The influence of the thymic environment on the CD4-versus-CD8 T lineage decision

Thymic-specific regulation of TCR signaling by Tespa1

Double-positive (DP) thymocytes undergo positive selection to become mature single-positive CD4⁺ and CD8⁺ T cells in response to T cell receptor (TCR) signaling. Unlike mature T cells, DP cells must respond to low-affinity self-peptide-MHC ligands before full upregulation of their surface TCR expression can occur. Thus, DP thymocytes must be more sensitive to ligands than mature T cells. A number of molecules have been found that are able to enhance the strength of the TCR signal to facilitate positive selection. However, almost all of these molecules are also active in mature T cells. Themis (thymocyte expressed molecule involved in selection) and Tespa1 (thymocyte expressed positive selection associated 1) are two recently discovered molecules essential for optimal TCR signaling and thymocyte development. A deficiency in both molecules leads to defects in positive selection. Here, we compared the relative contributions of Themis and Tespa1 to positive selection in thymocytes. We show that Tespa1 deficiency led to more limited and specific gene expression profile changes in cells undergoing positive selection. In mixed bone marrow transfer experiments, Tespa1^-/- cells showed more severe defects in thymocyte development than Themis^-/- cells. However, Tespa1^-/- cells showed a substantial degree of homeostatic expansion and became predominant in the peripheral lymphoid organs, suggesting that Tespa1 is a thymic-specific TCR signaling regulator. This hypothesis is further supported by our observations in Tespa1 conditional knockout mice, as Tespa1 deletion in peripheral T cells did not affect TCR signaling or cell proliferation. The different regulatory effects of Tespa1 and Themis are in accordance with their nonredundant roles in thymocyte selection, during which Tespa1 and Themis double knockouts showed additive defects.

A new mechanism shapes the naïve CD8+ T cell repertoire: the selection for full diversity

During thymic T cell differentiation, TCR repertoires are shaped by negative, positive and agonist selection. In the thymus and in the periphery, repertoires are also shaped by strong inter-clonal and intra-clonal competition to survive death by neglect. Understanding the impact of these events on the T cell repertoire requires direct evaluation of TCR expression in peripheral naïve T cells. Several studies have evaluated TCR diversity, with contradictory results. Some of these studies had intrinsic technical limitations since they used material obtained from T cell pools, preventing the direct evaluation of clonal sizes. Indeed with these approaches, identical TCRs may correspond to different cells expressing the same receptor, or to several amplicons from the same T cell. We here overcame this limitation by evaluating TCRB expression in individual naïve CD8⁺ T cells. Of the 2269 Tcrb sequences we obtained from 13 mice, 99% were unique. Mathematical analysis of the data showed that the average number of naïve peripheral CD8⁺ T cells expressing the same TCRB is 1.1 cell. Since TCRA co-expression studies could only increase repertoire diversity, these results reveal that the number of naïve T cells with unique TCRs approaches the number of naïve cells. Since thymocytes undergo multiple rounds of divisions after TCRB rearrangement and 3-5% of thymocytes survive thymic selection events the number of cells expressing the same TCRB was expected to be much higher. Thus, these results suggest a new repertoire selection mechanism, which strongly selects for full TCRB diversity.

Functional evidence for TCR-intrinsic specificity for MHCII

How T cells become restricted to binding antigenic peptides within class I or class II major histocompatibility complex molecules (pMHCI or pMHCII, respectively) via clonotypic T-cell receptors (TCRs) remains debated. During development, if TCR-pMHC interactions exceed an affinity threshold, a signal is generated that positively selects the thymocyte to become a mature CD4(+) or CD8(+) T cell that can recognize foreign peptides within MHCII or MHCI, respectively. But whether TCRs possess an intrinsic, subthreshold specificity for MHC that facilitates sampling of the peptides within MHC during positive selection or T-cell activation is undefined. Here we asked if increasing the frequency of lymphocyte-specific protein tyrosine kinase (Lck)-associated CD4 molecules in T-cell hybridomas would allow for the detection of subthreshold TCR-MHC interactions. The reactivity of 10 distinct TCRs was assessed in response to selecting and nonselecting MHCII bearing cognate, null, or "shaved" peptides with alanine substitutions at known TCR contact residues: Three of the TCRs were selected on MHCII and have defined peptide specificity, two were selected on MHCI and have a known pMHC specificity, and five were generated in vitro without defined selecting or cognate pMHC. Our central finding is that IL-2 was made when each TCR interacted with selecting or nonselecting MHCII presenting shaved peptides. These responses were abrogated by anti-CD4 antibodies and mutagenesis of CD4. They were also inhibited by anti-MHC antibodies that block TCR-MHCII interactions. We interpret these data as functional evidence for TCR-intrinsic specificity for MHCII.

Thymic epithelial requirement for γδ T cell development revealed in the cell ablation transgenic system with TSCOT promoter

In order to investigate the role of thymic epithelial cell (TEC) subsets during T-cell development, we established a new transgenic system, enabling inducible cell-specific ablation as well as marking the TEC subsets using bicistronic bacterial nitroreductase and EGFP genes. Two different lengths of the TSCOT promoter in transgenic mice, named 3.1T-NE and 9.1T-NE, drive EGFP expression into TECs. In adult life, EGFP expression was located in the medulla with a smaller 3.1 kb TSCOT promoter, while it was maintained in the cortex with a 9.1 kb promoter, suggesting putative TEC specific as well as compartment specific cis elements within two promoters. Nitroreductase induced cell death was specific without bystander killing upon the treatment of prodrugs such as nitrofurantoin and metronidazol. The degree of cell death was dependent on the dose of the prodrug in the cell and the fetal thymic organ cultures (FTOCs). Fetal thymic stromal populations were analyzed based on the expression levels of EpCAM, MHCII, CDR1 and/or UEA-1. EGFP expression patterns varied among subsets indicating the differential TSCOT promoter activity in each TEC subset. Prodrug treatment in FTOCs reduced the numbers of total and subsets of thymocytes. A CD4(+)CD8(+) double positive cell population was highly susceptible in both transgenic lines. Surprisingly, there was a distinct reduction in γδ T cell population only in the 9.1T-NE thymus, indicating that they require a NTREGFP expressing TEC population. Therefore, these results support a division of labor within TEC subsets for the αβ and γδ lineage specification.

Thymic and peripheral differentiation of regulatory T cells

The development of regulatory T (Treg) cells is essential for the maintenance of immune tolerance and homeostasis. Here, we review recent studies that have advanced our understanding of Treg cell differentiation. In the thymus, TCR specificity to self-antigen appears to be a primary determinant for Treg cell lineage commitment, with c-Rel being an important factor that links T cell receptor (TCR) engagement and Foxp3 expression, along with cytokines and costimulatory molecules. It is also clear that postthymic events shape the peripheral Treg cell population. This includes preferential maintenance of Treg cells specific to self-antigens presented in the periphery, as well as the de novo generation of Treg cells from conventional Foxp3(-) T cells. The process of peripheral Treg cell differentiation shares some features with thymic Treg cell development, but there are notable differences. Together, thymic and peripheral Treg cell differentiation appear to generate an "imprint" of both self- and foreign antigens in the peripheral Treg cell population to provide dominant tolerance.

Evolving concepts of specificity in immune reactions

Our goal is to provide a perspective on current understanding of the origins of specificity in immune reactions, a topic that has intrigued scientists for over a century. A fundamental property of adaptive immune responses is the ability to discriminate among an immense variety of substances by means of antibodies (Abs) and Ab-like receptors on T lymphocytes [T-cell receptors (TCRs)], each able to bind a particular chemical structure [the antigen (Ag)] and not, or only weakly, similar alternatives. Evidence has long existed, however, and has grown, especially recently, that while exhibiting remarkable specificity, many individual Abs and TCRs can also bind a variety of very different ligands. How can Ag recognition by these receptors exercise the great specificity for which they are renowned and yet react with a variety of different ligands (degeneracy)? We critically consider the mechanistic bases for this specificity/degeneracy enigma and also compare and contrast Ag recognition by Abs and TCRs.

Decision checkpoints in the thymus

The development of T cells in the thymus involves several differentiation and proliferation events, during which hematopoietic precursors give rise to T cells ready to respond to antigen stimulation and undergo effector differentiation. This review addresses signaling and transcriptional checkpoints that control the intrathymic journey of T cell precursors. We focus on the divergence of alphabeta and gammadelta lineage cells and the elaboration of the alphabeta T cell repertoire, with special emphasis on the emergence of transcriptional programs that direct lineage decisions.

The endocytic adaptor Numb regulates thymus size by modulating pre-TCR signaling during asymmetric division

Stem cells must proliferate and differentiate to generate the lineages that shape mature organs; understanding these 2 processes and their interaction is one of the central themes in current biomedicine. An intriguing aspect is asymmetric division, by which 2 daughter cells with different fates are generated. Several cell fate determinants participate in asymmetric division, with the endocytic adaptor Numb as the best-known example. Here, we have explored the role of asymmetric division in thymocyte development, visualizing the differential segregation of Numb and pre-TCR in thymic precursors. Analysis of mice where Numb had been inhibited by expressing a dominant negative revealed enhanced pre-T-cell receptor (TCR) signaling and a smaller thymus. Conversely, Numb overexpression resulted in loss of asymmetric division and a larger thymus. The conclusion is that Numb determines the levels of pre-TCR signaling in dividing thymocytes and, ultimately, the size of the pool from which mature T lymphocytes are selected.

Intraclonal competition limits the fate determination of regulatory T cells in the thymus

Because the deletion of self-reactive T cells is incomplete, thymic development of natural Foxp3⁺CD4⁺ regulatory T (T_reg) cells is required for preventing autoimmunity. However, the role of T cell receptor (TCR) specificity in thymic T_reg cell development remains controversial. To address this issue, we generated a transgenic line expressing a naturally occurring T_reg cell-derived TCR. Surprisingly, efficient thymic T_reg cell development occurred only when the antigen-specific T_reg cell precursors were present at low clonal frequency (<1%) within a normal thymus. Using retroviral vectors and bone marrow chimeras, we observed similar behavior with two other T_reg cell-derived TCRs. These data demonstrate that thymic T_reg cell development is a TCR-instructive process involving a niche which can be saturable at much lower clonal frequencies than the niche for positive selection.

Thymocyte-dendritic cell interactions near sources of CCR7 ligands in the thymic cortex

Little is known about the dynamics of the interactions between thymocytes and other cell types, as well as the spatiotemporal distribution of thymocytes during positive selection in the microenvironment of the cortex. We used two-photon laser scanning microscopy of the mouse thymus to visualize thymocytes and dendritic cells (DCs) and to characterize their interactions in the cortex. We show that thymocytes make frequent contacts with DCs in the thymic cortex and that these associations increase when thymocytes express T cell receptors that mediate positive selection. We also show that cortical DCs and the chemokine CCL21 expression are closely associated with capillaries throughout the cortex. The overexpression of the chemokine receptor CCR7 in thymocytes results in an increase in DC-thymocyte interactions, while the loss of CCR7 in the background of a positive-selecting TCR reduces the extent of DC-thymocyte interactions. These observations identify a vasculature-associated microenvironment within the thymic cortex that promotes interactions between DCs and thymocytes that are receiving positive selection signals.

Central tolerance: what have we learned from mice?

Producing a healthy immune system capable of defending against pathogens, while avoiding autoimmunity, is dependent on thymic selection. Positive selection yields functional T cells that have the potential to recognize both self and foreign antigens. Therefore, negative selection exists to manage potentially self-reactive cells. Negative selection results from the induction of anergy, receptor editing, clonal diversion (agonist selection), and/or clonal deletion (apoptosis) in self-reactive clones. Clonal deletion has been inherently difficult to study because the cells of interest are undergoing apoptosis and being eliminated quickly. Furthermore, analysis of clonal deletion in humans has proved even more difficult due to availability of samples and lack of reagents. Mouse models have thus been instrumental in achieving our current understanding of central tolerance, and the evolution of elegant model systems has led to an explosion of new data to be assimilated. This review will focus on recent advances in the field of clonal deletion with respect to three aspects: the development of physiological model systems, signaling pathways that lead to apoptosis, and antigen presenting cell types involved in the induction of clonal deletion.

Clonal deletion of thymocytes can occur in the cortex with no involvement of the medulla

The thymic medulla is generally held to be a specialized environment for negative selection. However, many self-reactive thymocytes first encounter ubiquitous self-antigens in the cortex. Cortical epithelial cells are vital for positive selection, but whether such cells can also promote negative selection is controversial. We used the HY^cd4 model, where T cell receptor for antigen (TCR) expression is appropriately timed and a ubiquitous self-antigen drives clonal deletion in male mice. We demonstrated unambiguously that this deletion event occurs in the thymic cortex. However, the kinetics in vivo indicated that apoptosis was activated asynchronously relative to TCR activation. We found that radioresistant antigen-presenting cells and, specifically, cortical epithelial cells do not efficiently induce apoptosis, although they do cause TCR activation. Rather, thymocytes undergoing clonal deletion were preferentially associated with rare CD11c⁺ cortical dendritic cells, and elimination of such cells impaired deletion.

A Stat5b transgene is capable of inducing CD8+ lymphoblastic lymphoma in the absence of normal TCR/MHC signaling

Stat5 proteins are critical signaling molecules activated by many cytokines. Within the immune system, Stat5 plays important roles related to the development of thymocytes and proliferation of T cells. Stat5 has been implicated in malignant transformation, and moreover, the activated tyrosine phosphorylated form of Stat5 is frequently observed in human lymphomas. We previously demonstrated the oncogenic potential of Stat5, with thymic lymphoblastic lymphomas developing in a significant proportion of transgenic (TG) mice overexpressing Stat5a or Stat5b in lymphocytes. In addition, immunization or expression of a T-cell receptor (TCR) transgene augmented the rate of tumor formation. Here, we investigate the mechanism of Stat5-mediated lymphomagenesis by exploring the contributions of major histocompatibility complex (MHC)/TCR and pre-TCR signals. We present data demonstrating that Stat5b TG mice unexpectedly develop CD8(+) lymphoma even in the absence of either pre-TCR signaling or normal thymic selection. Indeed, acceleration of Stat5b transgene-mediated lymphoma occurred on TCRalpha(-/-) and pre-TCRalpha(-/-) backgrounds. In light of these data, we propose a model in which alterations in T-cell development at the double-negative/double-positive (DN/DP) stages cooperate with cytokine-mediated pathways in immature thymocytes to give rise to lymphoblastic T-cell lymphomas in Stat5b TG mice.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

Stage-dependent reactivity of thymocytes to self-peptide--MHC complexes

In mice that express a transgene for the 2C T cell antigen-receptor (TCR) and lack a recombinase-activating gene (2C(+)RAG(-/-) mice) most of the peripheral T cells are CD8(+), a few are CD4(+), and a significant fraction are CD4(-)CD8(-) [double negative (DN)]. The DN 2C cells, like DN T cells that are abundant in various other alphabeta TCR-transgenic mice, appear to be derived directly from DN thymocytes that prematurely express the TCR transgene. The DN 2C cells are virtually absent in mice deficient in major histocompatibility complex class II (MHC-II) but more abundant in mice deficient in MHC-I, suggesting that the DN 2C thymocytes are positively selected by self-peptide-MHC-II (pMHC-II) complexes and negatively selected by self-pMHC-I complexes. The pMHC-I complexes, however, positively select CD8(+) 2C T cells in the same mice. The different effects of thymic pMHC-I on DN and CD8(+) thymocytes are consistent with the finding that DN 2C thymocytes are more sensitive than more mature CD4(+)CD8(+) [double positive (DP)] thymocytes to a weak pMHC-I agonist for the 2C TCR. Together with previous evidence that DP thymocytes respond more sensitively than T cells in the periphery to weak pMHC agonists, the findings suggest progressive decreases in responsiveness to self-pMHC-I complexes as thymocytes develop from DN to DP thymocytes and then to mature naïve T cells in the periphery.

RasGRP1 Transmits Prodifferentiation TCR Signaling That Is Crucial for CD4 T Cell Development

TCR signaling plays a governing role in both the survival and differentiation of bipotent double-positive thymocytes into the CD4(+) and CD8(+) single-positive T cell lineages. A central mediator of this developmental program is the small GTPase Ras, emitting cytoplasmic signals through downstream MAPK pathways and eventually affecting gene expression. TCR signal transduction orchestrates the activation of Ras by integrating at least two Ras-guanyl nucleotide exchange factors, RasGRP1 and Sos. In this study, we have characterized the relationship between RasGRP1 function and its potential roles in promoting ERK activity, cell survival, maturation, and lineage commitment. Investigations on RasGRP1(-/-) mice expressing a transgenic (Tg) MHC class II-restricted TCR revealed that the development of CD4 T cells expressing this Tg TCR is completely dependent on RasGRP1. Unexpectedly, a small number of functional CD8 single-positive thymocytes expressing the Tg MHC class II-restricted TCR exists in mutant mice. In addition, RasGRP1(-/-) double-positive thymocytes exhibit marked deficits in TCR-stimulated up-regulation of the positive selection marker CD69 and the antiapoptotic protein Bcl-2, whereas CD5 induction is unaffected. To evaluate the role of RasGRP1 in providing cellular survival signaling, we enforced Bcl-2 expression in RasGRP1(-/-) thymocytes. These studies demonstrate that RasGRP1 function cannot be fully complemented by Tg Bcl-2 expression. Therefore, we propose that RasGRP1 transmits differentiation signaling critically required for CD4 T cell development.

A conformational change senses the strength of T cell receptor-ligand interaction during thymic selection

T cell antigen receptor (TCR) triggering determines the fate of immature thymocytes. The affinity of the TCR for its endogenous peptide/MHC ligands serves as a signal for positive or negative selection through mechanisms that are still little understood. We have used a conformation-specific antibody to demonstrate that recognition of TCR ligands that lead to negative selection induces a conformational change in the TCR in situ. In contrast, this conformational change is elicited in only a small percentage of immature thymocytes during positive selection. Using a TUNEL assay, we demonstrate that the conformational change in the TCR is strongly linked to activation of programmed cell death in conditions leading to negative selection. Furthermore, the few conformational change-positive thymocytes detected in conditions that preferably lead to positive selection are also TUNEL-positive. Thus, the conformational change in the TCR may underlie the discrimination of ligands leading to positive and negative selection.

Phosphatidylinositol 3-kinase improves the efficiency of positive selection

We have generated transgenic mice expressing the amino-terminal fragment of the phosphatidylinositol 3-kinase (PI3K) catalytic subunit (p110ABD) in thymocytes. Expression of p110ABD results in constitutive activation of PI3K and in significant increases in the numbers of mature, single-positive thymocytes. We previously reported that the increase in mature cells was in part due to a defect in thymic emigration. In this study we identify another component to this phenotype. Expression of p110ABD results in an enhancement of positive selection, without alterations in thymocyte lifespan or negative selection. Since PI3K can affect activation of Btk, which in turn potentiates calcium fluxes, during B cell development, our results suggest that PI3K could play a role in the regulation of Itk kinases in T cells, and that both cell types share a common signaling network to modulate calcium responses downstream of their antigen receptor.

The quantitative assessment of MHC II on thymic epithelium: implications in cortical thymocyte development

The dynamics of MHC II expression in various thymic stromal compartments was investigated. By including MHC II in flow cytometry in addition to the cortical CDR1, medullary UEA-1 and pan-epithelial G8.8 markers, thymic stromal compartments were subdivided into at least six different populations. The total level of surface and cytoplasmic MHC II from fresh cortical thymic epithelial cells (cTECs) of normal mouse was as high as MHC II levels in medullary thymic epithelial cells (mTECs). MHC II levels as well as the percentages and cycling status of thymic epithelial cell populations expressing MHC II were not static during post-natal development, suggesting quantitative flexibility in presenting signals to the developing thymocytes. Although there was no evidence found for regulation of surface MHC II levels by TCR or by IFN-gamma, the absence of class II transactivator reduced both the level of MHC II expression and the number of MHC II+ cells. Surprisingly, MHC II molecules were found to form distinct focal aggregates on the surface of cTEC but not mTEC using high-resolution analysis by confocal microscopy. Moreover, these aggregates were formed independent of TCR or TCR-bearing cells in the thymus. These aggregates could potentially generate a functional unit containing a much higher local MHC II concentration to yield a higher avidity interaction. We discuss possible mechanisms for positive selection by weak interactions in the presence of such preformed MHC II aggregate units in cTEC.

Development of CD4+ T cells expressing a nominally MHC class I-restricted T cell receptor by two different mechanisms

Differences in T cell receptor (TCR) signaling initiated by interactions among TCRs, coreceptors, and self-peptide-MHC complexes determine the outcome of CD4 versus CD8 lineage of T cell differentiation. The H-2Ld and Kbm3 alloreactive 2C TCR is positively selected by MHC class I Kb and a yet-to-be identified nonclassical class I molecule to differentiate into CD8+ T cells. Here we describe two mechanisms by which CD4+ 2C T cells can be generated in 2C TCR-transgenic mice. In the RAG-/- background, development of CD4+ 2C T cells requires the expression of both I-Ab and the TAP genes, indicating that both MHC class I and II molecules are required for positive selection of these T cells. Notably, only some of the 2C+ RAG-/- mice (approximately 30%) develop CD4+ 2C T cells, with frequencies in individual mice varying from 0.5% to as high as approximately 50%. In the RAG+ background, where endogenous TCRalpha genes are rearranged and expressed, CD4+ 2C T cells are generated because these cells express the 2C TCR as well as additional TCRs, consisting of the 2C TCRbeta and endogenous TCRalpha chains. Similarly, T cells expressing the OT-1 TCR, which is nominally MHC class I-restricted, can also develop into CD4+ T cells through the same two mechanisms. Thus, expression of two TCRs by a single thymocyte, TCR recognition of multiple MHC molecules, and heterogeneity of TCR, coreceptors, and peptide-MHC interactions in the thymus all contribute to the outcome of CD4 versus CD8 lineage development.

Complex regulation of CCR9 at multiple discrete stages of T cell development

We have conducted a comprehensive assessment of CCR9 expression and function at the important milestone stages of murine thymocyte development. We reveal an unusually complex regulatory pattern, in which CCR9 influences T cell development at several widely dispersed stages. We find that CCR9 is not expressed within the thymus until the double-negative (DN)3 stage, although it appears to contribute to T cell precursor development prior to residence in the thymus. CCR9 expression is influenced by pre-T cell receptor signals, and is dramatically up-regulated in a population that appears to be transitional between the DN4 and double-positive stages. In the periphery, functional CCR9 is expressed by all naive CD8 T cells, but not by naive CD4 T cells. To our knowledge, this latter finding is the first difference observed in homing receptor expression between naive lymphocyte populations. This suggests that naive CD8 T cells might have access to lymphoid microenvironments from which naive CD4 T cells are excluded.

Premature Expression of Chemokine Receptor CCR9 Impairs T Cell Development

During thymocyte development, CCR9 is expressed on late CD4-CD8- (double-negative (DN)) and CD4+CD8+ (double-positive) cells, but is subsequently down-regulated as cells transition to the mature CD4+ or CD8+ (single-positive (SP)) stage. This pattern of expression has led to speculation that CCR9 may regulate thymocyte trafficking and/or export. In this study, we generated transgenic mice in which CCR9 surface expression was maintained throughout T cell development. Significantly, forced expression of CCR9 on mature SP thymocytes did not inhibit their export from the thymus, indicating that CCR9 down-regulation is not essential for thymocyte emigration. CCR9 was also expressed prematurely on immature DN thymocytes in CCR9 transgenic mice. Early expression of CCR9 resulted in a partial block of development at the DN stage and a marked reduction in the numbers of double-positive and SP thymocytes. Moreover, in CCR9-transgenic mice, CD25high DN cells were scattered throughout the cortex rather than confined to the subcapsular region of the thymus. Together, these results suggest that regulated expression of CCR9 is critical for normal development of immature thymocytes, but that down-regulation of CCR9 is not a prerequisite for thymocyte emigration.

The frequency of double-positive thymocytes expressing an alphabeta TCR clonotype regulates peripheral CD4 T cell compartment homeostasis

The present study aimed to determine whether the frequency of double positive (DP) thymocytes expressing alphabeta T-cell receptor (TCR) clonotypes at the time of selection regulates peripheral CD4 T-cell compartment size. Scid recipients were inoculated with various ratios of TCR Calpha(0/0) and wild-type bone marrow (BM) stem cells. Increasing the frequency of TCR Calpha(0/0) thymocytes at steady-state introduced a graded decrease in the maturation probability of the total DP thymocyte pool. At 12-14 weeks following BM inoculation, the frequency of TCR Calpha(0/0) DP thymocytes was inversely correlated with that of CD4 single positive (SP) thymocytes. Notwithstanding, a decreased frequency of wild-type DP thymocytes led to a marked increase in their transit efficiency from the DP to SP compartments. The frequency-dependent increase in thymocyte transit efficiency was associated with a CD4 SP cell surface phenotype indicative of increased antigenic experience. Importantly, the frequency of DP thymocytes capable of expressing TCR clonotypes dictated the steady-state size of the peripheral CD4 T cell compartment and its potential for homeostatic proliferation. Collectively, these results indicate that the efficiency of DP to CD4 SP transit is a frequency dependent process, which determines (1) the steady-state size of the peripheral T cell compartment and (2) the threshold for homeostatic expansion of peripheral CD4 T 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.

An Alternate Pathway for CD4 T Cell Development: Thymocyte-Expressed MHC Class II Selects a Distinct T Cell Population

Conventional understanding of CD4 T cell development is that the MHC class II molecules on cortical thymic epithelial cell are necessary for positive selection, as demonstrated in mouse models. Clinical data, however, show that hematopoietic stem cells reconstitute CD4 T cells in patients devoid of MHC class II. Additionally, CD4 T cells generated from human stem cells in immunocompromised mice were restricted to human, but not mouse, MHC class II. These studies suggest an alternative pathway for CD4 T cell development that does not normally exist in mice. MHC class II is expressed on developing human thymocytes, indicating a possible role of MHC II on thymocytes for CD4 T cell generation. Therefore, we created mice in which MHC class II is expressed only on T lineage cells. Remarkably, the CD4 compartment in such mice is efficiently reconstituted with unique specificity, demonstrating a novel thymocyte-driven pathway of CD4 T cell selection.

Receptor signals and nuclear events in CD4 and CD8 T cell lineage commitment

MHC specificity in positive selection is a major determinant in the CD4/CD8 T cell lineage decision. Previous studies support the view that quantitative differences in T cell receptor (TCR) signaling in immature CD4+CD8+ double positive thymocytes leads to an instructive bias in CD4/CD8 T cell lineage commitment that must be re-inforced in subsequent selection steps to ensure that MHC-restricted antigen recognition is linked to appropriate effector functions in mature T cells. Recent work has further defined the TCR signaling pathways involved in this process, but a major effort has been made to identify transcription factors and other regulators of CD4 and CD8 T cell lineage commitment. Methods and screens for detecting changes in gene expression, associated with TCR signaling in positive selection and lineage determination, are starting to provide a better understanding of these complex developmental processes.

Thymocyte-intrinsic genetic factors influence CD8 T cell lineage commitment and affect selection of a tumor-reactive TCR

Selection of immature CD4CD8 double-positive (DP) thymocytes for CD4 or CD8-lineage commitment is controlled by the interaction of the TCR with stromal cell-expressed peptide/MHC. We show that thymocyte-intrinsic genes influence the pattern of expression of a MHC class I-restricted transgenic (tg) TCR so that in DBA/2 mice, DP thymocytes with a characteristically high expression of tg TCR, infrequently transit to CD8 single-positive thymocytes. In contrast, in B10.D2 mice, the same tg TCR is expressed at lower levels on a subpopulation of DP thymocytes that more frequently transit to CD8 single-positive thymocytes. These characteristics were not influenced by thymic stromal components that control positive selection. Radiation chimeras reconstituted with a mixture of BM from tg TCR mice of the two genetic backgrounds revealed that the relative frequency of transit to the CD8 lineage remained thymocyte-intrinsic. Identifying the gene products whose polymorphism controls CD8 T cell development may shed new light on the mechanisms controlling T cell commitment/selection in mice other than the most studied "C57BL/6"-based strains.

Signalling in T-lymphocyte development: integration of signalling pathways is the key

alpha beta T-cell development is restricted to the thymus. Interactions between developing lymphocytes and the thymic stroma, together with bone-marrow-derived monocytes and dendritic cells, are critical for proper development of the T-cell lineage. The developmental sequence through which T-cell progenitors pass on their way to maturity is well established, and can be followed by the sequential acquisition and/or removal of cell surface molecules. Using the combination of modern genetic manipulations, such as transgenesis, gene ablation (knockouts) and targeted mutagenesis (knock-ins), with the ever-improving conditional and inducible manipulation of gene expression, we are beginning to gain an understanding of how intercellular interactions can be relayed via intracellular signalling cascades to bring about nuclear re-organisation and the differentiated mature CD4(+) and CD8(+) subpopulations.

Thymic selection can compensate for mutations affecting T cell activation and generate a normal T cell repertoire in mutant mice

Thymic selection adjusts the reactivity of the peripheral T cell repertoire to maximize recognition of pathogens and minimize stimulation by innocuous substances and self-antigen. The study of molecules implicated in T cell activation often involves the generation of knockout (-/-) mice. In many instances, knockout animals display revealing phenotypes. But should a lack of phenotype be interpreted as a lack of function? Bcl-xgamma was shown previously to affect T cell activation in vitro, and here we note that overexpression of this molecule increases cell cycling after T cell receptor ligation by antibody. It was therefore surprising that Bcl-xgamma(-/-), Bcl-xgamma transgenic, and WT T cells displayed similar levels of sensitivity to antigen according to ex vivo stimulation. Bcl-xgamma could be demonstrated to influence competitiveness and selection of thymocytes in a manner that counteracted the effects of Bcl-xgamma mutation on T cell activation. These findings suggest that thymic selection can overcome genetic defects in T cell activation to generate a T cell repertoire of normal reactivity.