The timing of TCR alpha expression critically influences T cell development and selection

The impact of circulating dendritic cells on the development and differentiation of thymocytes

Central tolerance is established through the negative selection of self-reactive thymocytes and the induction of T-regulatory cells (T-regs). A role for thymic epithelial cells in mediating both negative selection and T-reg induction has been clearly shown. The role of thymic dendritic cells (DCs) in these processes has not been clearly determined but has been the focus of recent studies. Thymic DCs include two conventional DC (cDC) subtypes, CD8(lo)Sirpalpha(hi/+) (CD8(lo)Sirpalpha(+) herein) and CD8(hi)Sirpalpha(lo/-) (CD8(hi)Sirpalpha(-) herein). It has been shown that these DC subsets have distinct developmental origins, the CD8(hi)Sirpalpha(-) cDCs developing intra-thymically and the CD8(lo)Sirpalpha(+) migrating into the thymus from the periphery. Furthermore, an important role for thymic DCs in the induction of T-regs has been shown. In this review, the role of DCs in the development and education of T cells in the thymus will be reviewed, with emphasis on the role of circulatory DCs in mediating these processes.

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.

Through regulation of TCR expression levels, an Idd7 region gene(s) interactively contributes to the impaired thymic deletion of autoreactive diabetogenic CD8+ T cells in nonobese diabetic mice

When expressed in NOD, but not C57BL/6 (B6) genetic background mice, the common class I variants encoded by the H2g7 MHC haplotype aberrantly lose the ability to mediate the thymic deletion of autoreactive CD8+ T cells contributing to type 1 diabetes (T1D). This indicated some subset of the T1D susceptibility (Idd) genes located outside the MHC of NOD mice interactively impair the negative selection of diabetogenic CD8+ T cells. In this study, using both linkage and congenic strain analyses, we demonstrate contributions from a polymorphic gene(s) in the previously described Idd7 locus on the proximal portion of Chromosome 7 predominantly, but not exclusively, determines the extent to which H2g7 class I molecules can mediate the thymic deletion of diabetogenic CD8+ T cells as illustrated using the AI4 TCR transgenic system. The polymorphic Idd7 region gene(s) appears to control events that respectively result in high vs low expression of the AI4 clonotypic TCR alpha-chain on developing thymocytes in B6.H2g7 and NOD background mice. This expression difference likely lowers levels of the clonotypic AI4 TCR in NOD, but not B6.H2g7 thymocytes, below the threshold presumably necessary to induce a signaling response sufficient to trigger negative selection upon Ag engagement. These findings provide further insight to how susceptibility genes, both within and outside the MHC, may interact to elicit autoreactive T cell responses mediating T1D development in both NOD mice and human patients.

The thymus chapter in the life of gut-specific intra epithelial lymphocytes

The intestinal intraepithelial lymphocytes (IEL) represent multi-lineage T cell populations. In addition to a major gammadeltaTCR(+) T cell subset, many IEL express alphabetaTCRs and they can be separated into alphabeta sublineages. Some TCRalphabeta(+)IEL have characteristics in common with conventional TCRalphabeta(+)T cells whereas others share an unconventional phenotype with their TCRgammadelta(+) counterparts. Because the latter are enriched for autoreactive TCRs and can be generated in the absence of a thymus, it has long been postulated that some IEL subsets develop locally in the intestine. Several new data however, indicate that under physiological conditions, IEL require a thymic education that directs lineage commitment and functional differentiation. This review will discuss the contributions of the thymus in shaping the various intestinal IEL sublineages.

Lineage diversion of T cell receptor transgenic thymocytes revealed by lineage fate mapping

Background

The binding of the T cell receptor (TCR) to major histocompatibility complex (MHC) molecules in the thymus determines fates of TCRαβ lymphocytes that subsequently home to secondary lymphoid tissue. TCR transgenic models have been used to study thymic selection and lineage commitment. Most TCR transgenic mice express the rearranged TCRαβ prematurely at the double negative stage and abnormal TCRαβ populations of T cells that are not easily detected in non-transgenic mice have been found in secondary lymphoid tissue of TCR transgenic mice.

Methodology and Principal Findings

To determine developmental pathways of TCR-transgenic thymocytes, we used Cre-LoxP-mediated fate mapping and show here that premature expression of a transgenic TCRαβ diverts some developing thymocytes to a developmental pathway which resembles that of gamma delta cells. We found that most peripheral T cells with the HY-TCR in male mice have bypassed the RORγt-positive CD4⁺8⁺ (double positive, DP) stage to accumulate either as CD4⁻8⁻ (double negative, DN) or as CD8α⁺ T cells in lymph nodes or gut epithelium. Likewise, DN TCRαβ cells in lymphoid tissue of female mice were not derived from DP thymocytes.

Conclusion

The results further support the hypothesis that the premature expression of the TCRαβ can divert DN thymocytes into gamma delta lineage cells.

Calcineurin sets the bandwidth for discrimination of signals during thymocyte development

At critical times in development, cells are able to convert graded signals into discrete developmental outcomes; however, the mechanisms involved are poorly understood. During thymocyte development, cell fate is determined by signals originating from the alphabeta T-cell receptor. Low-affinity/avidity interactions between the T-cell receptor and peptide-MHC complexes direct differentiation to the single-positive stage (positive selection), whereas high-affinity/avidity interactions induce death by apoptosis (negative selection). Here we show that mice deficient in both calcineurin and nuclear factor of activated T cells (NFAT)c2/c3 lack a population of preselection thymocytes with enhanced ability to activate the mitogen-activated protein kinase (Raf-MEK-ERK) pathway, and fail to undergo positive selection. This defect can be partially rescued with constitutively active Raf, indicating that calcineurin controls MAPK signalling. Analysis of mice deficient in both Bim (which is required for negative selection) and calcineurin revealed that calcineurin-induced ERK (extracellular signal-regulated kinase) sensitization is required for differentiation in response to 'weak' positive selecting signals but not in response to 'strong' negative selecting signals (which normally induce apoptosis). These results indicate that early calcineurin/NFAT signalling produces a developmental period of ERK hypersensitivity, allowing very weak signals to induce positive selection. This mechanism might be generally useful in the discrimination of graded signals that induce different cell fates.

Shaping of the autoreactive regulatory T cell repertoire by thymic cortical positive selection

The main function of regulatory T lymphocytes is to keep autoimmune responses at bay. Accordingly, it has been firmly established that the repertoire of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) is enriched in autospecific cells. Differences in thymic-positive and/or -negative selection may account for selection of the qualitatively distinct regulatory and conventional T cell (Tconv) repertoires. It has previously been shown that precursors for Tregs are less sensitive to negative selection than Tconv precursors. Studies with TCR/ligand doubly transgenic mice suggested that an agonist ligand might induce positive selection of Treg (but not Tconv) cells. However, massive deletion of Tconv (but not Treg) cell precursors observed in these mice renders interpretation of such data problematic and a potential role for positive selection in generation of the autospecific Treg repertoire has remained therefore incompletely understood. To study this important unresolved issue and circumvent use of TCR/ligand-transgenic mice, we have developed transgenic mice expressing a single MHC class II/peptide ligand on positively selecting thymic cortical epithelial cells. We found that functional Treg (but not Tconv) cells specific for the single ligand were preferentially selected from the naturally diverse repertoire of immature precursors. Our data therefore demonstrate that thymic cortical positive selection of regulatory and Tconv precursors is governed by distinct rules and that it plays an important role in shaping the autoreactive Treg repertoire.

BCL11B is required for positive selection and survival of double-positive thymocytes

Transcriptional control of gene expression in double-positive (DP) thymocytes remains poorly understood. We show that the transcription factor BCL11B plays a critical role in DP thymocytes by controlling positive selection of both CD4 and CD8 lineages. BCL11B-deficient DP thymocytes rearrange T cell receptor (TCR) α; however, they display impaired proximal TCR signaling and attenuated extracellular signal-regulated kinase phosphorylation and calcium flux, which are all required for initiation of positive selection. Further, provision of transgenic TCRs did not improve positive selection of BCL11B-deficient DP thymocytes. BCL11B-deficient DP thymocytes have altered expression of genes with a role in positive selection, TCR signaling, and other signaling pathways intersecting the TCR, which may account for the defect. BCL11B-deficient DP thymocytes also presented increased susceptibility to spontaneous apoptosis associated with high levels of cleaved caspase-3 and an altered balance of proapoptotic/prosurvival factors. This latter susceptibility was manifested even in the absence of TCR signaling and was only partially rescued by provision of the BCL2 transgene, indicating that control of DP thymocyte survival by BCL11B is nonredundant and, at least in part, independent of BCL2 prosurvival factors.

Impaired thymic selection in mice expressing altered levels of the SLP-76 adaptor protein

Intracellular signaling initiated by ligation of the TCR influences cell fate at multiple points during the lifespan of a T cell. This is especially evident during thymic selection, where the nature of TCR-dependent signaling helps to establish a MHC-restricted, self-tolerant T cell repertoire. The Src homology 2 domain-containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76) adaptor protein is a required intermediate in multiple signaling pathways triggered by TCR engagement, several of which have been implicated in dictating the outcome of thymic selection (e.g., intracellular calcium flux and activation of ERK family MAPKs). To determine if thymocyte maturation and selection at later stages of development are sensitive to perturbations in SLP-76 levels, we analyzed these crucial events using several transgenic (Tg) lines of mice expressing altered levels of SLP-76 in the thymus. In Tg mice expressing low levels of SLP-76 in preselection thymocytes, the CD4:CD8 ratio in the thymus and spleen was skewed in a manner consistent with impaired selection and/or maturation of CD4+ thymocytes. Low SLP-76 expression also correlated with reduced CD5 expression on immature thymocytes, consistent with reduced TCR signaling potential. In contrast, reconstitution of SLP-76 at higher levels resulted in normal thymic CD5 expression and CD4:CD8 ratios in the thymus and periphery. It is curious that thymic deletion of TCR-Tg (HY) thymocytes was markedly impaired in both lines of Tg-reconstituted SLP-76-/- mice. Studies using chimeric mice indicate that the defect in deletion of HY+ thymocytes is intrinsic to the developing thymocyte, suggesting that maintenance of sufficient SLP-76 expression from the endogenous locus is a key element in the selection process.

Multiple constraints at the level of TCRalpha rearrangement impact Valpha14i NKT cell development

CD1d-restricted NKT cells that express an invariant Valpha14 TCR represent a subset of T cells implicated in the regulation of several immune responses, including autoimmunity, infectious disease, and cancer. Proper rearrangement of Valpha14 with the Jalpha18 gene segment in immature thymocytes is a prerequisite to the production of a TCR that can be subsequently positively selected by CD1d/self-ligand complexes in the thymus and gives rise to the NKT cell population. We show here that Valpha14 to Jalpha rearrangements are temporally regulated during ontogeny providing a molecular explanation to their late appearance in the thymus. Using mice deficient for the transcription factor RORgamma and the germline promoters T early-alpha and Jalpha49, we show that developmental constraints on both Valpha and Jalpha usage impact NKT cell development. Finally, we demonstrate that rearrangements using Valpha14 and Jalpha18 occur normally in the absence of FynT, arguing that the effect of FynT on NKT cell development occurs subsequent to alpha-chain rearrangement. Altogether, this study provides evidence that there is no directed rearrangement of Valpha14 to Jalpha18 segments and supports the instructive selection model for NKT cell selection.

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.

Transcriptional analysis of clonal deletion in vivo

Engagement of the TCR on CD4(+)CD8(+) thymocytes initiates either a program of survival and differentiation (positive selection) or death (clonal deletion), which is dictated in large part by the affinity of the TCR for self-peptide-MHC complexes. Although much is known about the factors involved in positive selection, little is understood about the molecular mechanism leading to clonal deletion. To gain further insight into this process, we used a highly physiological TCR transgenic mouse model to compare gene expression changes under conditions of nonselection, positive selection, and negative selection. We identified 388 genes that were differentially regulated in negative selection compared with either nonselection or positive selection. These regulated genes fall into many functional categories including cell surface and intracellular signal transduction, survival and apoptosis, transcription and translation, and adhesion and migration. Additionally, we have compared our transcriptional profile to profiles of negative selection in other model systems in an effort to identify those genes with a higher probability of being functionally relevant. These included three up-regulated genes, bim, nur77, and ian1, and one down-regulated gene, lip1. Collectively, these data provide a framework for understanding the molecular basis of clonal deletion.

Early TCR expression and aberrant T cell development in mice with endogenous prerearranged T cell receptor genes

The factors that regulate the rate of production of T cells by the thymus remain incompletely defined. To test whether generation of functional T cell receptors limits the rate of thymic T cell export, we made use of a line of mice, LN3alphabeta, that have endogenously prerearranged TCR genes. The prerearranged TCR genes were expressed abnormally early in hemopoietic development, indicating that RAG-mediated recombination, rather than transcription factor expression, is the key determinant of the initiation of robust TCR transcription. Thymic T cell export rates were similar between wild-type (wt) and LN3alphabeta mice, indicating that T cell maturation rates in these mice are determined by factors other than TCR gene rearrangement. In competitive bone marrow chimeras, however, LN3alphabeta thymocytes were out-competed by wt cells and failed to develop beyond the double-negative 4 stage. Furthermore, wt progenitors transplanted intrathymically into LN3alphabeta mice proliferated excessively, suggesting that increased proliferative signals in the LN3alphabeta thymus compensate for faulty T cell development driven by early TCR expression.

The related adaptors, adaptor in lymphocytes of unknown function X and Rlk/Itk-binding protein, have nonredundant functions in lymphocytes

Adaptors play a critical role in regulating signaling pathways that control lymphocyte development and activation. Adaptor in lymphocytes of unknown function X (ALX) and Rlk/Itk-binding protein (RIBP) are adaptors related by structure and sequence, coexpressed in T cells. Mice deficient for each adaptor demonstrated that ALX and RIBP, respectively, negatively and positively regulate T cell activation in response to TCR/CD28 stimulation. However, these results did not preclude that they may function redundantly in other cell populations, or in response to other stimuli. Therefore, to understand the relationship between these related adaptors, ALX/RIBP-deficient mice were generated. We demonstrate that although ALX and RIBP are expressed throughout T cell development, T cell development occurs normally in these mice. Using the H-Y TCR transgenic model, positive and negative selection were found to proceed unimpeded in the absence of ALX and RIBP. We demonstrate that RIBP is also expressed in B cells; however, RIBP- and ALX/RIBP-deficient mice had normal B cell development, and responded equivalently to wild type in response to IgM, CD40, B cell-activating factor/B lymphocyte stimulator, CpG, and LPS. Interestingly, T cells deficient in both ALX and RIBP behaved similarly to those deficient in ALX alone during T cell activation in response to TCR/CD28, exhibiting increased IL-2 production, CD25 expression, and proliferation, thus showing that ALX deficiency masked the effect of RIBP deficiency. ALX/RIBP-deficient T cells did not have any alterations in either activation-induced cell death or Th1/2 polarization. Therefore, we did not find any functional redundancy or synergy during lymphocyte development, selection, activation, or survival in ALX/RIBP-deficient mice, demonstrating that these molecules function independently.

Signalling in T-cell development: is it all location, location, location?

During thymocyte development, signals through the pre-T-cell receptor induce proliferation and differentiation at early stages of maturation; thereafter, signals through the alphabeta T-cell receptor complex mediate positive and negative selection and commitment to the CD4 or CD8 lineage. How these signals are initiated, the transduction pathways involved, and ultimately how gene transcription is regulated are current active areas of research. Recent literature highlights the importance of geography to thymocyte differentiation: first, in relation to the ability of the developing thymocyte to traffic to localities within the thymus in which particular selection events occur and which are crucial for successful T-cell maturation; and, second, in respect to the subcellular localisation of intracellular signalling molecules, which might provide the key to understanding how similar signals can be translated into dramatically distinct fates, such as positive selection, lineage commitment and negative selection.

Intraepithelial lymphocytes: their shared and divergent immunological behaviors in the small and large intestine

At the front line of the body's immunological defense system, the gastrointestinal tract faces a large number of food-derived antigens, allergens, and nutrients, as well as commensal and pathogenic microorganisms. To maintain intestinal homeostasis, the gut immune system regulates two opposite immunological reactions: immune activation and quiescence. With their versatile immunological features, intraepithelial lymphocytes (IELs) play an important role in this regulation. IELs are mainly composed of T cells, but these T cells are immunologically distinct from peripheral T cells. Not only do IELs differ immunologically from peripheral T cells but they are also comprised of heterogeneous populations showing different phenotypes and immunological functions, as well as trafficking and developmental pathways. Though IELs in the small and large intestine share common features, they have also developed differences as they adjust to the two different environments. This review seeks to shed light on the immunological diversity of small and large intestinal IELs.

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.

The unconventional role of LT alpha beta in T cell differentiation

Lymphotoxin (LT)alphabeta, a member of the tumor necrosis factor cytokine superfamily, and its receptor, the LTbeta receptor (LTbetaR), have a well defined role in secondary lymphoid organogenesis but an unexpected function in T cell differentiation. Although earlier studies indicated that conventional T cell subsets were normal in mice deficient in the LTbetaR pathway, accumulating evidence indicates that the LTalphabeta-LTbetaR pathway has a pivotal role in the ontogeny of unconventional T cells, including gammadelta T cells and invariant natural killer T cells. The LTbetaR pathway seems to operate at distinct levels during thymic development. Double positive thymocytes regulate the differentiation of early thymocyte progenitors and gammadelta T cells through a mechanism dependent on LTbetaR. In addition, LTbetaR signaling in thymic stroma was proposed to affect central tolerance to peripherally restricted antigens. These findings highlight the complex cellular crosstalk between lymphoid and stromal compartments during thymic differentiation.

The extrathymic T-cell differentiation in the murine gut

The gut epithelial border is in continuous contact with exogenous antigens and harbors a distinctive and very abundant CD8 alpha alpha intraepithelial T-lymphocyte effector population. We describe here the characteristics of these cells that distinguish them from all other T-cell types in the body as well as their functions in local protection. We also describe how these cells differentiate from local precursors present in the gut cryptopatches (CPs) following a pathway of T-cell differentiation unique to the gut wall. Finally, we describe the origin of the precursors of CD8 alpha alpha T cells, which come from the bone marrow in athymic mice but are first imprinted in the thymus in euthymic mice. Indeed, CD3(-)CD4(-)CD8(-) T-cell-committed precursors can leave the thymus before T-cell receptor rearrangements and then colonize the gut CPs, proceeding with their differentiation within the gut wall.

Thymic differentiation of TCRαβ+CD8αα+IELs

Intraepithelial lymphocytes (IELs) contain several subsets, but the origin of the T-cell receptor (TCR)alphabeta(+) CD8 alpha alpha(+) IELs has been particularly controversial. Here we provide a synthesis, based on recent work, that attempts to unify the divergent views. The intestine has a primordial function in lymphopoiesis, and precursors with the potential to differentiate into T cells are found both in the epithelium and underlying lamina propria. Moreover, the thymus has been reported to export cells to the intestine that are not fully differentiated. TCR alpha beta(+) CD8 alpha alpha(+) IELs can differentiate in the intestine from each of these sources, but in normal euthymic mice, the thymus appears to be the major source for TCR alpha beta(+) CD8 alpha alpha(+) IELs. This unique IEL subset is a self-reactive population that requires exposure to self-agonists for selection in the thymus, similar to other regulatory T-cell populations. IELs transition through a double-positive (DP) intermediate in the thymus, but they originate from a subset of the DP cells that can be identified by its expression of CD8 alpha alpha homodimers. The agonist-selected cells in the thymus are TCRbeta(+) but CD4 and CD8 double negative. The evidence suggests that reacquired expression of CD8 alpha alpha and downregulation of CD5 occur after thymus export, perhaps in the intestine under the influence of interleukin-15. As a result of agonist exposure, a new gene expression program is activated. Therefore, the increased understanding of the developmental origin of TCR alpha beta(+) CD8 alpha alpha(+) IELs may help us to understand how they participate in immune regulation and protection in the intestine.

Activation of the Hedgehog signaling pathway in T-lineage cells inhibits TCR repertoire selection in the thymus and peripheral T-cell activation

TCR signal strength is involved in many cell fate decisions in the T-cell lineage. Here, we show that transcriptional events induced by Hedgehog (Hh) signaling reduced TCR signal strength in mice. Activation of Hh signaling in thymocytes in vivo by expression of a transgenic transcriptional-activator form of Gli2 (Gli2DeltaN(2)) changed the outcome of TCR ligation at many stages of thymocyte development, allowing self-reactive cells to escape clonal deletion; reducing transgenic TCR-mediated positive selection; reducing the ratio of CD4/CD8 single-positive (SP) cells; and reducing cell surface CD5 expression. In contrast, in the Shh(-/-) thymus the ratio of CD4/CD8 cells and both positive and negative selection of a transgenic TCR were increased, demonstrating that Shh does indeed influence TCR repertoire selection and the transition from double-positive (DP) to SP cell in a physiological situation. In peripheral T cells, Gli2DeltaN(2) expression attenuated T-cell activation and proliferation, by a mechanism upstream of ERK phosphorylation.

Receptor editing in lymphocyte development and central tolerance

The specificities of lymphocytes for antigen are generated by a quasi-random process of gene rearrangement that often results in non-functional or autoreactive antigen receptors. Regulation of lymphocyte specificities involves not only the elimination of cells that display 'unsuitable' receptors for antigen but also the active genetic correction of these receptors by secondary recombination of the DNA. As I discuss here, an important mechanism for the genetic correction of antigen receptors is ongoing recombination, which leads to receptor editing. Receptor editing is probably an adaptation that is necessitated by the high probability of receptor autoreactivity. In both B cells and T cells, the genes that encode the two chains of the antigen receptor seem to be specialized to promote, on the one hand, the generation of diverse specificities and, on the other hand, the regulation of these specificities through efficient editing.

Identification of pre- and postselection TCRalphabeta+ intraepithelial lymphocyte precursors in the thymus

The immune system preserves and makes use of autoreactive lymphocytes with specialized functions. Here we showed that one of these populations, CD8alphaalpha(+)TCRalphabeta(+) intestinal intraepithelial lymphocytes (IELs), arose from a unique subset of double-positive thymocytes. This subset of cells was precommitted to preferentially give rise to CD8alphaalpha(+)TCRalphabeta(+) IELs, but they required exposure to self-agonist peptides. The agonist-selected TCRalphabeta(+) thymocytes are CD4 and CD8 double-negative, and their final maturation, including the induction of CD8alphaalpha expression, appeared to occur only after thymus export in the IL-15-rich environment of the gut. These developmental steps, including precommitment of immature thymocytes, TCR-mediated agonist selection, and postthymic differentiation promoted by cytokines, define a unique pathway for the generation of CD8alphaalpha(+)TCRalphabeta(+) IEL.

Double-negative T regulatory cells can develop outside the thymus and do not mature from CD8+ T cell precursors

Recent studies have demonstrated that activated peripheral alphabeta TCR+ CD3+ CD4- CD8- NK1.1- (double-negative, DN) regulatory T cells (Tregs) from both mice and humans are able to down-regulate immune responses in vitro and in vivo. However, the origin and developmental requirements of functional DN Tregs remain unclear. In this study, we investigated the requirement for CD8 expression as well as the presence of a thymus for the development of functional DN Tregs. We demonstrate that DN Tregs exist in CD8-deficient mice and that stimulation of CD8+ T cells in vivo with TCR-specific Ag does not convert CD8+ T cells into DN Tregs. In addition, we found that DN T cells are present in the spleens and lymph nodes of thymectomized mice that are irradiated and reconstituted with T cell-depleted bone marrow cells. Interestingly, DN Tregs that develop in thymectomized mice can suppress syngeneic CD8+ T cells more effectively than those that develop in sham-thymectomized mice. Taken together, our data suggest that DN Tregs are not derived from CD8+ T cell precursors and that functional DN Tregs may preferentially develop outside of the thymus. These data suggest that DN Tregs may represent a developmentally and functionally unique cell population.

Negative selection of the T-cell repertoire: where and when does it occur?

Because of the use of somewhat artificial models for the elucidation of negative selection [superantigen, T-cell receptor (TCR) transgenic mice], there is still considerable uncertainty at what stages of T-cell development negative selection can occur and whether it becomes manifest as developmental arrest, lineage diversion, or induction of apoptotic cell death. Here, experimental evidence is reviewed that excludes developmental arrest and lineage diversion as the sole mechanisms of negative selection. The data emphasize that both CD4+ CD8+ double-positive cortical as well as semi-mature, single-positive, medullary thymocytes are targets of deletion in experimental models employing superantigen and TCR transgenic mice with premature as well as 'timely' onset of TCR expression.

Central tolerance: good but imperfect

T-cell development is a highly coordinated process that depends on interactions between thymocytes, thymic epithelium, and bone marrow (BM)-derived dendritic cells (DCs). Before entering the peripheral T-cell pool, thymocytes are subject to negative selection, a process that eliminates (or deletes) T cells with high affinity toward self-antigens and therefore promotes self-tolerance. These self-antigens include those that are broadly expressed ubiquitous antigens and those whose expression is restricted to a few tissues, tissue-specific antigens (TSAs). Expression of TSAs in the thymus is mostly a property of medullary thymic epithelial cells (mTECs), and because these cells may be less capable than BM-derived DCs at mediating negative selection to ubiquitous antigens, we investigated the roles of both of these cell types in tolerance to TSAs. Here, we review our studies in which we found that mTECs were competent mediators of negative selection to a subset of TSA-reactive T cells, while thymic DCs extend the range of TSA-reactive T cells that undergo negative selection by capturing TSAs from mTECs. In addition, we recently investigated the efficiency of central tolerance to TSA during ontogeny, and we report that this process was less efficient in neonates than adult animals.

The TCR C beta FG loop regulates alpha beta T cell development

The TCRbeta chain constant domain contains an unusually elongated, solvent-exposed FG loop. This structural element forms one component of an alphabeta TCR cavity against which CD3epsilongamma may abut to facilitate Ag-specific signaling. Consistent with this notion, in the present study we show that N15alphabeta TCR transfectants expressing a FG loop-deleted chain (betaDeltaFG) stimulate less tyrosine protein phosphorylation than those bearing a wild-type beta-chain (betawt) upon TCR cross-linking. Furthermore, coimmunoprecipitation studies suggest a weakened association between the CD3epsilongamma heterodimer and the beta-chain in TCR complexes containing the betaDeltaFG variant. To further investigate the biologic role of the Cbeta FG loop in development, we competitively reconstituted the thymus of Ly5 congenic or RAG-2-/- mice using bone marrow cells from betawt or betaDeltaFG transgenic C57BL/6 (B6) mice. Both betawt and betaDeltaFG precursor cells generate thymocytes representative of all maturational stages. However, betaDeltaFG-expressing thymocytes dominate during subsequent development, resulting in an excess of betaDeltaFG-expressing peripheral T cells with reduced proliferative and cytokine production abilities upon TCR stimulation. Collectively, our results show that the unique Cbeta FG loop appendage primarily controls alphabeta T cell development through selection processes.

Generation of T-cell receptor retrogenic mice

T-cell receptor (TCR) transgenic (Tg) mice have revolutionized our understanding of many aspects of T-cell biology. Whereas they provide an almost unlimited source of T cells with a single specificity, breeding them onto different backgrounds and/or new knockout/knock-in mouse models is often time-consuming (6 months to several years), which can make the process costly and can significantly delay research. This protocol describes a new method for expressing defined TCR-alpha and TCR-beta proteins from a single 2A peptide-linked multicistronic retroviral vector in mice, using retrovirus-mediated stem cell gene transfer. We refer to these as 'retrogenic' (Rg) mice ('retro' from retrovirus and 'genic' from Tg) to avoid confusion with traditional transgenic mice. We have successfully used this approach to express over 50 different TCRs on several different mouse backgrounds in as little as 6 weeks.

The E delta enhancer controls the generation of CD4- CD8- alphabetaTCR-expressing T cells that can give rise to different lineages of alphabeta T cells

It is well established that the pre–T cell receptor for antigen (TCR) is responsible for efficient expansion and differentiation of thymocytes with productive TCRβ rearrangements. However, Ptcra- as well as Tcra-targeting experiments have suggested that the early expression of Tcra in CD4⁻CD8⁻ cells can partially rescue the development of αβ CD4⁺CD8⁺ cells in Ptcra-deficient mice. In this study, we show that the TCR Eδ but not Eα enhancer function is required for the cell surface expression of αβTCR on immature CD4⁻CD8⁻ T cell precursors, which play a crucial role in promoting αβ T cell development in the absence of pre-TCR. Thus, αβTCR expression by CD4⁻CD8⁻ thymocytes not only represents a transgenic artifact but occurs under physiological conditions.

Mechanisms imposing the Vbeta bias of Valpha14 natural killer T cells and consequences for microbial glycolipid recognition

Mouse and human natural killer T (NKT) cells recognize a restricted set of glycosphingolipids presented by CD1d molecules, including self iGb3 and microbial α-glycuronosylceramides. The importance of the canonical Vα14-Jα18 TCR α chain for antigen recognition by NKT cells is well recognized, but the mechanisms underlying the Vβ8, Vβ7, and Vβ2 bias in mouse have not been explored. To study the influences of thymic selection and the constraints of pairing with Vα14-Jα18, we have created a population of mature T cells expressing Vα14-Jα18 TCR α chain in CD1d-deficient mice and studied its recognition properties in vitro and in vivo. Transgenic cells expressed a diverse Vβ repertoire but their recognition of endogenous ligands and synthetic iGb3 was restricted to the same biased Vβ repertoire as expressed in natural NKT cells. In contrast, α-GalCer, a synthetic homologue of microbial α-glycuronosylceramides, was recognized by a broader set of Vβ chains, including the biased NKT set but also Vβ6, Vβ9, Vβ10, and Vβ14. These surprising findings demonstrate that, whereas Vβ8, Vβ7, and Vβ2 represent the optimal solution for recognition of endogenous ligand, many Vβ chains that are potentially useful for the recognition of foreign lipids fail to be selected in the NKT cell repertoire.

MR1-restricted V alpha 19i mucosal-associated invariant T cells are innate T cells in the gut lamina propria that provide a rapid and diverse cytokine response

Mucosal-associated invariant T (MAIT) cells reside primarily in the gut lamina propria and require commensal flora for selection/expansion. They are restricted by the highly conserved MHC class I-related molecule MR1 and, like most NK T cells, express an invariant TCRalpha chain. Although they probably contribute to gut immunity, MAIT cells have not been functionally characterized because they are so rare. To create a model in which they are more abundant, we generated transgenic mice expressing only the TCRalpha chain (Valpha19i) that defines MAIT cells. By directly comparing Valpha19i transgenic mice on MR1+/+ and MR1-/- backgrounds, we were able to distinguish and characterize a population of Valpha19i T cells dependent on MR1 for development. MR1-restricted Valpha19i transgenic T cells recapitulate what is known about MAIT cell development. Furthermore, a relatively high proportion of transgenic MAIT cells express NK1.1, and most have a cell surface phenotype similar to that of Valpha14i NK T cells. Finally, MR1-restricted Valpha19i T cells secrete IFN-gamma, IL-4, IL-5, and IL-10 following TCR ligation, and we provide evidence for what may be two functionally distinct MAIT cell populations. These data strongly support the idea that MAIT cells contribute to the innate immune response in the gut mucosa.

The TCRalpha locus control region specifies thymic, but not peripheral, patterns of TCRalpha gene expression

The molecular mechanisms ensuring the ordered expression of TCR genes are critical for proper T cell development. The mouse TCR alpha-chain gene locus contains a cis-acting locus control region (LCR) that has been shown to direct integration site-independent, lymphoid organ-specific expression of transgenes in vivo. However, the fine cell type specificity and developmental timing of TCRalpha LCR activity are both still unknown. To address these questions, we established a transgenic reporter model of TCRalpha LCR function that allows for analysis of LCR activity in individual cells by the use of flow cytometry. In this study we report the activation of TCRalpha LCR activity at the CD4-CD8-CD25-CD44- stage of thymocyte development that coincides with the onset of endogenous TCRalpha gene rearrangement and expression. Surprisingly, TCRalpha LCR activity appears to decrease in peripheral T cells where TCRalpha mRNA is normally up-regulated. Furthermore, LCR-linked transgene activity is evident in gammadelta T cells and B cells. These data show that the LCR has all the elements required to reliably reproduce a developmentally correct TCRalpha-like expression pattern during thymic development and unexpectedly indicate that separate gene regulatory mechanisms are acting on the TCRalpha gene in peripheral T cells to ensure its high level and fine cell type-specific expression.

The thymus exports long-lived fully committed T cell precursors that can colonize primary lymphoid organs

Thymic export of cells is believed to be restricted to mature T cells. Here we show that the thymus also exports fully committed T cell precursors that colonize primary lymphoid organs. These precursor cells exited the thymus before T cell receptor rearrangements and colonized lymphoid organs such as the thymus and the gut. Migration of the thymic T cell-committed precursors led to permanent colonization of the gut precursor compartment, improved the capacity of gut precursors to further differentiate into T cells and was sufficient for the generation of 'euthymic like' CD8alphaalpha(+) intraepithelial lymphocytes. These data demonstrate a new function for the thymus in peripheral seeding with T cell precursors that become long lived after thymus export.

Central tolerance: learning self-control in the thymus

In the past few years, there has been a flurry of discoveries and advancements in our understanding of how the thymus prepares T cells to exist at peace in normal healthy tissue: that is, to be self-tolerant. In the thymus, one of the main mechanisms of T-cell central tolerance is clonal deletion, although the selection of regulatory T cells is also important and is gaining enormous interest. In this Review, we discuss the emerging consensus about which models of clonal deletion are most physiological, and we review recent data that define the molecular mechanisms of central tolerance.

The role of apoptosis in the development and function of T lymphocytes

Apoptosis plays an essential role in T cell biology. Thymocytes expressing nonfunctional or autoreactive TCRs are eliminated by apoptosis during development. Apoptosis also leads to the deletion of expanded effector T cells during immune responses. The dysregulation of apoptosis in the immune system results in autoimmunity, tumorogenesis and immunodeficiency. Two major pathways lead to apoptosis: the intrinsic cell death pathway controlled by Bcl-2 family members and the extrinsic cell death pathway controlled by death receptor signaling. These two pathways work together to regulate T lymphocyte development and function.