Replacement of pre-T cell receptor signaling functions by the CD4 coreceptor

Overexpression of RhoH Permits to Bypass the Pre-TCR Checkpoint

RhoH, an atypical small Rho-family GTPase, critically regulates thymocyte differentiation through the coordinated interaction with Lck and Zap70. Therefore, RhoH deficiency causes defective T cell development, leading to a paucity of mature T cells. Since there has been no gain-of-function study on RhoH before, we decided to take a transgenic approach to assess how the overexpression of RhoH affects the development of T cells. Although RhoH transgenic (RhoH^tg) mice expressed three times more RhoH protein than wild-type mice, β-selection, positive, and negative selection in the thymus from RhoH^tg mice were unaltered. However, transgenic introduction of RhoH into Rag2 deficient mice resulted in the generation of CD4⁺CD8⁺ (DP) thymocytes, indicating that overexpression of RhoH could bypass β-selection without TCRβ gene rearrangement. This was confirmed by the in vitro development of DP cells from Rag2^-/-RhoH^tg DN3 cells on TSt-4/Dll-1 stroma in an Lck dependent manner. Collectively, our results indicate that an excess amount of RhoH is able to initiate pre-TCR signaling in the absence of pre-TCR complexes.

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.

Transcription factor AP4 modulates reversible and epigenetic silencing of the Cd4 gene

CD4 coreceptor expression is negatively regulated through activity of the Cd4 silencer in CD4(-)CD8(-) double-negative (DN) thymocytes and CD8(+) cytotoxic lineage T cells. Whereas Cd4 silencing is reversed during transition from DN to CD4(+)CD8(+) double-positive stages, it is maintained through heritable epigenetic processes following its establishment in mature CD8(+) T cells. We previously demonstrated that the Runx family of transcription factors is required for Cd4 silencing both in DN thymocytes and CD8(+) T cells. However, additional factors that cooperate with Runx proteins in the process of Cd4 silencing remain unknown. To identify collaborating factors, we used microarray and RNAi-based approaches and found the basic helix-loop-helix ZIP transcription factor AP4 to have an important role in Cd4 regulation. AP4 interacts with Runx1 in cells in which Cd4 is silenced, and is required for Cd4 silencing in immature DN thymocytes through binding to the proximal enhancer. Furthermore, although AP4-deficient CD8(+) T cells appeared to normally down-regulate CD4 expression, AP4 deficiency significantly increased the frequency of CD4-expressing effector/memory CD8(+) T cells in mice harboring point mutations in the Cd4 silencer. Our results suggest that AP4 contributes to Cd4 silencing both in DN and CD8(+) T cells by enforcing checkpoints for appropriate timing of CD4 expression and its epigenetic silencing.

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.

MHC-restricted T cell receptor signaling is required for alpha beta TCR replacement of the pre T cell receptor

A developmental block is imposed on CD25(+)CD44(-) thymocytes at the beta-selection checkpoint in the absence of the pre T cell receptor (preTCR) alpha-chain, pTalpha. Early surface expression of a transgenic alphabeta TCR has been shown to partially circumvent this block, such that thymocytes progress to the CD4(+)CD8(+) double-positive stage. We wanted to analyze whether a restricting MHC element is required for alphabeta TCR-expressing double-negative (DN) thymocytes to overcome the developmental block in pTalpha-deficient animals. We used the HY-I knock-in model that endows thymocytes with alphabeta TCR expression in the DN compartment but has the advantage of physiological expression levels, in contrast to conventional TCR transgenes. On a pTalpha-deficient background, this HY-I TCR transgene 'rescued' CD25(+)CD44(-) thymocytes from apoptosis and enabled progression to later differentiation stages. On a non-selecting MHC background, however, pTalpha-deficient HY-I mice presented a pronounced reduction in numbers of splenocytes and thymocytes when compared to animals of selecting MHC genotype, showing that MHC restriction is necessary to drive HY-TCR-mediated rescue of pTalpha-deficient thymocytes.

The proline-rich sequence of CD3epsilon controls T cell antigen receptor expression on and signaling potency in preselection CD4+CD8+ thymocytes

Antigen recognition by T cell antigen receptors (TCRs) is thought to 'unmask' a proline-rich sequence (PRS) present in the CD3epsilon cytosolic segment, which allows it to trigger T cell activation. Using 'knock-in' mice with deletion of the PRS, we demonstrate here that elimination of the CD3epsilon PRS had no effect on mature T cell responsiveness. In contrast, in preselection CD4+CD8+ thymocytes, the CD3epsilon PRS acted together with the adaptor protein SLAP to promote CD3zeta degradation, thereby contributing to downregulation of TCR expression on the cell surface. In addition, analysis of CD4+CD8+ thymocytes of TCR-transgenic mice showed that the CD3epsilon PRS enhanced TCR sensitivity to weak ligands. Our results identify previously unknown functions for the evolutionarily conserved CD3epsilon PRS at the CD4+CD8+ developmental stage and suggest a rather limited function in mature T cells.

Nucleoprotein structure of the CD4 locus: implications for the mechanisms underlying CD4 regulation during T cell development

The CD4 gene is regulated in a stage-specific manner during T cell development, being repressed in CD4(-)CD8(-) double-negative (DN) and CD8 cells, but expressed in CD4(+)CD8(+) double-positive (DP) and CD4 cells. Furthermore, the expression/repression pattern is reversible in developing (DN and DP) thymocytes, but irreversible in mature (CD4 and CD8) T cells. Here, we explored the molecular mechanisms underlying this complex mode of regulation by examining the nucleoprotein structure of the CD4 locus throughout T cell development and in DN cells lacking the CD4 silencer. In DN cells, the CD4 enhancer is preloaded with multiple transcription activators, but p300 recruitment is impaired by the silencer that is associated with the repressor Runx1. DP cells achieve high-level CD4 expression via a combination of CD4 derepression and true activation, but Runx1 remains bound to the silencer that retains an open chromatin configuration. In CD4 cells, Runx1 dissociates from the silencer that has become less accessible, and CD4 transcription appears to be achieved via a mechanism distinct from that operating in DP cells. In CD8 cells, the CD4 promoter becomes incorporated into heterochromatin-like structure. Our data shed light on the molecular basis of CD4 regulation and provide a conceptual framework for understanding how the same regulatory elements can mediate both reversible and irreversible CD4 regulation.

Regulated costimulation in the thymus is critical for T cell development: dysregulated CD28 costimulation can bypass the pre-TCR checkpoint

Expression of CD28 is highly regulated during thymic development, with CD28 levels extremely low on immature thymocytes but increasing dramatically as CD4- CD8- cells initiate expression of TCRbeta. B7-1 and B7-2, the ligands for CD28, have a restricted distribution in the thymic cortex where immature thymocytes reside and are more highly expressed in the medulla where the most mature thymocytes are located. To determine the importance of this regulated CD28/B7 expression for T cell development, we examined the effect of induced CD28 signaling of immature thymocytes in CD28/B7-2 double-transgenic mice. Strikingly, we found that differentiation to the CD4+ CD8+ stage in CD28/B7-2 transgenics proceeds independent of the requirement for TCRbeta expression manifest in wild-type thymocytes, occurring even in Rag- or CD3epsilon- knockouts. These findings indicate that signaling of immature thymocytes through CD28 in the absence of TCR- or pre-TCR-derived signals can promote an aberrant pathway of T cell differentiation and highlight the importance of finely regulated physiologic expression of CD28 and B7 in maintaining integrity of the "beta" checkpoint for pre-TCR/TCR-dependent thymic differentiation.

Function of the Src-family kinases, Lck and Fyn, in T-cell development and activation

The function of the Src-family kinases (SFKs) Lck and Fyn in T cells has been intensively studied over the past 15 years. Animal models and cell line studies both indicate a critical role for Lck and Fyn in proximal T-cell antigen receptor (TCR) signal transduction. Recruited SFKs phosphorylate TCR ITAMs (immunoreceptor tyrosine-based activation motifs) in the CD3 and zeta chains, which then serve as docking sites for Syk-family kinases. SFKs then phosphorylate and activate the recruited Syk-family kinase. Lck and Fyn are spatially segregated in cell membranes due to differential lipid raft localization, and may undergo sequential activation. In addition to the CD4 and CD8 coreceptors, a recently described adaptor, Unc119, may link SFKs to the TCR. CD45 and Csk provide positive and negative regulatory control of SFK functions, respectively, and Csk is constitutively bound to the transmembrane adapter protein, PAG/Cbp. TCR-based signaling is required at several stages of T-cell development, including at least pre-TCR signaling, positive selection, peripheral maintenance of naive T cells, and lymphopenia-induced proliferation. SFKs are required for each of these TCR-based signals, and Lck seems to be the major contributor.

Surface expression of Notch1 on thymocytes: correlation with the double-negative to double-positive transition

Notch1 plays a critical role in regulating T lineage commitment during the differentiation of lymphoid precursors. The physiological relevance of Notch1 signaling during subsequent stages of T cell differentiation has been more controversial. This is due in part to conflicting data from studies examining the overexpression or targeted deletion of Notch1 and to difficulties in distinguishing between the activities of multiple Notch family members and their ligands, which are expressed in the thymus. We employed a polyclonal antiserum against the extracellular domain of Notch1 to study surface expression during thymopoiesis. We found high levels of Notch1 on the cell surface only on double negative (DN) stage 2 through the immature single-positive stage of thymocyte development, before the double-positive (DP) stage. The Notch signaling pathway, as read out by Deltex1 expression levels, is highly active in DN thymocytes. When an active Notch1 transgene, Notch1IC, is exogenously introduced into thymocytes of recombinase-activating gene 2-deficient mice, it promotes proliferation and development to the DP stage following anti-CD3 treatment without apparently affecting the intensity of pre-TCR signaling. In addition, a stromal cell line expressing the Notch ligand, Delta-like-1, promotes the in vitro expansion of wild-type DN3 thymocytes in vitro. Consistent with other recent reports, these data suggest a role for Notch1 during the DN to DP stage of thymocyte maturation and suggest a cellular mechanism by which Notch1IC oncogenes could contribute to thymoma development and maintenance.

Decreased binding of peptides-MHC class I (pMHC) multimeric complexes to CD8 affects their binding avidity for the TCR but does not significantly impact on pMHC/TCR dissociation rate

The CD8 coreceptor plays a crucial role in both T cell development in the thymus and in the activation of mature T cells in response to Ag-specific stimulation. In this study we used soluble peptides-MHC class I (pMHC) multimeric complexes bearing mutations in the CD8 binding site that impair their binding to the MHC, together with altered peptide ligands, to assess the impact of CD8 on pMHC binding to the TCR. Our data support a model in which CD8 promotes the binding of TCR to pMHC. However, once the pMHC/TCR complex is formed, the TCR dominates the pMHC/TCR dissociation rates. As a consequence of these molecular interactions, under physiologic conditions CD8 plays a key role in complex formation, resulting in the enhancement of CD8 T cell functions whose specificity, however, is determined by the TCR.

CD8+ T cells induce medullary thymic epithelium and CD4+CD8+CD25+ TCRbeta- thymocytes in SCID mice

During T-cell development the transition in the thymus of CD4-CD8- double negative (DN) progenitor T cells into CD4+CD8+ double positive (DP) cells is dependent on the expression of a T-cell receptor (TCR)-beta-chain protein. In this study purified peripheral CD4+ and CD8+ T lymphocytes from the C.B-17 strain of mice were adoptively transferred into syngeneic, neonatal SCID mice, where donor cells resided at constant numbers in thymus from 2 weeks until 10 weeks post cell transfer. In the recipient thymus the CD8+ donor cells outnumbered the CD4+ cells by a factor of three to five and both subsets contained a large fraction of activated cells. During the late phase of treatment, CD8+ T cells induced high numbers of DP thymocytes in the SCID mice, a process accompanied by the maturation of medullary epithelial cells. Such thymic development in the SCID mouse was inhibited by coresiding CD4+ donor T cells. These results indicate a regulatory role by mature peripheral T cells on medullary epithelial growth and thymocyte development in the treated SCID mice.

Scheduled kinetics of cell proliferation and phenotypic changes during immature thymocyte generation

Precursor CD4-CD8- (DN) thymocytes rearrange their TCR-beta genes, and only those which succeed in beta-selection subsequently expand and differentiate into immature CD4+CD8+ (DP) thymocytes. The cell subsets corresponding to the successive steps of this transition can be defined in terms of CD44 and CD25 expression. We partially synchronized the differentiation process by eliminating cycling cells with the anti-mitotic agent demecolcine. Using in vivo pulse labeling with bromodeoxyuridine, we determined the order of entry into DNA synthesis of the different DN and transitory (CD4-/lo CD8+) cell subsets. Two independent proliferation phases were identified. The first cells to enter the cell cycle were CD44-CD25lo, and CD4/CD8/TCR-/BrdU four-color staining showed that they all expressed a low density of the TCR-beta chain, an element of the pre-TCR (the TCR-alpha locus is still in germ-line configuration at this stage). Cycling of CD44+CD25+ cells was detected later, and no starting point was observed at the CD44-CD25hi stage. CD8 expression was immediately detectable in cycling cells, but they took 24 h to reach the DP stage. The study of TCR-Calpha-deficient mice showed that beta gene rearrangement occurred once proliferation had ceased at the DP stage, and that it had no influence on the DN-DP transition. These data show that precursor thymocytes undergo two independent waves of expansion, and that the second wave is restricted to cells capable of pre-TCR expression.

Early onset of CD8 transgene expression inhibits the transition from DN3 to DP thymocytes

In this paper we show that the effects of transgenic coreceptor expression on thymocyte development depend on the onset of transgene expression. Thus, a CD8 transgene expressed on CD44+CD25+ (DN2) and CD44-CD25+ (DN3) cells causes a partial block at the stage when TCRbeta selection takes place and diminishes expansion at the subsequent developmental stages, resulting in increased DN3 and markedly reduced double-positive (DP) thymocyte numbers. This effect is evident on a polyclonal TCR repertoire as well as in TCR-transgenic mice (F5). By contrast, a CD8 transgene that leads to the same degree of overexpression on DP thymocytes, but is not expressed on double-negative subsets, has no effect on thymus size or composition. Therefore, the reduction of DP thymocyte numbers in CD8 TCRtg mice can be attributed to interferences at early developmental stages rather than to increased negative selection of DP cells.

Molecular requirements for lineage commitment in the thymus--antibody-mediated receptor engagements reveal a central role for lck in lineage decisions

Recent experiments in our laboratory have focused on the receptor engagements required for the differentiation of fully mature, single positive thymocytes from their double positive precursors. We have used a novel approach which involves the ligation of surface receptors on immature thymocytes with genetically engineered F(ab1)2 reagents, which, unlike conventional antibodies, do not aggregate the CD3 complex to such an extent as to induce extensive deletion of these cells. The experimental data presented in this review indicate that differentiation of the two mature CD4 and CD8 lineages occurs in response to distinct intracellular signals induced by particular receptor engagements. The data suggest that the tyrosine kinase p56lck (lck) plays a crucial role in determining lineage choice, in that maturation of thymocytes into the CD4 lineage occurs upon recruitment of active lck to the T-cell receptor (TCR)/CD3 complex, whereas CD8 maturation can be induced by CD3 ligation in the absence of co-receptor-mediated lck recruitment. A central role for lck activity in determining the threshold for differentiation of the CD4 lineage is revealed in experiments with thymi deficient for a regulator of lck activity, CD45. A model of thymocyte differentiation is presented in which we propose that the relative balance of signals delivered by TCR engagement and lck activation determines lineage choice.

Csk controls antigen receptor-mediated development and selection of T-lineage cells

The development and function of alphabetaT lymphocytes depend on signals derived from pre-T and alphabetaT cell receptors (preTCR and alphabetaTCR) (reviewed in refs 1, 2). The engagement of these receptors leads to the activation of Lck and Fyn, which are protein tyrosine kinases (PTKs) of the Src family. It remains unclear to what extent the activation of Src-family PTKs can direct the differentiation steps triggered by preTCR and alphabetaTCR. Here we show that the inactivation of the negative regulator of Src-family PTKs, carboxy-terminal Src kinase (Csk), in immature thymocytes abrogates the requirement for preTCR, alphabetaTCR and major histocompatibility complex (MHC) class II for the development of CD4+ 8+ double-positive and CD4+ single-positive thymocytes as well as peripheral CD4 alphabetaT-lineage cells. These data show that Csk and its substrates are required to establish preTCR/alphabetaTCR-mediated control over the development of alphabetaT cells.

HD mice: a novel mouse mutant with a specific defect in the generation of CD4(+) T cells

We have identified a spontaneous mutation in mice, which we term HD for "helper T cell deficient." This mouse is distinguished by the virtual absence of peripheral T cells of the CD4(+)8(-) major histocompatibility complex (MHC) class II-restricted T helper subset due to a specific block in thymic development. The developmental defect is selective for CD4(+)8(-) cells; the maturation of CD4(-)8(+) and gamma delta T cells is normal. The autosomal recessive mutation underlying the HD phenotype is unrelated to MHC class II, since it segregates independently of the MHC class II locus. Moreover, the HD phenotype is not caused by a defect of the CD4 gene. Bone marrow transfer experiments demonstrate that the defect is intrinsic to cells of the hematopoietic lineage, i.e., most likely to developing thymocytes themselves. The frequency of CD4(+)8(low) intermediate cells is markedly increased in HD mice, suggesting that class II-restricted thymocytes are arrested at this stage. This is the first genetic defect of its kind to be described in the mouse and may prove highly informative in understanding the molecular pathways underlying lineage commitment.

T cell receptor gamma gene regulatory sequences prevent the function of a novel TCRgamma/pTalpha pre-T cell receptor

Expression of a TCRgamma transgene in RAG-1-/- mice resulted in the development of a limited number of CD4+CD8+ (DP) thymocytes. In vivo treatments with anti-TCRgamma antibody enhanced the number of DP thymocytes, demonstrating that TCRgamma chains were expressed on the cell surface in the absence of delta, alpha, or beta chains. Mutations in pTalpha or CD3epsilon genes abolished transgene-induced DP cell development, indicating that TCRgamma can associate with pTalpha and CD3 to form a novel pre-TCR. With a transgene containing additional regulatory sequences, TCRgamma expression was down-regulated in DP cells, and little DP cell development occurred. Thus, the function of the endogenous TCRgamma/pTalpha is limited by the transcriptional down-regulation of TCRgamma genes that normally accompanies DP cell development.

CD4-CD8-C.B-17 SCID thymocytes enter the CD4+CD8+ stage in the presence of neonatally grafted T cells

The aim of this work was to study the selection of donor T cells and their influence on thymic development in C.B-17 scid/scid (severe combined immunodeficient; SCID) mice during chronic graft-versus-host disease (GVHD). Recipient SCID mice (H-2d), neonatally grafted with allogeneic peripheral T cells from CBA/J strain (H-2k) of mice, only developed a mild acute GVHD, and were, at the chronic stage, devoid of pathological symptoms. Thymic cell numbers of injected mice differed from 10(5) to 1.2 x 10(7) at 2-3 weeks post-injection (p.i.), and from 4 x 10(5) to 8.5 x 10(7) at 2 months p.i. In these mice, the thymus size was correlated to the CD4-CD8- (double negative; DN) to CD4+CD8+ (double positive; DP) cell ratio, where at 2 months p.i., 8 out of 16 treated SCID mice contained 5 x 10(6) cells or more and also possessed the highest frequencies of endogenous DP cells (25-95%). In contrast to previous findings, peripheral donor T cells from allogeneic and syngeneic mice, infiltrating the host thymus, had a positive effect on the development of endogenous DP thymocytes. Furthermore, these thymocytes were developmentally blocked at the DP stage, occasionally in combination with the expression of CD25, CD44 and CD117 but in the absence of T-cell receptor (TCR) expression. Also, at this time-point, the CBA/J donor TCR Vbeta repertoire was equal to that of normal CBA/J mice, but purified responding donor cells were proliferatively inhibited against H-2d stimulators in ex vivo mixed lymphocyte cultures. In contrast, the same responders showed a pronounced proliferation against syngeneic H-2Kk stimulators, suggesting either a reversion from anergy of autoreactive CBA/J T cells or a vast expansion of multiple self-reactive T-cell clones, when parked in a milieu with a lower concentration of self-antigens.

Protein tyrosine kinases in thymocyte development

Much has been learned over the past few years about how protein tyrosine kinases mediate pre-TCR and mature alphabetaTCR function. The highlights include understanding the roles and the distinct effects of the Src and Syk families of protein tyrosine kinases in thymocyte development and function.