverage Vβ Complementarity Determining Regions (CDRs) and Hydrophobic Patch in Mechanosensing Thymic Self-ligands

The pre-T cell receptor (pre-TCR) is a pTα-β heterodimer functioning in early αβ T cell development. Although once thought to be ligand-autonomous, recent studies show that pre-TCRs participate in thymic repertoire formation through recognition of peptides bound to major histocompatibility molecules (pMHC). Using optical tweezers, we probe pre-TCR bonding with pMHC at the single molecule level. Like the αβTCR, the pre-TCR is a mechanosensor undergoing force-based structural transitions that dynamically enhance bond lifetimes and exploiting allosteric control regulated via the Cβ FG loop region. The pre-TCR structural transitions exhibit greater reversibility than TCRαβ and ordered force-bond lifetime curves. Higher piconewton force requires binding through both complementarity determining region loops and hydrophobic Vβ patch apposition. This patch functions in the pre-TCR as a surrogate Vα domain, fostering ligand promiscuity to favor development of β chains with self-reactivity but is occluded by α subunit replacement of pTα upon αβTCR formation. At the double negative 3 thymocyte stage where the pre-TCR is first expressed, pre-TCR interaction with self-pMHC ligands imparts growth and survival advantages as revealed in thymic stromal cultures, imprinting fundamental self-reactivity in the T cell repertoire. Collectively, our data imply the existence of sequential mechanosensor αβTCR repertoire tuning via the pre-TCR.

Direct detection of FoxP3 expression in thymic double-negative CD4-CD8- cells by flow cytometry

Foxp3 expression is a marker of regulatory T cells (Treg), but how early it is expressed in the thymus is still not fully defined. In this study, we examined Foxp3 expression in double-negative (DN) CD4(-)CD8(-) T cell precursors in the thymus by flow cytometry. By increasing the number of collected cells from the conventional 10(4) cells up to more than 10(6) cells during flow cytometry, we found that DN cells exhibited higher Foxp3 expression than double-positive (DP) CD4(+)CD8(+) and single-positive (SP) CD4(+) or CD8(+) (SP) T cells. CD44(+) expression positively correlated with Foxp3 in thymic DN cells. Furthermore, TCR-β(-)CD25(+) DN cells exhibited the highest frequency of Foxp3-expressing cells. Almost all Foxp3(+) cells expressed CD25in DN cells. These results suggest that Foxp3 expression in DN cells can directly be detected by flow cytometry and it was positively corelated with CD25 and CD44 in DN cells.

WITHDRAWN: The majority of T cells, including Treg cells, are developed from CD4-CD8-T progenitor cells without the involvement of the CD4+CD8+ stage in the thymus

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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.

OX40 ligand and CD30 ligand are expressed on adult but not neonatal CD4+CD3- inducer cells: evidence that IL-7 signals regulate CD30 ligand but not OX40 ligand expression

In this report, we have examined the expression of the T cell survival signals, OX40 ligand (OX40L) and CD30 ligand (CD30L) on CD4(+)CD3(-)CD11c(-)B220(-)IL-7Ralpha(+) inducer cells from birth to adulthood in mice. We found that adult but not neonatal inducer cells expressed high levels of OX40L and CD30L, whereas their expression of TNF-related activation-induced cytokine (TRANCE) and receptor activator of NF-kappaB (RANK) was comparable. The failure of neonatal inducer cells to express the ligands that rescue T cells helps to explain why exposure to Ag in neonatal life induces tolerance rather than immunity. The expression of OX40L and CD30L on inducer cells increased gradually in the first few weeks of life achieving essentially normal levels around the time mice were weaned. We found that IL-7 signaling through the common cytokine receptor gamma-chain was critical for the optimal expression of both TNF-related activation-induced cytokine and CD30L but not OX40L. Furthermore, glucocorticoids, which potently suppress T effector function, did not influence the expression of OX40L and CD30L in the presence of IL-7.

Localization of the domains in Runx transcription factors required for the repression of CD4 in thymocytes

The runt family transcription factors Runx1 and Runx3 are expressed in developing murine thymocytes. We show that enforced expression of full-length Runx1 in CD4(-)CD8(-) thymocytes results in a profound suppression of immature CD4/CD8 double-positive thymocytes and mature CD4 single-positive thymocytes compared with controls. This effect arises from Runx1- or Runx3-mediated repression of CD4 expression, and is independent of positively selecting signals. Runx1 is able to repress CD4 in CD4/CD8 double-positive thymocytes, but not in mature splenic T cells. Runx-mediated CD4 repression is independent of association with the corepressors Groucho/TLE or Sin3. Two domains are required for complete Runx-mediated CD4 repression. These are contained within Runx1 aa 212-262 and 263-360. The latter region contains the nuclear matrix targeting sequence, which is highly conserved among runt family transcription factors across species. The presence of the nuclear matrix targeting sequence is required for Runx-mediated CD4 repression, suggesting that Runx transcription factors are stabilized on the CD4 silencer via association with the nuclear matrix.

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.

Thymic selection revisited: how essential is it?

Intrathymic T cell development represents one of the best studied paradigms of mammalian development. Lymphoid committed precursors enter the thymus and the Notch1 receptor plays an essential role in committing them to the T cell lineages. The pre-T cell receptor (TCR), as an autonomous cell signaling receptor, commits cells to the alphabeta lineage while its rival, the gammadeltaTCR, is involved in generating the gammadelta lineage of T cells. Positive and negative selection of immature alphabetaTCR-expressing cells are essential mechanisms for generating mature T cells, committing them to the CD4 and CD8 lineages and avoiding autoimmunity. Additional lineages of alphabetaT cells, such as the natural killer T cell lineage and the CD25+ regulatory T cell lineage, are formed when the alphabetaTCR encounters specific ligands in suitable microenvironments. Thus, positive selection and receptor-instructed lineage commitment represent a hallmark of the thymus. Ectopically expressed organ-specific antigens contribute to thymic self-nonself discrimination, which represents an essential feature for the evolutionary fitness of mammalian species.

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.

Self-recognition promotes the foreign antigen sensitivity of naive T lymphocytes

Major histocompatibility complex (MHC) class I and II molecules are highly polymorphic proteins that bind and present foreign peptides to the clonally distributed alphabeta receptors (TCR) of T lymphocytes. As a population, the immature T lymphocytes generated in the thymus express a very diverse set of TCR specificities. A process of positive selection filters this broad repertoire to optimize peripheral T cells for antigen recognition in the context of available MHC products. Only those precursor T cells whose TCRs generate an adequate but not excessive signalling response to self-peptides bound to the expressed MHC proteins undergo successful maturation. Here we show that post-thymic self-recognition facilitates the antigen reactivity of mature T cells. Both experimental and physiological interruption of T-cell contact with self-peptide MHC ligands leads to a rapid decline in signalling and response sensitivity to foreign stimuli. Because the adaptive immune system must be recruited early in an infectious process when antigen is limiting, these findings suggest that positive selection ensures predictable T-cell recognition of available self-ligands, which in turn promotes efficient responses to pathogens.

Fas-dependent elimination of nonselected CD8 cells and lpr disease

MHC/self peptide interactions with cognate coreceptor/TCR complexes are central to homeostasis of the T cell repertoire. Recent reports have also underlined the critical role of IL-15/IL-2 cytokines in regulating this homeostatic process. In this study, we investigate mechanisms that regulate potentially autoreactive CD8 cells that have escaped intrathymic selection. These cells, upon exit from the thymus, express high levels of CD44, B220, and the IL-15R/IL-2R, and undergo fas-dependent apoptosis. Defects in fas signaling allow increased IL-15/IL-2-dependent survival of these CD44/B220(+) CD8(+) as well as the double-negative T cells characteristic of lpr disease.

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.

Deletion of the CD4 silencer element supports a stochastic mechanism of thymocyte lineage commitment

The mechanism of T cell lineage commitment remains controversial; to examine it we deleted the CD4-silencer element in the germ line of a mouse using a combination of gene targeting and Cre/LoxP-mediated recombination. We found that these mice were unable to extinguish CD4 expression either in immature thymocytes or mature CD8+ cytotoxic T cells (CTLs), which resulted in the development of major histocompatibility complex class II-restricted double-positive CTLs in the periphery. This finding strongly supports a stochastic over an instructive mechanism of coreceptor down-regulation.

Maturation versus death of developing double-positive thymocytes reflects competing effects on Bcl-2 expression and can be regulated by the intensity of CD28 costimulation

Immature double-positive (DP) thymocytes mature into CD4(+)CD8(-) cells in response to coengagement of TCR with any of a variety of cell surface "coinducer" receptors, including CD2. In contrast, DP thymocytes are signaled to undergo apoptosis by coengagement of TCR with CD28 costimulatory receptors, but the molecular basis for DP thymocyte apoptosis by TCR plus CD28 coengagement is not known. In the present study, we report that TCR plus CD28 coengagement does not invariably induce DP thymocyte apoptosis but, depending on the intensity of CD28 costimulation, can induce DP thymocyte maturation. We demonstrate that distinct but interacting signal transduction pathways mediate DP thymocyte maturation signals and DP thymocyte apoptotic signals. Specifically, DP maturation signals are transduced by the extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway and up-regulate expression of the antiapoptotic protein Bcl-2. In contrast, the apoptotic response stimulated by CD28 costimulatory signals is mediated by ERK/MAPK-independent pathways. Importantly, when TCR-activated thymocytes are simultaneously coengaged by both CD28 and CD2 receptors, CD28 signals can inhibit ERK/MAPK-dependent Bcl-2 protein up-regulation. Thus, there is cross-talk between the signal transduction pathways that transduce apoptotic and maturation responses, enabling CD28-initiated signal transduction pathways to both stimulate DP thymocyte apoptosis and also negatively regulate maturation responses initiated by TCR plus CD2 coengagement.

Inactivation of misselected CD8 T cells by CD8 gene methylation and cell death

Misselected CD8 cells that express T cell receptors (TCRs) that do not recognize class I major histocompatibility complex (MHC) protein can emerge from thymic selection. A postthymic quality control mechanism that purges these cells from the repertoire is defined here. The failure of mature CD8 cells to simultaneously engage their TCR and CD8 coreceptor triggers an activation process that begins with inhibition of CD8 gene expression through remethylation and concludes with up-regulation of surface Fas and Fas ligand and cellular apoptosis. Thus, inhibition of a death signal through continued TCR-CD8 coengagement of MHC molecules is a key checkpoint for the continued survival of correctly selected T cells. Molecular defects that prevent delivery of the death signal to mistakenly selected T cells underlie the expansion of double-negative T cells, which is the cellular signature of a subset of systemic autoimmune diseases.

CD4+ T Cells Mature in the Absence of MHC Class I and Class II Expression in Ly-6A.2 Transgenic Mice

The TCRs expressed on T lymphocytes recognize foreign peptides bound to MHC molecules. This reactivity is the basis of specific immune response to the foreign Ag. How such specificities are generated in the thymus is still being debated. Signals generated through TCR upon interaction with self MHC-peptide complexes are critical for maturation of the CD4+ helper and CD8+ cytotoxic subsets. We have observed maturation of CD4+ but not CD8+ T cells in Ly-6A.2 transgenic MHC null mice. Since there can be no interactions with MHC molecules in these mice, these CD4+ cells must express the T cell repertoire that exists before positive and negative selection. Interestingly, despite an absence of selection by MHC molecules, the CD4+ cells that mature recognize MHC molecules at a frequency as high as in CD4+ cells in normal mice. These results demonstrate that: 1) the germline sequences encoding TCRs are biased toward reactivity to MHC molecules; and 2) CD4+ cells as opposed to CD8+ cells have distinct lineage commitment signals. These results also suggest that signals originating from Ly-6 can promote or substitute for signals generated from TCR that are required for positive selection. Moreover, this animal model offers a system to study T cell development in the thymus that can provide insights into mechanisms of lineage commitment in developing T cells.

A novel Bcl-x isoform connected to the T cell receptor regulates apoptosis in T cells

We define a novel Bcl-x isoform, Bcl-x gamma, that is generated by alternative splicing and characterized by a unique 47 amino acid C-terminus. Bcl-x gamma is expressed primarily in thymocytes, where it may depend on an interaction between the TCR and host MHC products, and in mature T cells, where its expression is associated with ligation of the T cell receptor. Overexpression of Bcl-x gamma in T cells inhibits activation-induced apoptosis; inhibition of Bcl-x gamma, after stable expression of Bcl-x gamma antisense cDNA, enhances activation-induced apoptosis. In contrast to other Bcl-x isoforms, cells that fail to express Bcl-x gamma after CD3 ligation undergo programmed cell death, while activated T cells that express Bcl-x gamma are spared. Identification of Bcl-x gamma helps provide a molecular explanation of T cell activation and death after antigen engagement.

Two separable T cell receptor signals reconstitute positive selection of CD4 lineage T cells in vivo

Positive selection is an obligatory step during intrathymic T cell differentiation. It is associated with rescue of short-lived, self major histocompatibility complex (MHC)-restricted thymocytes from programmed cell death, CD4/CD8 T cell lineage commitment, and induction of lineage-specific differentiation programs. T cell receptor (TCR) signaling during positive selection can be closely mimicked by targeting TCR on immature thymocytes to cortical epithelial cells in situ via hybrid antibodies. We show that selection of CD4 T cell lineage cells in mice deficient for MHC class I and MHC class II expression can be reconstituted in vivo by two separable T cell receptor signaling steps, whereas a single TCR signal leads only to induction of short-lived CD4+CD8lo intermediates. These intermediates remain susceptible to a second TCR signal for 12-48 h providing an estimate for the duration of positive selection in situ. While both TCR signals induce differentiation steps, only the second one confers long-term survival on immature thymocytes. In further support of the two-step model of positive selection we provide evidence that CD4 T cell lineage cells rescued by a single hybrid antibody pulse in MHC class II-deficient mice are pre-selected by MHC class I.

Commitment of immature CD4+8+ thymocytes to the CD4 lineage requires CD3 signaling but does not require expression of clonotypic T cell receptor (TCR) chains

As a consequence of positive selection in the thymus, immature CD4(+)8(+) double-positive, [DP] thymocytes selectively terminate synthesis of one coreceptor molecule and, as a result, differentiate into either CD4(+) or CD8(+) T cells. The decision by individual DP thymocytes to terminate synthesis of one or the other coreceptor molecule is referred to as lineage commitment. Previously, we reported that the intrathymic signals that induced commitment to the CD4 versus CD8 T cell lineages were markedly asymmetric. Notably, CD8 commitment appeared to require lineage-specific signals, whereas CD4 commitment appeared to occur in the absence of lineage-specific signals by default. Consequently, it was unclear whether CD4 commitment, as revealed by selective termination of CD8 coreceptor synthesis, occurred in all DP thymocytes, or whether CD4 commitment occurred only in T cell receptor (TCR)-CD3-signaled DP thymocytes. Here, we report that selective termination of CD8 coreceptor synthesis does not occur in DP thymocytes spontaneously. Rather, CD4 commitment in DP thymocytes requires signals transduced by either CD3 or zeta chains, which can signal CD4 commitment even in the absence of clonotypic TCR chains.

From stem cells to lymphocytes: biology and transplantation

We review the development of the hematopoietic system, focusing on the transition from hematopoietic stem cells (HSCs) to T cells. This includes the isolation of HSCs, and recent progress in understanding their ontogeny, homing properties, and differentiation. HSC transplantation is reviewed, including the kinetics of reconstitution, engraftment across histocompatibility barriers, the facilitation of allogeneic engraftment, and the mechanisms of graft rejection. We describe progress in understanding T-cell development in the bone marrow and thymus as well as the establishment of lymph nodes. Finally, the role of bcl-2 in regulating homeostasis in the hematopoietic system is discussed.

Signals through CD8 or CD4 can induce commitment to the CD4 lineage in the thymus

Differentiation of thymocytes into mature single-positive T cells is an ordered process involving sequential interactions between T cell receptor (TCR), co-receptors (CD4 or CD8) and their appropriate major histocompatibility complex-encoded ligands. Precisely how these receptor/co-receptor engagements determine lineage commitment is still controversial, but recently it has been suggested that quantitative differences in the signal transmitted by co-ligation of CD4 versus CD8 with TCR might provide the discriminating signal. We examine this hypothesis, using bispecific F(ab')2 antibodies to mimic TCR/ co-receptor engagement during thymocyte differentiation. These bispecific antibodies lack Fc and can engage surface molecules without extensive cross-linking or targeting to Fc receptor-bearing cells. We show that TCR/CD3 co-ligation with CD4 induces efficient differentiation of mature CD4 lineage cells, irrespective of their TCR specificity. Interestingly, TCR/CD3 co-ligation with CD8 also induces maturation of CD4 T cells, although less efficiently, but not of CD8 T cells. Thus, although the signals delivered by co-ligation of TCR and CD8 appear weaker than from co-ligation of TCR and CD4, the outcome from either engagement is the same. These data suggest that differences in signal intensity alone do not determine lineage commitment in the thymus, but that distinct signals are required for CD4 and CD8 single-positive cell differentiation.

Positive selection of thymocytes bearing alpha beta T cell receptors

The past year has been marked by the application of new technologies to study thymocyte development. These studies have shown that positive selection affects a surprisingly large proportion of thymocytes, and now are beginning to investigate the affinities/avidities of the reactions involved. The lengthy controversy over how thymocytes decide whether to express CD4 or CD8 has taken a new turn with the discovery that levels of expression of the fate-determining protein Notch may affect the decision.

Thymocytes can become mature T cells without passing through the CD4+ CD8+, double-positive stage

T cells bearing the class II-restricted, DO-T cell receptor (TCR) are CD4+ if their thymocyte precursors are positively selected on the class II protein, IAd, but they are almost all CD4- after positive selection on a class II for which they have higher avidity, IAb. DO-TCR+ T cells mature in H-2b mice lacking CD4. CD4- DO-TCR+ T cells appear in H-2b mice at the same rate as their CD4+ counterparts appear in H-2d animals, suggesting that the CD4- cells are not the product of some minor pathway of thymocyte development and selection. In H-2b CD4 knock out mice expressing human CD2 under the control of the mouse CD4 promoter, mature DO-TCR+ cells did not express human CD2. These results suggest that the CD4-CD8-, DO-TCR+ mature T cells have developed without ever passing through the equivalent of a CD4+,CD8+ stage. The early expression of alpha/beta receptors (TCRs) on thymocytes in TCR transgenic mice may allow maturation of this type. Passage through the equivalent of the CD4+ CD8+, double-positive stage is not essential for differentiation of thymocytes into mature T cells.

Unexpectedly complex regulation of CD4/CD8 coreceptor expression supports a revised model for CD4+CD8+ thymocyte differentiation

CD4+ CD8+ TCRlo thymocytes are the precursors of CD4+ and CD8+ mature T cells, whose receptors show specific recognition of peptide-MHC class II and MHC class I complexes, respectively. How T cells emerge from the intrathymic differentiation process with selective expression of either CD8 molecule or CD4 molecule coordinated with the MHC class specificity of the TCR has been the subject of intense examination. Many previous studies of this question have been based on the assumption that extinction of CD4 or CD8 expression by the precursor thymocytes was a steady, uninterrupted process. Here we show that this is an incorrect assumption, with CD4 and CD8 expression undergoing an unexpectedly complex series of expression changes involving down-modulation, kinetically asymmetric up-regulation, and then selective loss. Based on these data, we propose a model for the differentiation pathway of alphabeta TCR thymocytes that explains previous, apparently contradictory findings and establishes useful parameters for future studies at the cellular and gene level.

Requirement for p56lck tyrosine kinase activation in T cell receptor-mediated thymic selection

The nonreceptor protein tyrosine kinase p56lck (Lck) serves as a fundamental regulator of thymocyte development by delivering signals from the pre-T cell receptor (pre-TCR) that permit subsequent maturation. However, considerable evidence supports the view that Lck also participates in signal transduction from the mature TCR. We have tested this conjecture by expressing a dominant-negative form of Lck under the control of a promoter element (the distal lck promoter) that directs high expression in CD4+CD8+ thymocytes, mature thymocytes, and peripheral T cells, thereby avoiding, complications that result from the well-documented ability of dominant-negative Lck to block very early events in thymocyte maturation. Here we report that expression of the catalytically inactive Lck protein at twice normal concentrations inhibits thymocyte positive selection by as much as 80%, while leaving other aspects of cell maturation intact. This effect was studied in more detail in mice simultaneously bearing the male-specific H-Y alpha/beta TCR transgene and ovalbumin-specific DO10 alpha/beta TCR transgene, where even equimolar expression of the dominant-negative Lck protein substantially vitiated the positive selection process. Although deletion of H-Y alpha/beta thymocytes proceeded normally in male mice despite the presence of catalytically inactive Lck, modest inhibition of superantigen-mediated deletion was in some cases observed. These data further implicate Lck in the propagation of all TCR-derived signals, and indicate that even very modest deficiencies in the representation of functional Lck molecules could in humans, profoundly alter the character of the peripheral TCR repertoire.

The c-kit+ maturation pathway in mouse thymic T cell development: lineages and selection

Positive selection of T cells begins with TCR alpha beta lo thymic progenitors. Here, we show that the most efficient TCRlo progenitors are c-kit+ with intermediate levels of CD4 and CD8 (DPint). Positive selection of DPint TCRlo c-kit+ cells results in TCRmed CD69+ c-kit+ transitional intermediates that show increased TCRV beta frequencies to selecting superantigen (SAg) that are committed to the CD4 or CD8 pathway. The cells on the c-kit+ maturation pathway maintain Bcl-2 expression. Most DPint c-kit+ progenitors fail positive selection, and become DPhi c-kit- cells that lose Bcl-2 expression. Some DPhi c-kit blast cells can be salvaged to produce mature single-positive (SP) cells. DPint c-kit+ maturation to SP cells can occur in <12 hr in vitro on thymic stromal monolayers.

Positive selection of CD4+ and CD8+ T cells

Significant progress has been made in characterizing intermediates and defining individual steps of positive selection, providing important insights into mechanisms of CD4/CD8 lineage commitment. New evidence suggests that specific recognition of peptides may be important for positive selection of CD4+ T cells. Several studies have defined signal-transduction pathways important for positive selection and have provided evidence that distinct signaling pathways may regulate positive versus negative selection.

MHC class II-specific T cells can develop in the CD8 lineage when CD4 is absent

The generation of mature CD4 T cells from CD4+CD8+ precursor thymocytes usually requires corecognition of class II MHC by a TCR and CD4, while the production of mature CD8 T cells requires corecognition of class I MHC by a TCR and CD8. To assess the role of the CD4 coreceptor in development and lineage commitment, we generated CD4-deficient mice expressing a transgenic class II-specific TCR. Surprisingly, in the absence of CD4 a large number of T cells mature, but these cells appear in the CD8 lineage. Thus, when CD4 is present, the majority of immature T cells with this class II-specific TCR choose the CD4 lineage but develop in the CD8 pathway when CD4 is absent. The results indicate that even for TCRs that are not dependent on coreceptor for MHC recognition, the coreceptor can influence the lineage choice. These findings are considered in terms of a quantitative signaling model for CD4/CD8 lineage commitment.

CD8/CD4 lineage commitment occurs by an instructional/default process followed by positive selection

In the present study, we investigated the developmental potential of purified populations of transitional CD4inCD8hi and CD4hiCD8in thymocytes that were further defined according to their differentiation stage by their levels of T cell receptor (TCR) expression into TCRlo, TCRin and TCRhi subpopulations. The differentiation potential of each of these subsets was tested in vitro in a single-cell suspension culture assay that showed that CD4inCD8hiTCRhi are precursors of CD8 single-positive cells, whereas CD4hiCD8inTCRin/hi are precursors of both CD4 and CD8 single-positive thymocytes. The analysis of transitional subsets in mutant mice for either beta 2-microglobulin or major histocompatibility complex (MHC) class II further revealed that lineage commitment to the CD8 lineage requires a TCR-MHC class I engagement, presumably at the immature double-positive stage of thymic development, while CD4 commitment does not require an MHC class II-mediated signal, but rather occurs by default. Using the addition of MHC class I- or class II-expressing cells or the addition of total thymocytes to purified sorted transitional precursors for the duration of the cultures in vitro, we identified an additional stage of differentiation for both CD4 and CD8 lineages that requires a positive selection signal. Examination of protein tyrosine phosphorylation of transitional precursors revealed that CD4inCD8hi transitional cells contain a high level of a 70-kDa phosphorylated protein consistent with a role for ZAP70 in the signal transduction during the positive selection of CD8+ cells.

The regulation and function of the CD4 coreceptor during T lymphocyte development

The data reviewed in this chapter suggest that the primary developmental function of the CD4 and CD8 coreceptors is to improve the efficacy by which a thymocyte recognizes peptide/MHC. During positive selection, DP thymocytes down-regulate expression of either CD4 or CD8 in response to signals that originate from the TCR/coreceptor complex. Experiments with transgenic and MHC-null mice have shown that coreceptor down-regulation and lineage commitment can occur stochastically in a manner that is independent of TCR specificity for MHC. Nevertheless, the positive selection of a given thymocyte is contingent on sustained expression of the coreceptor that is appropriate for the MHC specificity of its TCR. In most cases, loss of the required coreceptor blocks developmental progression and results in thymocyte apoptosis. CD4 expression is controlled by both positive and negative regulatory sequences embedded in the CD4 gene and it is likely that similar sequences regulate the CD8 gene. The down-regulation of coreceptor expression is coupled to a functional commitment which ensures that mature CD4+ T cells have a helper phenotype and CD8+ T cells have a cytotoxic phenotype. The molecular basis for this coupling and the identity of the switching mechanism which governs coreceptor regulation remain to be determined.

Stochastic coreceptor shut-off is restricted to the CD4 lineage maturation pathway

Kinetics of mature T cell generation in the thymus of normal or major histocompatibility complex (MHC) class I- or II-deficient mice were studied by the bromodeoxyuridine pulse labeling method. As previously described, the early activation and final maturation phases were found to be synchronous for the two T cell lineages, but CD4+8- cells were generated faster than CD4-8+ cells in MHC class I- and II-deficient mice, respectively. CD8 downregulation started on day 2 after cell proliferation even in the absence of MHC class II expression. CD8 downregulation thus appears to be stochastic at its beginning. By contrast, CD4 shut-off was found totally instructive, as the generation of CD4lo8+ cells with a high TCR density was not observed in class I-deficient mice. The analysis of the V beta 14 TCR frequencies in CD4/8 subsets in normal and MHC-deficient mice confirmed that CD4 and CD8 generation pathways are not symmetrical. These findings show that commitment towards the CD4+8- or CD4-8+ phenotype is controlled at the CD8lo step for the former and at the CD4+8+ double-positive stage for the latter.

Intermediate steps in positive selection: differentiation of CD4+8int TCRint thymocytes into CD4-8+TCRhi thymocytes

The differentiation potential of putative intermediates between CD4+8+ thymocytes and mature T cells has been examined. Such intermediate populations were sorted, in parallel with CD4+8+ thymocytes, from three types of C57BL/6 mice: major histocompatibility complex (MHC) class II-deficient mice, mice transgenic for an alpha/beta T cell receptor (TCR) restricted by class I MHC and normal mice. The sorted populations were then transferred into the thymus of nonirradiated C57BL/Ka mice differing in Thy 1 allotype, and the progeny of the transferred cells were analyzed 2 d later. Surprisingly, with all three types of donor mice, a major proportion of the CD4+8intTCRint-derived progeny were found to be CD4-8+TCRhi cells, thus delineating a new alternative pathway for development of the CD8 lineage. In contrast, the transfer of CD4int8+TCRint thymocytes produced CD4-8+TCRhi cells but no significant proportion of CD4+8-TCRhi cells, suggesting that there is no equivalent alternative pathway for the CD4 lineage. The results negate some of the evidence for a stochastic/selective model of lineage commitment, and point to an asymmetry in the steps leading to CD4-8+ versus CD4+8- T cells.

Intrathymic T cell receptor (TcR) targeting in mice lacking CD4 or major histocompatibility complex (MHC) class II: rescue of CD4 T cell lineage without co-engagement of TcR/CD4 by MHC class II

A critical step during intrathymic T cell development, termed positive selection, is associated with rescue of short-lived, immature thymocytes from programmed cell death, T cell lineage commitment, and induction of lineage-specific differentiation programs. T cell receptor (TcR)-major histocompatibility complex (MHC) interactions during positive selection can be closely mimicked by targeting TcR on immature thymocytes to cortical epithelial cells in situ via hybrid antibodies. Here, we show that antibody-mediated TcR signaling in mice deficient for CD4 or MHC class II expression induces polyclonal differentiation of the CD4 T cell lineage. Following a single TcR signal pulse in situ, a temporal sequence of phenotype changes can be discerned: CD69 up-regulation (< 1 day), CD8 down-regulation, TcR up-regulation (1-1.5 days) and down-regulation of the heat-stable antigen (1.5-2 days). Differentiation of phenotypically and functionally mature CD4 T cells in situ is attained within 3 days. Rescue of CD4 lineage T cells in the absence of TcR/CD4 co-engagement by MHC class II in this experimental system supports the stochastic/selective model of T cell lineage commitment.

T cell receptor-mediated signaling events in CD4+CD8+ thymocytes undergoing thymic selection: requirement of calcineurin activation for thymic positive selection but not negative selection

The goal of this study was to identify the differences of intracellular signals between the processes of thymic positive and negative selection. The activation of calcineurin, a calcium- and calmodulin-dependent phosphatase, is known to be an essential event in T cell activation via the T cell receptor (TCR). The effect of FK506, an inhibitor of calcineurin activation, on positive and negative selection in CD4+CD8+ double positive (DP) thymocytes was examined in normal mice and in a TCR transgenic mouse model. In vivo FK506 treatment blocked the generation of mature TCRhighCD4+CD8- and TCRhighCD4-CD8+ thymocytes, and the induction of CD69 expression on DP thymocytes. In addition, the shutdown of recombination activating gene 1 (RAG-1) transcription and the downregulation of CD4 and CD8 expression were inhibited by FK506 treatment suggesting that the activation of calcineurin is required for the first step (or the very early intracellular signaling events) of TCR-mediated positive selection of DP thymocytes. In contrast, FK506-sensitive calcineurin activation did not appear to be required for negative selection based on the observations that negative selection of TCR alpha beta T cells in the H-2b male thymus (a negative selecting environment) was not inhibited by in vivo treatment with FK506 and that there was no rescue of the endogenous superantigen-mediated clonal deletion of V beta 6 and V beta 11 thymocytes in FK506-treated CBA/J mice. DNA fragmentation induced by TCR activation of DP thymocytes in vitro was not affected by FK506. In addition, different effects of FK506 from Cyclosporin A on the T cell development in the thymus were demonstrated. The results of this study suggest that different signaling pathways work in positive and negative selection and that there is a differential dependence on calcineurin activation in the selection processes.

The function of the CD4 coreceptor in the development of T cells

T cells with helper activity can be found in mice that lack expression of the CD4 glycoprotein. The CD4 promoter is active in these cells; they respond to antigens presented by MHC class II molecules; they do not express CD8 and they do not depend on MHC class I for their development. By such criteria, these CD8- T cells resemble normal CD4+ helper T cells. The development of the helper lineage in CD4-null mice can be potentiated by expression of transgenes that encode either wild type CD4, or a deletion mutant of CD4 that lacks the cytoplasmic tail and therefore cannot interact with the tyrosine kinase p56lck. These observations suggest that CD4 is not absolutely required for the specification of the helper cell lineage. The role of the CD4 molecule in the development of T cells and possible mechanisms by which it achieves its functions are discussed.

The majority of postselection CD4+ single-positive thymocytes requires the thymus to produce long-lived, functional T cells

We have previously isolated, and characterized in vitro, two subsets of CD4hi T cell receptor (TCR)hi single positive (SP) thymocytes: CD8- and CD8lo. In this report, we have analyzed phenotypic, functional, and developmental characteristics of these "late" CD4hi SP thymocyte subsets. The TCRhi phenotype and the elimination of T cells expressing TCR V beta segments reactive with endogenous mouse mammary tumor virus (MMTV) products suggested that both subsets had undergone positive and negative selection. CD8-4hi thymocytes were functional, as judged by their ability to: (a) induce lethal graft versus host disease (GVHD); (b) survive and expand in peripheral lymphoid organs; and (c) proliferate, rather than undergo apoptosis, in response to in vitro TCR cross-linking. By contrast, CD8lo4hi cells could not induce GVHD, were unable to expand (and perhaps even survive) in peripheral organs and underwent apoptosis upon TCR cross-linking. However, when reintroduced into the thymus, these cells matured into functional, long-lived CD8-4hi lymphocytes. These results document an obligatory requirement for the thymic microenvironment in the final maturation of the majority of CD4hi SP postselection thymocytes, and demonstrate the existence of a previously unrecognized control point in T cell development.

Bcl-2 expression during T-cell development: early loss and late return occur at specific stages of commitment to differentiation and survival

During T-cell development CD3-CD4-CD8- (double-negative) thymocytes proliferative and produce an enormous number of CD3loCD4+CD8+ (double-positive) thymocytes which are destined to die intrathymically unless rescued by positive selection. Those which survive become mature CD3hiCD4/8+ (single-positive) cells and are the precursor of peripheral blood lymphocytes. The product of the bcl-2 protooncogene has been implicated in preventing programmed cell death and is required for prolonged lymphocyte survival following maturation. Previously we and others have reported that Bcl-2 protein expression is biphasic, being high in proliferating double-negative stem cells, low in all double-positive thymocytes except for 1-5% of these cells, and restored in mature, single-positive thymocytes. However, it remained unclear which signaling and selection events regulate Bcl-2 during T-cell maturation. Now we have utilized four-color flow cytometry in normal and genetically altered mice for a detailed analysis of Bcl-2 expression as it relates to T-cell receptor (TCR) expression and positive selection. These studies show that (i) expression of a transgenic TCR in double-negative thymocytes does not lead to premature loss of Bcl-2; thus, Bcl-2 downregulation is not solely due to TCR expression; (ii) Bcl-2 expression is lost at the early transitional CD3-/loCD4-CD8+ stage, prior to expression of CD4; (iii) the Bcl-2+ double-positive thymocytes are those which have undergone positive selection; and (iv) upregulation of Bcl-2 during positive selection requires participation of the CD4 or CD8 co-receptor. These results demonstrate that Bcl-2 and TCR expression are regulated independently during T-cell development, and suggest a role for the CD4 or CD8 co-receptor in Bcl-2 induction during positive selection.

Evidence for a selective and multi-step model of T cell differentiation: CD4+CD8low thymocytes selected by a transgenic T cell receptor on major histocompatibility complex class I molecules

We have characterized a prominent (15-20%) thymocyte population expressing CD4 at a high and CD8 at a low level (CD4+8lo) in mice transgenic for a T cell receptor (TCR) restricted by major histocompatibility complex (MHC) class I molecules. The results demonstrate that the CD4+8lo population is an intermediate stage between immature CD4+8+ and end-stage CD4+8- thymocytes and that the survival of these cells crucially depends on the successful interaction of the transgenic TCR with self MHC class I molecules. In addition we demonstrate that the avidity of the interaction between TCR and self MHC class I molecules determines whether CD4+8lo thymocytes are found in significant numbers in this transgenic model. Our findings support a selective and multi-step model of T cell differentiation in the thymus.