Early events in T lymphocyte genesis in the fetal thymus

Mechanisms of thymus organogenesis and morphogenesis

The thymus is the primary organ responsible for generating functional T cells in vertebrates. Although T cell differentiation within the thymus has been an area of intense investigation, the study of thymus organogenesis has made slower progress. The past decade, however, has seen a renewed interest in thymus organogenesis, with the aim of understanding how the thymus develops to form a microenvironment that supports T cell maturation and regeneration. This has prompted modern revisits to classical experiments and has driven additional genetic approaches in mice. These studies are making significant progress in identifying the molecular and cellular mechanisms that control specification, early organogenesis and morphogenesis of the thymus.

Thymic nurse cells: a microenvironment for thymocyte development and selection

Thymic nurse cells (TNCs) represent a unique microenvironment in the thymus for MHC restriction and T cell repertoire selection composed of a cortical epithelial cell surrounding 20-200 immature thymocytes. TNCs have been isolated from many classes of animals from fish to humans. Studies performed using TNC lines showed that TNCs bind viable alphabetaTCRlow CD4(+)CD8(+)CD69(-) thymocytes. A subset of the bound cells is internalized, proliferates within the TNC, and matures to the alphabetaTCRhigh CD4(+)CD8(+)CD69(+) stage, indicative of positive selection. A subset of the internalized population is released while cells that remain internalized undergo apoptosis and are degraded by lysosomes within the TNC. A TNC-specific monoclonal antibody added to fetal thymic organ cultures resulted in an 80% reduction in the number of thymocytes recovered, with a block at the double positive stage of development. Together these data suggest a critical role for TNC internalization in thymocyte selection as well as the removal and degradation of negatively selected thymocytes. Recent studies have shown that in addition to thymocytes, peripheral circulating macrophages are also found within the TNC complex and can present antigens to the developing thymocytes. These circulating macrophages could provide a source of self-antigens used to ensure a self-tolerant mature T cell repertoire. A reduction in TNC numbers is associated with a variety of autoimmune diseases including thyroiditis and systemic lupus erythematosis.

Cytokine requirement for the development of T-lymphoid lineage potential in clonal lymphohaematopoietic progenitors in vitro

The early process of T-cell development prior to thymic colonization has been poorly investigated because of the lack of a sensitive assay. We have developed a two-step in vitro culture system by combining a clonal culture with a fetal thymus organ culture (FTOC) and analysed the early development of T cells from lymphohaematopoietic progenitors. Cells of immature colonies derived from bone marrow cells of 5-fluorouracil (5FU)-treated mice using various combinations of early acting cytokines were transferred into a FTOC. All the combinations of stem cell factor (SCF), interleukin (IL)-3 and IL-6 capable of inducing colony formation supported T-cell generation. IL-11 and the Flt3 ligand possessed T-lineage promotional effects similar to IL-6 and SCF respectively. However, there were some quantitative differences in the final T-cell yield among cytokine combinations. Thus, the commitment towards T lineage in lymphohaematopoietic progenitors may be an event determined intrinsically rather than induced by specific stimuli, but there may be a hierarchy between the activity of cytokines in further development. Furthermore, we examined the T-lineage potential of individual colonies derived from Lin(-)c-Kit(+)Sca-1(+) cells clone-sorted from post-5FU marrow cells. No colonies that contained only myelocytic progenitors showed T-lineage potential, but 23.3% of colonies with a haematopoietic multipotentiality did. Therefore, the divergence of the T lineage from other lineages such as myeloid potential may occur at an early stage of the hierarchy of haematopoiesis. The proposed method should prove valuable for exploring the molecular and cellular changes that occur during early T-cell development before thymic colonization.

Stepwise development of thymic microenvironments in vivo is regulated by thymocyte subsets

T-cell development is under the tight control of thymic microenvironments. Conversely, the integrity of thymic microenvironments depends on the physical presence of developing thymocytes, a phenomenon designated as ‘thymic crosstalk’. We now show, using three types of immunodeficient mice, i.e. CD3(epsilon) transgenic mice, RAG(null) mice and RAG(null)-bone-marrow-transplanted CD3(epsilon) transgenic mice, that the control point in lymphoid development where triple negative (CD3(−),CD4(−),CD8(−)) thymocytes progress from CD44(+)CD25(−) towards CD44(−)CD25(+), influences the development of epithelial cells, critically inducing the extra, third dimension in the organization of the epithelial cells in the cortex. This tertiary configuration of the thymic epithelium is a typical feature for the thymus, enabling lymphostromal interaction during T-cell development. Crosstalk signals at this control point also induce the formation of thymic nurse cells. Moreover, our data indicate that establishment of a thymic cortex is a prerequisite for the development of the thymic medulla. Thus, differentiating thymocytes regulate the morphogenesis of thymic microenvironments in a stepwise fashion.

Regulation of NK1.1 Expression During Lineage Commitment of Progenitor Thymocytes

We recently identified a stage in fetal ontogeny (NK1.1+/CD117+) that defines committed progenitors for T and NK lymphocytes. These cells are found in the fetal thymus as early as day 13 of gestation, but are absent in the fetal liver. Nonetheless, multipotent precursors derived from both the fetal thymus and fetal liver are capable of rapidly differentiating to the NK1.1+ stage upon transfer into fetal thymic organ culture (FTOC). This suggests that expression of NK1.1 marks a thymus-induced lineage commitment event. We now report that a subset of the most immature fetal thymocytes (NK1.1−/CD117+) is capable of up-regulating NK1.1 expression spontaneously upon short-term in vitro culture. Interestingly, fetal liver-derived CD117+ precursors remain NK1.1− upon similar culture. Spontaneous up-regulation of NK1.1 surface expression is minimally affected by transcriptional blockade, mitogen-induced activation, or exposure of these cells to exogenous cytokines or stromal cells. These data suggest that induction of NK1.1 expression on cultured thymocytes may be predetermined by exposure to the thymic microenvironment in vivo. Importantly, multipotent CD117+ thymocytes subdivided on the basis of NK1.1 expression after short-term in vitro culture show distinct precursor potential in lymphocyte lineage reconstitution assays. This demonstrates that even the earliest precursor thymocyte population, although phenotypically homogeneous, contains a functionally heterogeneous subset of lineage-committed progenitors. These findings characterize a thymus-induced pathway in the control of lymphocyte lineage commitment to the T and NK cell fates.

Early Intrathymic Precursor Cells Acquire a CD4low Phenotype

CD4low cells are a population of lymphoid lineage-restricted progenitor cells representing the earliest precursors present in the adult thymus. Paradoxically, thymic progenitors with a similar phenotype in fetal mice and adult RAG-2-deficient (RAG-2−/−) mice lack this characteristic low-level expression of CD4. We now show that radiation-induced differentiation of CD4+CD8+ double positive thymocytes in RAG-2−/− mice results in the appearance of low levels of CD4 on thymocytes that are phenotypically identical to CD4low progenitor cells present in the normal adult thymus. This suggests that CD4 surface expression can be passively transferred from double positive cells to early progenitor thymocytes. Analysis of mixed bone marrow chimeras, reconstituted with hematopoietic stem cells from both CD4−/− (CD45.2) and CD4wt (CD45.1) congenic mice, revealed a CD4low phenotype on cells derived from CD4−/− bone marrow cells. Furthermore, these CD4−/−-derived “CD4low” progenitors were capable of reconstituting lymphocyte-depleted fetal thymi, with all thymocytes displaying a CD4−/− phenotype. This directly demonstrates that genetically CD4-deficient thymic progenitor cells can passively acquire a CD4low phenotype. Moreover, CD4 expression on CD4low progenitor thymocytes is sensitive to mild acid treatment, indicating that CD4 may not exist as an integral cell surface molecule on this thymocyte population. Our findings demonstrate that low-level CD4 surface expression can be passively acquired by intrathymic progenitor cells from the surrounding thymic microenvironment, suggesting that other cell surface molecules expressed at low levels may also result from an acquired phenotype.

Inductive role of fibroblastic cell lines in development of the mouse thymus anlage in organ culture

Previously, we have shown that embryonic day 12 thymus anlage cultured alone cannot develop into the mature organ but degenerates. In the present study, we investigated the cause of this insufficient organogenesis of embryonic day 12 thymus anlage in organ culture. We cocultured embryonic day 12 thymus anlages with various cell lines as pellets formed by centrifugation. In coculture with fibroblastic cell lines, but not with thymic epithelial cell lines, embryonic day 12 thymus anlages developed to support full T cell differentiation, and expressed mature stromal cell markers, Ia and Kb. By pellet culture of thymus anlages and fibroblastic cell lines transfected with a beta-galactosidase expression vector, we analyzed the distribution of added fibroblastic cells in pellets. The added fibroblastic cells constituted neither thymic capsule nor septa but disappeared after about 2 weeks in culture. Moreover, immunohistochemical studies indicated that added fibroblastic cells were adjacent to mesenchymal cells of thymus anlage. Our results strongly suggest that added fibroblastic cells support the development of the thymus anlage through interaction with its mesenchymal cells.

Subtractive isolation of phage-displayed single-chain antibodies to thymic stromal cells by using intact thymic fragments

In the murine thymus, the stroma forms microenvironments that control different steps in T cell development. To study the architecture of such microenvironments and more particularly the nature of communicative signals in lympho-stromal interaction during T cell development, we have employed the phage antibody display technology, with the specific aim of isolating thymic stromal cell-specific single-chain antibodies from a semisynthetic phage library. A subtractive approach using intact, mildly fixed thymic fragments as target tissue and lymphocytes as absorber cells generated monoclonal phages (MoPhabs) detecting subsets of murine thymic stromal cells. In the present paper we report on the reactivity of single-chain antibodies derived from three MoPhabs, TB4-4, TB4-20, and TB4-28. While TB4-4 and TB4-20 are both epithelium specific, TB4-28 detects an epitope expressed on both epithelial- and mesenchymal-derived stromal cells. TB4-4 reacts with all cortical epithelial cells and with other endoderm-derived epithelia, but this reagent leaves the majority of medullary epithelial cells unstained. In contrast, MoPhab TB4-20 detects both cortical and medullary thymic epithelial cells, as well as other endoderm- and ectoderm-derived epithelial cells. Cross-reaction of single-chain antibodies to human thymic stromal cells shows that our semisynthetic phage antibody display library, in combination with the present subtractive approach, permits detection of evolutionary conserved epitopes expressed on subsets of thymic stromal cells.

Thymocyte development in vitro: implications for studies of ageing and thymic involution

Functional defects that accumulate in the T cell compartment are thought to be responsible for the pronounced immunodeficiency that develops during ageing, and reduced production of T cells by the thymus as it undergoes involution has been suggested to contribute to this phenomenon. Understanding the mechanisms responsible for thymic involution requires a thorough knowledge of how thymopoiesis is regulated. Obtaining such information is dependent upon the availability of defined experimental systems that permit analysis of thymopoiesis at the cellular and molecular levels. Recent advances have been made in the development of such human and murine in vitro systems, and their analysis has the potential to identify thymic microenvironmental signals that regulate T cell production. This information should, in turn, provide a basis for understanding changes in thymopoiesis that occur during ageing. The features of these culture systems are reviewed in this article, and their potential application to the study of T cell production during ageing is discussed.

Involvement of transcription factors TCF-1 and GATA-3 in the initiation of the earliest step of T cell development in the thymus

Flow cytometric and immunocytochemical analyses of murine fetal thymus (FT) cells with antibodies to various surface markers and transcription factors reveal that the synthesis of TCF-1 and GATA-3 protein begins simultaneously in a fraction of the most immature population of FT cells, which have the phenotype of CD4-CD8-CD44+CD25-. No TCF-1-producing cells is found in the fetal liver (FL). In CD44+CD25- FT cells, the production of TCF-1 is immediately followed by intracellular expression of CD3 epsilon. It is also found that the T cell development from FL, but not FT, progenitors in the FT organ culture system is severely inhibited by the addition of antisense oligonucleotides for either TCF-1 or GATA-3. These results strongly suggest that TCF-1 and GATA-3 play essential roles in the initiation of the earliest steps of T cell development in the thymus.

Regulation of thymocyte development from immature progenitors

T lymphocytes differentiate from hematopoietic stem cells that settle in the microenvironment of the thymus. The earliest stages of mouse alpha/beta T-cell differentiation occurring before surface expression of the TCR include three important events: proliferation, commitment to the T lineage, and rearrangement and expression of the TCR loci. Recent evidence suggests that the survival as well as differentiation of early thymocytes depends critically on molecular signals such as those generated by the recently described pre-TCR complex.

Limited development capacity of the earliest embryonic murine thymus

Previous studies have demonstrated that murine thymus separates from the pharynx during 11.5-12 days of gestation, and that the proliferation of thymic cells starts at this age. We characterized embryonic day 12 thymus in terms of the surface phenotype of the thymus cells, the function of the lobe in supporting T cell development in organ culture, and the precursor activity of the thymus cells in a mixed culture with deoxyguanosine-treated lobes. The phenotype of the major population of embryonic day 12 thymus cells was HSA+, CD44+, c-kit+, Thy-1-, CD25-, CD4-, CD8-, TcR-, and Sca-1-. In organ culture of embryonic day 12 thymus lobes, most of the lobes did not develop well and failed to generate CD4+CD8+, CD4+CD8-, or CD4-CD8+ cells, even when embryonic day 14 thymus cells were added. However, thymus cells on embryonic day 12 contained T cell precursors that developed into mature T cells in co-culture with deoxyguanosine-treated fetal thymic lobes. The majority of the stromal cells in deoxyguanosine-treated embryonic day 14 thymus lobes expressed the surface molecules I-A and H-2D, whereas these cells in embryonic day 12 thymus lobes were negative for these surface molecules. Thus, our findings suggest that the embryonic day 12 thymus lobe contains T cell precursors, but that the undeveloped thymic stromal cells are insufficient to support full T cell development.

Constitutive activation of integrin alpha 4 beta 1 defines a unique stage of human thymocyte development

Our understanding of thymocyte development and of the positive and negative selection events involved in shaping the repertoire of mature T lymphocytes has been greatly facilitated by the use of transgenic and gene knockout animals. Much less is known about the factors that control the homing and population of the thymus by T cell precursors and the subsequent migration of developing thymocytes through the thymic architecture. As the integrins represent a candidate group of cell surface receptors that may regulate thymocyte development, we have analyzed the expression and function of alpha 4 beta 1 and alpha 5 beta 1 on human thymocytes. A major portion of double positive (CD4+ CD8+) human thymocytes express alpha 4 beta 1 in a constitutively active form and adhere to fibronectin and vascular cell adhesion molecule 1. alpha 4 beta 1 expression is similar on adherent and nonadherent populations, thus, activity reflects the receptor state and not simple expression. The adherent cells are immature, expressing high levels of CD4/CD8 and low levels of CD3 and CD69. In contrast, nonadherent cells possess the phenotype of thymocytes after positive selection, expressing intermediate levels of CD4 and/or CD8 and high levels of CD3 and CD69. The adherent population fails to respond to activation with anti-CD3 and fibronectin, whereas nonadherents exhibit an alpha 5 beta 1-dependent proliferation. Differential regulation of alpha 4 beta 1 and alpha 5 beta 1 receptors may provide a mechanism controlling cellular traffic, differentiation, and positive selection of thymocytes.

The role of the thymus in myasthenia gravis

The experimental work discussed here supports the hypothesis that in the pathogenesis of MG the initial and essential steps take place within the thymus. Most if not all thymuses of MG patients contain B cells capable of producing AChR specific autoantibody along with appropriate stroma elements. Hyperplastic thymuses characteristically contain germinal centers with cellular complexes of AChR-producing MC and surrounding interdigitating dendritic cells. In thymomas, the source of the myasthenogenic autoantigen is less obvious. There are data suggesting that thymoma epithelium expresses a protein sharing certain peptide epitopes with the AChR alpha chain, although there is no further molecular similarity. A unique type of 'molecular self-mimicry' cold be involved in the initiation of thymoma-associated MG.

Differentiation of CD3-4-8- human fetal thymocytes in vivo: characterization of a CD3-4+8- intermediate

Human thymocyte differentiation was examined by injecting fetal thymic progenitor populations into human thymic xenografts in SCID-hu mice. Thymic progenitors were fluorescently labeled with the lipophilic dye PKH2. The phenotypes of their progeny could be identified by flow cytometric analysis of cells with a very high fluorescent PKH2 signal. Intrathymic injection of purified triple negative (TN) CD3-4-8- thymocytes resulted in the sequential appearance of CD3-4+8-, CD3-4+8+, and CD3+4+8+ cells, with the subsequent appearance of small numbers of phenotypically mature CD3+4+8- and CD3+4-8+ cells over a 4-d period. Sorted CD3-4+8- thymocytes injected intrathymically rapidly differentiated to CD4+8+ cells. CD4+8+ fetal thymocytes in cell cycle differentiated into phenotypically mature CD3+4+8- and CD3+4-8+ populations, whereas nondividing CD4+8+ cells failed to differentiate after intrathymic transfer. The number of cell divisions that occurred between the injection of TN thymocytes and their progeny at different time points was estimated based on the decrease in the intensity of the PKH2 label. The average length of the cell cycle for the TN population was calculated to be 24 h. The SCID-hu model thus provides a useful tool for studying the kinetics of cell division and differentiation of human thymocytes in vivo.

Phenotypically diverse mouse thymic stromal cell lines which induce proliferation and differentiation of hematopoietic cells

The heterogeneity of the thymic stroma has made careful characterization of particular thymic stromal cell types difficult. To this end, we have derived a panel of cloned thymic stromal cell lines from simian virus 40 T antigen (SV40-T antigen) transgenic mice. Based on their analysis with monoclonal antibodies that distinguish among subsets of thymic stroma cells, and on the morphology and ultrastructural features of the different clones, we suggest that our panel includes representatives of the thymic subcapsular cortex or thymic nurse cells (427.1), the deep cortex or cortical reticular cells (1308.1) and the medulla including medullary interdigitating (IDC)-like cells (6.1.1) and medullary epithelial cells (6.1.7). A fifth cell type of undesignated but apparent medullary origin (6.1.11) was also isolated. All of the cell lines constitutively express the SV40 T antigen transgene and the class I antigens of the major histocompatibility complex (MHC), and they can be induced to express MHC class II antigens upon stimulation with recombinant interferon-gamma (IFN-gamma). These cell lines elaborate a factor(s) that induces the proliferation of cells from the fetal liver and bone marrow, but not from the neonatal thymus. A factor(s) elaborated by the 1308.1 cell line also induces the proliferation of fetal thymocytes in the absence of mitogens, phorbol esters or calcium ionophore which is augmented with the addition of recombinant interleukin-2 (IL-2). Analysis by reverse transcription polymerase chain reaction with primers for some mouse cytokines reveals that each of these cell lines contain granulocyte-macrophage colony-stimulating factor (GM-CSF) transcripts and that 1308.1, 6.1.1 and 6.1.7 produce IL-6 mRNA. Cell lines 1308.1 and 6.1.1 also produce IL-7; 6.1.1 produces IL-1 beta and tumor necrosis factor (TNF)-alpha while the 427.1 cell line produces IL-5 and IFN-gamma mRNA. None of the cell lines tested express the IL-2 receptor, IL-2, IL-3, IL-4, TNF-beta or macrophage inflammatory proteins mRNA. Conditioned medium (CM) from 1308.1 and 6.1.11 induced differentiation of cells purified from the mouse fetal liver into granulocytes; 1308.1 CM also induced differentiation of the mouse hematopoietic stem cell line 32DCl3(G) suggesting that the CM contains granulocyte (G)-CSF activity. Each cell line produces GM-CSF but the greatest activity is associated with 1308.1 and 6.1.11 CM. The availability of these well-characterized, functional, cloned thymic stromal cells will allow a more detailed analysis of the role of each cell type in both myeloid and T cell development.

Differential eicosanoid synthesis by murine fetal thymic non-lymphoid cells

The temporal patterns of synthesis of prostaglandin (PG)E2 and PGI2 by organ-cultured fetal thymic lobes and the cell population(s) responsible for synthesis of such products within the murine fetal thymus have been investigated. Embryonic day 14 thymic lobes were organ-cultured in defined media for 14 days and the media were collected every 24 h and replaced with fresh media. Collected media were processed for quantitation of either PGE2 or PGI2. Lobes were also cultured in 2'-deoxyguanosine (1.35 mmol/L) to produce an enriched non-lymphoid population. The per cent cyclooxygenase-positive cells within non-lymphoid cell-enriched lobes as well as the capacity of such lobes to synthesize either PGE2 or PGI2 were determined and compared with that of intact thymic lobes. Results demonstrate that fetal thymic lobes, in vitro, differentially synthesize PGI2 and synthesize PGE2 at a constant rate. Moreover, lobes enriched for non-lymphoid cells contain a greater percentage of cyclooxygenase-positive cells and synthesize increased amounts of eicosanoids per 10(4) cells compared with controls.

A murine thymic stromal cell line which may support the differentiation of CD4-8- thymocytes into CD4+8- alpha beta T cell receptor positive T cells

A fibroblastoid cell line TSt-4 was established from fetal thymus tissue of C57BL/6 mice. When fetal thymus (FT) cells or CD4-8- (DN) cells of adult thymuses were cultured on the monolayer of TSt-4, a considerable proportion of lymphocytes expressed CD4 or both CD4 and CD8 within 1 day, and the CD4+CD8- cells were maintained further while the CD4+8+ cells disappeared by Day 5. A large proportion of cells generated from DN cells but not FT cells was shown to express CD3 and T cell receptor alpha beta. Addition of recombinant interleukin (IL)-7 into the cultures resulted in a marked increase of cell recovery without virtual change in differentiation process of alpha beta lineage. The present work strongly suggests that thymic fibroblasts play an important role in T cell differentiation and IL-7 contributes to supporting proliferation of differentiated cells.

Cytokines in T-cell development

The thymus provides a unique environment for the development of T cells, supporting both precursor cell proliferation and differentiation. The control of these processes is unknown but they may be mediated by cytokines, or other soluble factors, or by interactions with specific elements of the thymic stroma. Here, Simon Carding, Adrian Hayday and Kim Bottomly describe cellular, immunochemical and molecular studies of the production and action of cytokines within the human and mouse thymus and demonstrate their essential role in T-cell development.

Thymic modulation of IL-2 and IL-4 synthesis by peripheral T cells

In this paper we provide several lines of evidence to support the hypothesis that the thymus can exert regulatory influences on the functional capabilities of mature recirculating T cells. Our studies demonstrate that while the IL-2-producing potential of T cells that repopulate the secondary lymphoid organs of lethally irradiated and stem cell-reconstituted mice is significantly reduced compared to that of T cells harvested from normal mice, the amount of IL-4 produced by the T cells of these experimental animals is equivalent to, or greater than, the amount produced by T cells from control animals. In addition, we determined that the amount of biologically active IL-2 and IL-4 secreted by T cells harvested from lethally irradiated animals who reconstitute their hematopoietic and immune systems under the influence of nonirradiated thymic epithelial grafts is essentially identical to the amount produced by T cells harvested from nonirradiated control animals. Collectively, these findings suggest that: (1) the alterations observed in the lymphokine-producing potential of T cells harvested from lethally irradiated and stem cell-reconstituted mice is not due to a direct effect of ionizing radiation on the T lymphocytes themselves, and (2) the exposure of the epithelial cells of the thymus to ionizing radiation during marrow-ablative regimens abrogates or modifies a component of thymic function which can influence the lymphokine-secreting potential of recirculating T cells. Further evidence for thymic involvement in the regulation of lymphokine production by peripheral T cells comes from our finding of a post-thymectomy time-dependent reduction in the capacity of T cells from animals to produce IL-2. Coincident with this reduction, T cells harvested from peripheral lymphoid organs of thymectomized animals demonstrated an augmentation in their IL-4-producing capabilities. The finding that treatment of thymectomized animals with the androgen steroid hormone dehydroepiandrosterone reestablished a normal IL-2-producing potential by their T cells makes it unlikely that the reduced capacity to produce IL-2 was secondary to a loss in fresh thymic emigrants.

Disorganization and restoration of thymic medullary epithelial cells in T cell receptor-negative scid mice: evidence that receptor-bearing lymphocytes influence maturation of the thymic microenvironment

In order to assess the possible role of lymphoid cells in the development of thymic epithelium, we compared the organization and maturation of thymic epithelium in scid mice lacking T cell receptor (TcR)-positive cells with that in scid mice containing TcR+ cells. Immunohistologic examination revealed that thymi from TcR- scid mice were deficient in thymic medullary epithelial cells recognized by the monoclonal antibody ER-TR5, and that the few thymic medullary epithelial cells present were not organized into discrete medullary areas. In contrast, thymi from scid mice containing TcR+ cells possessed ER-TR5+ thymic medullary epithelial cells and these cells were organized normally into discrete medullary regions. Thus, normal organization and maturation of thymic medullary epithelial cells did not occur in the absence of TcR+ cells, but did occur upon introduction of TcR+ cells. We conclude that lympho-stromal cell interactions in the thymus are not unidirectional, and that a symbiotic relationship exists between maturing epithelial cells and developing lymphocytes.

Effect of cytokines on thymic hematopoietic precursors. Phenotypic and electron-microscopic study

We have previously shown that the interaction of thymocytes with thymic accessory cells (macrophages and/or interdigitating cells) is one of the factors required for thymocyte activation. Precursors of both thymic accessory cells and thymocytes are included in the CD4- CD8- Mac-1- Ia- subpopulation, and their respective maturation and/or activation may be modulated by granulocyte-macrophage colony-stimulating factor, interleukin 1 and interleukin 2. When CD4- CD8- thymic cells are activated with granulocyte-macrophage colony-stimulating factor plus interleukin 2, both macrophages and interdigitating-like cells are present, as shown by electron microscopy. When activated with interleukin 1 plus interleukin 2, the interdigitating-like cell is the only accessory cell present. In both culture conditions, large clusters are formed between interdigitating cells and lymphoid cells. These results have led us to propose two-step signals for thymocyte proliferation: first, the maturation of macrophages under granulocyte-macrophage colony-stimulating factor control and the production of interleukin 1, and secondly, the maturation of interdigitating cells under interleukin 1 control, their clustering with thymocytes which are then activated.

Requirement of dendritic cells and B cells in the clonal deletion of Mls-reactive T cells in the thymus

The present study was performed to identify cells responsible for the elimination of T cells reactive with minor lymphocyte-stimulating (Mls) antigens during T cell development. Experiments were carried out in a fetal thymus organ culture (FTOC) system. To examine the tolerance-inducing activity, various populations of cells from adult CBA/J (Mls-1a) mice were injected into deoxyguanosine (dGuo)-treated FTOC of C3H/He (Mls-1b) mice with a microinjector, and 2 d later, the thymus lobes were injected with fetal thymus cells from C3H/He mice as T cell precursors. After 14 d of cultivation, cells were harvested and assayed for the expression of the T cell receptor V beta 6 element. The absence or marked reduction of T cells expressing V beta 6 at high levels (V beta 6high) was regarded as indicating the deletion of Mls-1a-reactive T cells. T cell-depleted populations of thymic as well as splenic cells from CBA/J mice were able to induce clonal deletion. Further characterization of the effector cells was carried out by fractionating the spleen cells before injecting them into dGuo-FTOC. None of the dish-adherent population, dish-nonadherent population, or purified B cells alone were able to induce clonal deletion, whereas the addition of purified B cells to adherent cells restored tolerance inducibility. It was further shown that a combination of CBA/J B cells and C3H/He dendritic cells was effective in eliminating Mls-reactive clones. These results indicate that for the deletion of clones reactive with Mls antigens during T cell development in the thymus, both DC and B cells are required.

Thymic dendritic cells: phenotype and function

Interdigitating (IDC) cells of the thymus have been characterized in situ by their ultrastructure and phenotype. Thymic dendritic cells (DC), thought to represent their in vitro correlate, resemble splenic DC in their ability to initiate peripheral T cell responses. In vivo, however, DC of the thymus have been implicated in tolerance induction, although at one time they were thought to impart MHC-restriction on developing T cells. Our present understanding of these areas is reviewed here. An in vitro model has been developed to address directly the function of DC in the thymus. Mature DC and immature thymocytes migrate into deoxyguanosine-treated thymus lobes where they adopt a reciprocal distribution, DC homing primarily to the medulla while the thymocytes remain in the cortex. These observations support the close relationship between thymic DC and IDC and provide a powerful tool to examine the role of DC in thymocyte ontogeny.

T lymphocyte populations in fetal alcohol syndrome

There are reports that fetal alcohol syndrome (FAS) is associated with immune deficiency or DiGeorge syndrome. To investigate the effect of prenatal alcohol exposure on the immune system, we used a mouse model of FAS in which C57BL/6J female mice were fed a complete liquid diet containing 25% ethanol-derived calories (EDC) from gestational day (g.d.) 1 to 18. Thymus cell numbers were markedly reduced in 18-day fetuses exposed to ethanol. Thymocytes from fetuses from the 25% EDC diet group and from pair-fed and ad-libitum control diet groups were compared by flow cytometry for expression of T cell differentiation antigens. The proportions of L3T4- and Lyt-2 positive thymus cells were significantly reduced in alcohol-exposed fetuses compared to controls; however, the number of Thy-1-positive cells did not differ among any of the groups. Six-day old neonates exposed prenatally to ethanol from g.d. 1 to 13 had thymus and spleen T cell populations similar to those of controls in almost all cases, indicating a "catch-up" of T cell numbers in most animals. Spleen T cell function, assessed by response to Concanavalin A (Con A), or Con A plus T cell growth factors, was somewhat depressed in ethanol-exposed 6-day pups.

Stromal cell types in the developing thymus of the normal and nude mouse embryo

The anatomical distribution of various nonlymphoid cell types in the embryonic mouse thymus in vivo and in vitro, as well as in the thymic rudiment of the nude mouse embryo, has been studied. For this purpose a panel of monoclonal antibodies, ER-TR3, 4, 5, 6 and 7, directed to various types of stromal cells of the mouse thymus, was used in combination with immunoperoxidase labeling on frozen sections. It was shown that as early as day 13 in thymic ontogeny distinction of TR4+ cortical epithelial cells and TR5+ medullary epithelial cells is possible. Thus, as far as stromal components are concerned, the thymus at day 13 in ontogeny is already subdivided into cortex and medulla. At day 13, Ia (TR3) was expressed in a focal pattern in the medulla subsequently appearing throughout both cortex and medulla by day 16. The thymic rudiment of the nude mouse embryo differs markedly from the normal embryonic thymus in its lack of demonstrable Ia antigen. Furthermore, TR4 and TR5 were only expressed on occasional epithelial cells lining the cysts of the nude thymus in a mutually exclusive fashion. The majority of stromal cells of the nude thymus, however, is negative for all ER-TR antibodies tested. In addition, we have shown that in organ cultures the organization of the stroma of thymic lobes remains intact, at least for a period of 11 days. Embryonic thymi cultured in the presence of deoxyguanosine, which causes depletion of lymphoid cells, also contain cortical and medullary areas as identified by the presence of TR3,4+ and TR5+ stromal cells. This indicates that the lack of organization in the nude thymus is not simply due to the absence of lymphoid cells.

Thymic non-lymphoid cells

In formulating this summary of our simon-pure knowledge of the structure/function relationships in the thymus, we decided that the time may have come to introduce a suitable dose of cynicism to balance the sometimes hopeless optimism of the past. Are the non-lymphoid cells of the thymus necessary for thymic function? Probably, but not to the extent or uniqueness that some authors including ourselves have previously claimed; T cells can probably differentiate in other tissues but may acquire their preference for MHC class II in the thymus. Mouse thymic lymphoid cell traffic and surface phenotype has recently been summarized pictorally by Scollay and Shortman [95]. Briefly stated, within the thymus, cells are hatched, matched and then dispatched. Minimally, the non-lymphoid cells act either as scenically varied obstacles along the way, nurseries for newborn T cells, or as tombstones for life's disenfranchized, effete and autoaggressive thymocytes. Hassall's corpuscles are morphological structures unique to the thymus, which are most useful to medical students for identification of this tissue. Their function remains one of life's great mysteries. Morphologically, they are suitable companions to the more recently described strange multicellular complexes of lymphocytes and epithelial cells which might be functionally important. The thymus of the much studied inbred, environmentally mollycoddled, laboratory mouse has been often and majestically described. It is probably typical for that of man and most mammals. It may, however, be unrepresentative of the thymus of stressed and parasitized wild animals. Diseases of the thymus generally can be categorized as not having enough thymus, having a neoplastic thymus or having a thymus which does not work properly. The bottom line in our knowledge of thymic nonlymphoid cells is that if you are born without them, you get sick and die; unless, of course, you are a nude mouse in Omaha, in which case you just freeze to death.