Tracing interactions of thymocytes with individual stromal cell partners

Receptor-mediated T cell absorption of antigen presenting cell-derived molecules

T cells tend to acquire a variety of cell surface molecules derived from antigen presenting cells (APCs). The molecule uptake occurs mainly during direct T/APC contact and is instigated by specific receptor/ligand interactions, such as T cell receptor (TCR) with a cognate peptide/MHC complex (pMHC) or CD28 with B7. The acquired molecules are targeted for internalization and degradation in the lysosome. Nevertheless, those molecules are expressed on the surface of T cells for a period of time. The presentation of APC-derived ligands by T cells exhibited a multitude of immunological effects via antigen-specific T/T interaction upon recognition of the absorbed antigens by contact with other T cells. Ligand uptake also occurs via absorption of membrane vesicles shed from APCs prior to contact (e.g., exosomes and plasma membrane-derived vesicles). As in ligand absorption via direct T/APC interaction, the absorption of pre-formed membrane vesicles is also dependent on specific receptor/ligand interactions. In this review, biological mechanisms underlying the ligand absorption process as well as the biological significance and application of the event will be discussed.

Peptide-MHC heterodimers show that thymic positive selection requires a more restricted set of self-peptides than negative selection

T cell selection and maturation in the thymus depends on the interactions between T cell receptors (TCRs) and different self-peptide-major histocompatibility complex (pMHC) molecules. We show that the affinity of the OT-I TCR for its endogenous positively selecting ligands, Catnb-H-2Kb and Cappa1-H-2Kb, is significantly lower than for previously reported positively selecting altered peptide ligands. To understand how these extremely weak endogenous ligands produce signals in maturing thymocytes, we generated soluble monomeric and dimeric peptide-H-2Kb ligands. Soluble monomeric ovalbumin (OVA)-Kb molecules elicited no detectable signaling in OT-I thymocytes, whereas heterodimers of OVA-Kb paired with positively selecting or nonselecting endogenous peptides, but not an engineered null peptide, induced deletion. In contrast, dimer-induced positive selection was much more sensitive to the identity of the partner peptide. Catnb-Kb-Catnb-Kb homodimers, but not heterodimers of Catnb-Kb paired with a nonselecting peptide-Kb, induced positive selection, even though both ligands bind the OT-I TCR with detectable affinity. Thus, both positive and negative selection can be driven by dimeric but not monomeric ligands. In addition, positive selection has much more stringent requirements for the partner self-pMHC.

Functional development of the T cell receptor for antigen

For over three decades now, the T cell receptor (TCR) for antigen has not ceased to challenge the imaginations of cellular and molecular immunologists alike. T cell antigen recognition transcends every aspect of adaptive immunity: it shapes the T cell repertoire in the thymus and directs T cell-mediated effector functions in the periphery, where it is also central to the induction of peripheral tolerance. Yet, despite its central position, there remain many questions unresolved: how can one TCR be specific for one particular peptide-major histocompatibility complex (pMHC) ligand while also binding other pMHC ligands with an immunologically relevant affinity? And how can a T cell's extreme specificity (alterations of single methyl groups in their ligand can abrogate a response) and sensitivity (single agonist ligands on a cell surface are sufficient to trigger a measurable response) emerge from TCR-ligand interactions that are so low in affinity? Solving these questions is intimately tied to a fundamental understanding of molecular recognition dynamics within the many different contexts of various T cell-antigen presenting cell (APC) contacts: from the thymic APCs that shape the TCR repertoire and guide functional differentiation of developing T cells to the peripheral APCs that support homeostasis and provoke antigen responses in naïve, effector, memory, and regulatory T cells. Here, we discuss our recent findings relating to T cell antigen recognition and how this leads to the thymic development of foreign-antigen-responsive alphabetaT cells.

Low ligand requirement for deletion and lack of synapses in positive selection enforce the gauntlet of thymic T cell maturation

Immature double-positive (CD4(+)CD8(+)) thymocytes respond to negatively selecting peptide-MHC ligands by forming an immune synapse that sustains contact with the antigen-presenting cell (APC). Using fluorescently labeled peptides, we showed that as few as two agonist ligands could promote APC contact and subsequent apoptosis in reactive thymocytes. Furthermore, we showed that productive signaling for positive selection, as gauged by nuclear translocation of a green fluorescent protein (GFP)-labeled NFATc construct, did not involve formation of a synapse between thymocytes and selecting epithelial cells in reaggregate thymus cultures. Antibody blockade of endogenous positively selecting ligands prevented NFAT nuclear accumulation in such cultures and reversed NFAT accumulation in previously stimulated thymocytes. Together, these data suggest a "gauntlet" model in which thymocytes mature by continually acquiring and reacquiring positively selecting signals without sustained contact with epithelial cells, thereby allowing them to sample many cell surfaces for potentially negatively selecting ligands.

Ltbetar signaling does not regulate Aire-dependent transcripts in medullary thymic epithelial cells

Thymic medullary epithelial cells (mTECs) play a major role in central tolerance induction by expressing tissue-specific Ags (TSAs). The expression of a subset of TSAs in mTECs is under the control of Aire (autoimmune regulator). Humans defective for AIRE develop a syndrome characterized by autoimmune disease in several endocrine glands. Aire has been proposed to be regulated by lymphotoxin beta receptor (Ltbetar) signaling and there is evidence that, additionally, Aire-independent transcripts may be regulated by this pathway. Given the potential clinical importance of Aire regulation in mTECs for the control of autoimmunity, we investigated the relation between Ltbetar signaling and TSA expression by whole genome transcriptome analysis. In this study, we show that the absence of Ltbetar has no effect on the expression of Aire and Aire-dependent TSAs. Also, the lack of Ltbetar signaling does not disturb regulatory T cells or the distribution of dendritic cells in the thymus. However, mTECs in Ltbetar-deficient mice show an aberrant distribution within the thymic medulla with disruption of their three-dimensional architecture. This is predicted to impair the interaction between mTECs and thymocytes as shown by the reduced surface uptake of MHCII by mature thymocytes in Ltbetar-deficient mice. We propose that the physiological medullary architecture ensures negative-selection by supporting lympho-epithelial interaction through a large epithelial cell surface distributed evenly across the medulla.

Capture of membrane components via trogocytosis occurs in vivo during both dendritic cells and target cells encounter by CD8(+) T cells

Cytotoxic T lymphocytes recently stimulated by antigen-presenting cells (APC) display major histocompatibility class (MHC) I and II molecules inherited from APC. We have previously reported that, in vitro, transfer of MHC molecules and several other proteins occurs through trogocytosis, i.e. the active acquisition of target cell membrane fragments by T lymphocytes. Here, using the model of viral antigen LCMVgp33-41 recognition in transgenic P14 mice, we show that CD8(+) T cells perform trogocytosis in vivo, as detected by the capture of biotin- or fluorescence-labeled components of the APC surface. Trogocytosis occurs during interactions of CD8(+) T cells with at least two kinds of cells: target cells and dendritic cells (DC). In lymph nodes, CD8(+) T cells having performed trogocytosis with DC express the CD69 activation marker indicating that trogocytosis detects recently activated cells. Taken together, our findings suggest that trogocytosis may be a new in vivo marker of the recent interaction between a CD8(+) T cell and its cellular partners or targets.

Intercellular transfer of cell-surface proteins is common and can affect many stages of an immune response

Cells can extend the limits of their transcriptome by using proteins captured from other cells. Through an exchange of specific proteins, tools and information can be shared to establish integrated communities of cells that are better able to coordinate stages of an immune response. Transferred proteins can also contribute to pathology by allowing, for example, infection of cell types not otherwise infected. Here, I present the case for considering the intercellular transfer of cell-surface proteins between immune cells as commonplace and important.

Real-time measurement of signaling and motility during T cell development in the thymus

Intracellular signals arising from interactions of immature thymocytes with distinct populations of stromal cells in the thymus are central to T cell development. The characteristics of these signals and the mechanisms underlying thymocyte migration between stromal cell compartments have been difficult to identify from static measurements of fixed tissue. Recent advances in two-photon microscopy and the development of three-dimensional models for real-time studies of T cell development have shed light on how single cells navigate the thymus. These studies reveal crosstalk between thymocyte signaling and motility that may integrate the search for potentially rare self-antigens with the requirement for sustained signaling in T cell maturation.

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

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

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.

Swapping molecules during cell-cell interactions

Interactions between cells of the immune system can lead to intercellular exchange of cell-surface molecules. Recent studies indicate that such exchange can be bidirectional when natural killer cells interact with their target cells. Sprent reviews the mechanisms involved in cell-cell molecular transfer and the receptor-ligand interactions involved.

Foreignness as a matter of degree: the relative immunogenicity of peptide/MHC ligands

The ability of T lymphocytes (T cells) to recognize and attack foreign invaders while leaving healthy cells unharmed is often analysed as a discrete self/non-self dichotomy, with each peptide/MHC ligand classified as either self or non-self. We argue that the ligand immunogenicity is more naturally treated as a continuous quantity, and show how to define and quantitate relative ligand immunogenicity. In our theory, self-tolerance is acquired through reduction of the relative immunogenicity of autoantigens, whereas xenoantigens, typically not presented during induction of deletional tolerance, retain a high degree of relative immunogenicity. Autoantigens that are not prominently presented in deletional tolerance likewise retain a high relative immunogenicity and remain essentially foreign. According to our analysis, any given autoantigen can attain a high level of relative immunogenicity, provided it is presented at sufficiently high levels. Our theory provides a quantitative tool to analyse the immunogenicity of tumour-associated neoantigens and the aetiology of autoimmune disease.

Calcium oscillations regulate thymocyte motility during positive selection in the three-dimensional thymic environment

The three-dimensional thymic microenvironment and calcium signaling pathways are essential for driving positive selection of developing T cells. However, the nature of calcium signals and the diversity of their effects in the thymus are unknown. We describe here a thymic slice preparation for visualizing thymocyte motility and signaling in real time with two-photon microscopy. Naive thymocytes were highly motile at low intracellular calcium concentrations, but during positive selection cells became immobile and showed sustained calcium concentration oscillations. Increased intracellular calcium was necessary and sufficient to arrest thymocyte motility. The calcium dependence of motility acts to prolong thymocyte interactions with antigen-bearing stromal cells, promoting sustained signaling that may enhance the expression of genes underlying positive selection.

Peptide-Specific Intercellular Transfer of MHC Class II to CD4+ T Cells Directly from the Immunological Synapse upon Cellular Dissociation

The transfer of membrane proteins from APC to T cells was initially described in the 1970s, and subsequent work has described two mechanisms of transfer: APC-derived exosomes and direct transfer of small packets, while cells remain conjugated. Using fibroblast APC expressing a GFP-tagged I-E(k) molecule with covalently attached antigenic peptide, we observed a third mechanism in live cell imaging: T cells spontaneously dissociating from APC often capture MHC:peptide complexes directly from the immunological synapse. Using two I-E(k)-restricted murine TCR transgenic T cells with different peptide specificity, we show in this study that the MHC transfer is peptide specific. Using blocking Abs, we found that MHC:peptide transfer in this system requires direct TCR-MHC:peptide interactions and is augmented by costimulation through CD28-CD80 interactions. Capture of the GFP-tagged MHC:peptide complexes correlates with an activated phenotype of the T cell, elevated CD69 with down-modulated TCR. The transferred MHC:peptide molecules transferred to the T cell are associated with molecules that imply continued TCR signaling; p56(lck), phosphotyrosine, and polarization of the actin cytoskeleton.

Visualizing thymocyte motility using 2-photon microscopy

Our view of a thymocyte based on its behavior in tissue culture and appearance in fixed tissue sections was of a round sessile cell. Its travel through the thymus might occur slowly, perhaps even passively, leaving it in contact with the support cells that happened to be in its immediate environment. However, when we got our first look at the behavior of thymocytes in a 3D cellular stromal cell environment, that picture changed dramatically. Instead we found that thymocytes are actively crawling, allowing them to explore their environment over relatively long distances and interact with peptide-major histocompatibility complex (pMHC)-bearing thymic stromal cells in both dynamic and stable modes. In this review, we discuss the implications of thymocyte motility for T-cell repertoire selection and for the mechanisms that determine the spatial organization of thymocyte subsets within the thymus.

Promoter IV of the class II transactivator gene is essential for positive selection of CD4+ T cells

Major histocompatibility complex class II (MHCII) expression is regulated by the transcriptional coactivator CIITA. Positive selection of CD4(+) T cells is abrogated in mice lacking one of the promoters (pIV) of the Mhc2ta gene. This is entirely due to the absence of MHCII expression in thymic epithelia, as demonstrated by bone marrow transfer experiments between wild-type and pIV(-/-) mice. Medullary thymic epithelial cells (mTECs) are also MHCII(-) in pIV(-/-) mice. Bone marrow-derived, professional antigen-presenting cells (APCs) retain normal MHCII expression in pIV(-/-) mice, including those believed to mediate negative selection in the thymic medulla. Endogenous retroviruses thus retain their ability to sustain negative selection of the residual CD4(+) thymocytes in pIV(-/-) mice. Interestingly, the passive acquisition of MHCII molecules by thymocytes is abrogated in pIV(-/-) mice. This identifies thymic epithelial cells as the source of this passive transfer. In peripheral lymphoid organs, the CD4(+) T-cell population of pIV(-/-) mice is quantitatively and qualitatively comparable to that of MHCII-deficient mice. It comprises a high proportion of CD1-restricted natural killer T cells, which results in a bias of the V beta repertoire of the residual CD4(+) T-cell population. We have also addressed the identity of the signal that sustains pIV expression in cortical epithelia. We found that the Jak/STAT pathways activated by the common gamma chain (CD132) or common beta chain (CDw131) cytokine receptors are not required for MHCII expression in thymic cortical epithelia.

Acquisition of CD80 by human T cells at early stages of activation: functional involvement of CD80 acquisition in T cell to T cell interaction

The interaction between CD28 on T cells and CD80 on APCs intensifies the linkage between TCR and MHC at the site of contact between T cells and APCs. In this study, we demonstrate that during human T cell/human APC interaction, the autologous or allogeneic human CD4(+) T cells become positive for the detection of CD80 at an early stage of activation (24 h). This detection of CD80 is attributable to the acquisition of CD80 from APCs, as opposed to the up-regulation of endogenous CD80, as demonstrated by CD4(+) T cells treated with cyclohexamide. Furthermore, no CD80 mRNA could be detected at 24 h in T cells that had acquired CD80 from APCs. CD80 acquisition by T cells from APCs was enhanced upon TCR engagement. The amount of CD80 acquisition by CD4(+) T cells was shown to be related to the expression of CD80 on APCs. Using soluble fusion proteins (soluble CTLA-4, CD28, and CD80) to block either CD28 on the surface of T cells or CD80 on the surface of APCs, it was demonstrated that CD80 acquisition by T cells is mediated through its receptors, possibly CD28 interaction. Moreover, we demonstrate that T cells that have acquired CD80 have the ability to stimulate other T cells. These data thus suggest that CD80 acquisition by human T cells might play a role in the immunoregulation of T cell responses.

Genetic dissection of systemic lupus erythematosus pathogenesis: evidence for functional expression of Sle3/5 by non-T cells

On the non-autoimmune C57BL/6 (B6) background, the chromosome 7-derived lupus susceptibility loci Sle3 and Sle5 have been shown to mediate an elevated CD4:CD8 ratio with an increase in activated CD4(+) T cells, decreased susceptibility to apoptosis, and a break in humoral tolerance. Development of subcongenic strains has subsequently shown that the elevated CD4:CD8 ratio is due to Sle3 but that both loci contribute to the development of autoantibodies. To elucidate the functional expression patterns of these loci, adoptive transfer experiments were conducted. All possible combinations of bone marrow reconstitution, including syngenic, were conducted between the congenic B6 and B6.Sle3/5 strains. It was found that the Sle3/5 locus was functionally expressed by bone marrow-derived cells, but not by host cells, and that the elevated CD4:CD8 phenotype could be reconstituted in radiation chimeras. Using Ly5-marked congenic strains and B6 host mice, additional experiments surprisingly demonstrated that the elevated CD4:CD8 ratio was neither an intrinsic property of the T cells nor of single positive thymocytes. Allotype-marked chimeras indicated that autoantibody production by B cells was also an extrinsic property, as shown by the fact that B cells without the Sle3/5 interval contributed to autoantibody production. These experiments strongly suggest that a gene within the B6.Sle3/5 interval was expressed by a bone marrow-derived, nonlymphocyte population in the thymus and periphery and was affecting T cell selection and/or survival.

Competition for specific intrathymic ligands limits positive selection in a TCR transgenic model of CD4+ T cell development

Efficient positive selection of a broad repertoire of T cells is dependent on the presentation of a diverse array of endogenous peptides on MHC molecules in the thymus. It is unclear, however, whether the development of individual TCR specificities is influenced by the abundance of their selecting ligands. To examine this, we analyzed positive selection in a transgenic mouse carrying a TCR specific for the human CLIP:I-Ab class II complex. We found that these mice exhibit significantly reduced CD4+ T cell development compared with two other transgenic mice carrying TCRs selected on I-Ab. Moreover, many of the selected cells in these mice express endogenous and transgenic receptors as a consequence of dual TCRalpha expression. Dramatic enhancement of the selection efficiency is observed, however, when fewer transgenic cells populate the thymus in mixed bone marrow chimeras. These results suggest that positive selection is limited by the availability of selecting peptides in the thymus. This becomes apparent when large numbers of thymocytes compete for such peptides in TCR transgenic animals. Under such conditions, thymocytes appear to undergo further TCRalpha gene rearrangement to produce a receptor that may be selected more efficiently by other thymic self-peptides.

T cells can use either T cell receptor or CD28 receptors to absorb and internalize cell surface molecules derived from antigen-presenting cells

At the site of contact between T cells and antigen-presenting cells (APCs), T cell receptor (TCR)-peptide-major histocompatibility complex (MHC) interaction is intensified by interactions between other molecules, notably by CD28 and lymphocyte function-associated antigen 1 (LFA-1) on T cells interacting with B7 (B7-1 and B7-2), and intracellular adhesion molecule 1 (ICAM-1), respectively, on APCs. Here, we show that during T cell-APC interaction, T cells rapidly absorb various molecules from APCs onto the cell membrane and then internalize these molecules. This process is dictated by at least two receptors on T cells, namely CD28 and TCR molecules. The biological significance of T cell uptake of molecules from APCs is unclear. One possibility is that this process may allow activated T cells to move freely from one APC to another and eventually gain entry into the circulation.

Self-antigen presentation by thymic stromal cells: a subtle division of labor

Self-antigen-MHC complexes expressed by thymic stromal cells serve as ligands for TCR-mediated positive and negative selection, resulting in a self-MHC-restricted, self-tolerant T cell repertoire. It has recently become apparent that thymic stromal cells differ in their accessibility to antigen as well as their ability to process and present antigen. These differences result in the sampling by thymic stromal cells of largely nonoverlapping self-antigen pools and the display of self-peptide profiles specific for each cell type. In conjunction with single or serial cell-cell interactions between thymocytes and stromal cells, such differences in self-antigen display allow for maximal (re)presentation of 'self' in the thymus and optimize the efficacy of positive and negative selection.

Intrathymic deletion of MHC class I-restricted cytotoxic T cell precursors by constitutive cross-presentation of exogenous antigen

Cross-priming of cytotoxic T lymphocytes by professional antigen-presenting cells (APC) is a potential hazard to self tolerance because it exposes naive T cells to tissue-specific self antigens in the context of co-stimulatory signals. Here we show that cross-presentation of exogenous material occurs constitutively within the thymus. Although efficient cross-presentation is a property of relatively few APC it results in thymocyte deletion both in vitro and in vivo, suggesting that intrathymic cross-presentation can operate as an effective component of tolerance to circulating self antigens. The capacity of minor cell populations to mediate thymocyte deletion but not positive selection reflects an underlying difference in the biology of these two processes.

Normal Thymic Selection of TCR Transgenic CD4 T Cells, but Impaired Survival in the Periphery Despite the Presence of Selecting MHC Molecules

In this paper, we investigate selection in the thymus and survival in the periphery of CD4 T cells, which carry a major histocompatibility class II-restricted transgenic TCR (A18 TCRtg) specific for a natural self Ag, the fifth component of complement (C5). A18 TCRtg thymocytes develop normal numbers of CD4 single-positive (SP) thymocytes, but do not show pronounced overselection as do some other TCR transgenic strains. CD4 SP cells are mature as judged by termination of CD8 synthesis, resistance to cortisone, and functional competence. The kinetics of positive selection, determined by BrdU labeling, are very fast. CD4 SP thymocytes are demonstrable within 2 days of labeling, and within 8 days after labeling a large proportion (20%) of lymph node T cells are recent thymic emigrants. The high number of recent thymic emigrants suggests rapid turnover of CD4 T cells in the periphery, which was confirmed by thymectomy and determination of CD4 T cell life spans. A18 TCRtg T cells have a t1/2 of ∼6 wk, despite the presence of selecting MHC molecules. This explains the failure to accumulate high numbers of peripheral T cells and suggests that the MHC-bound ligand(s) responsible for initiating survival signals is limiting for the selection and maintenance of A18 transgenic CD4 T cells.

Homeostasis Limits the Development of Mature CD8+ But Not CD4+ Thymocytes

The involvement of a variety of clonal selection processes during the development of T lymphocytes in the thymus has been well established. Less information, however, is available on how homeostatic mechanisms may regulate the generation and maturation of thymocytes. To investigate this question, mixed radiation bone marrow chimeras were established in which wild-type T cell precursors capable of full maturation were diluted with precursors deficient in maturation potential because of targeted mutations of the RAG1 or TCR-α genes. In chimeras in which the majority of thymocytes are blocked at the CD4−CD8−CD25+ stage (RAG1 deficient), and only a small proportion of T cell precursors are of wild-type origin, we observed no difference in the maturation of wild-type CD4−CD8−CD25+ cells to the CD4+CD8+ stage as compared with control chimeras. Therefore, the number of cell divisions occurring during this transition is fixed and not subject to homeostatic regulation. In contrast, in mixed chimeras in which the majority of thymocytes are blocked at the CD4+CD8+ stage (TCR-α deficient), an increased efficiency of development of wild-type mature CD8+ cells was observed. Surprisingly, the rate of generation of mature CD4+ thymocytes was not affected in these chimeras. Thus, the number of selectable CD8 lineage thymocytes apparently saturates the selection mechanism in normal mice while the development of CD4 lineage cells seems to be limited only by the expression of a suitable TCR. These data may open the way to the identification of homeostatic mechanisms regulating thymic output and CD4/CD8 lineage commitment, and the development of means to modulate it.

The CD3-gammadeltaepsilon and CD3-zeta/eta modules are each essential for allelic exclusion at the T cell receptor beta locus but are both dispensable for the initiation of V to (D)J recombination at the T cell receptor-beta, -gamma, and -delta loci

The pre-T cell receptor (TCR) associates with CD3-transducing subunits and triggers the selective expansion and maturation of T cell precursors expressing a TCR-beta chain. Recent experiments in pre-Talpha chain-deficient mice have suggested that the pre-TCR may not be required for signaling allelic exclusion at the TCR-beta locus. Using CD3-epsilon- and CD3-zeta/eta-deficient mice harboring a productively rearranged TCR-beta transgene, we showed that the CD3-gammadeltaepsilon and CD3-zeta/eta modules, and by inference the pre-TCR/CD3 complex, are each essential for the establishment of allelic exclusion at the endogenous TCR-beta locus. Furthermore, using mutant mice lacking both the CD3-epsilon and CD3-zeta/eta genes, we established that the CD3 gene products are dispensable for the onset of V to (D)J recombination (V, variable; D, diversity; J, joining) at the TCR-beta, TCR-gamma, and TCR-delta loci. Thus, the CD3 components are differentially involved in the sequential events that make the TCR-beta locus first accessible to, and later insulated from, the action of the V(D)J recombinase.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.