Cellular interactions in thymocyte development

T-cell-specific expression of kinase-defective Eph-family receptor protein, EphB6 in normal as well as transformed hematopoietic cells

Although most kinase-defective growth factor receptor proteins are associated with pathogenic conditions, a kinase-defective Eph-family receptor protein, EphB6, is expressed in normal human tissues. We generated monoclonal antibodies specific for human EphB6 to characterize its expression on human hematopoietic cells. A very small population of normal human peripheral white blood cells (0.57 +/- 0.07%, n = 12) expressed EphB6. The EphB6-positive cells were CD2+, CD7+, CD3+ and CD4+ or CD8+ lymphocytes, but they did not express CD19 or CD11b. In human bone marrow, only 1.5 +/- 0.19% of lymphocytes expressed EphB6. Compared with the expression in peripheral lymphocytes, prominent expression of EphB6 protein was demonstrated in CD4+CD8+ double-positive mouse thymocytes. The T-cell lineage-specific expression was strictly conserved in human leukemia/lymphoma cells. Among T-cell-derived leukemia cells, the expression level of EphB6 seemed to decrease with maturation of the cells. These results suggest that EphB6 expression is regulated in T-cell development.

Green fluorescent protein-transgenic mice: immune functions and their application to studies of lymphocyte development

Green fluorescent protein (GFP) transgenic (GFP+) mice express GFP in most tissues except erythrocytes and hair. Immune responses of GFP+ mouse and their application to studies of lymphocyte development were investigated. Flow cytometric analyses revealed that differentiation patterns of lymphocytes from GFP+ mice are equivalent to those from parental C57BL/6 mice. There was no difference in mature T-cell proliferative ability in response to allogeneic stimulator cells or anti-CD3epsilon stimulation between GFP+ and C57BL/6 mice. Furthermore, the anti-OVA antibody response of GFP+ mice was also the same as that of C57BL/6 mice. Taken together, these results show no immunological differences between GFP+ and C57BL/6 mice. Bone marrow transplantation and in vitro thymus reconstitution experiments were performed in an attempt to apply the GFP+ mice to the analysis of lymphocyte development. When bone marrow cells from GFP+ mice were transplanted. T and B lymphocytes containing GFP developed normally in scid recipients. Next we examined intrathymic T-cell development by hanging drop culture methods. GFP+ and CD4+8+ immature T-cells developed normally from bone marrow cells in the reconstituted thymus. The experimental system using hematopoietic cells from GFP+ mice is a powerful tool for visualizing lymphocyte development.

Age- and disease-related decline in immune function: an opportunity for "thymus-boosting" therapies

The thymus is the site of production of mature T lymphocytes and thus is indispensable for the development and maintenance of the T cell-mediated arm of the immune system. Thymic production of mature T cells is critically dependent on an influx of bone marrow-derived progenitor T cells that undergo replication and selection within the thymus. Thymus cellularity and thymic hormone secretion reach a peak during the first year of life and then decline gradually until the age of 50-60 years, a process known as "thymic involution." A rapid reduction of thymus cellularity occurs in young patients following injuries, chemotherapy, and other forms of stress. The mechanisms underlying the involution process appear to be dependent on factors intrinsic to the thymic tissue, such as the local production of cytokines and chemoattractants, promoting the recruitment, growth, and differentiation of bone marrow-derived T cell progenitors in the thymus, as well as extrinsic factors, such as systemic levels of endocrine hormones and mediators released by intrathymic nerves of the autonomic nervous system. Knowledge of these factors provides a rational basis for the development of an approach based on tissue engineering that could be used to provide either temporary or permanent reconstitution of thymic function.

Immunohistochemical study of tyrosine phosphorylation signaling in Hassall's corpuscles of the human thymus

Tyrosine phosphorylation signaling has been reported to play a key role in thymocyte development. However, the physiological role of signaling in thymus stroma is poorly understood, and there is lack of information on the in situ localization of elements of the signaling pathway in thymus stroma. In the present study, we have found by immunohistochemical analysis that tyrosine-phosphorylated proteins are present in high amounts in Hassall's corpuscles of the thymus medulla. Hassall's corpuscles represent end stages of maturation of thymic medullary epithelium. We have also investigated the localization of the src family that is involved in tyrosine phosphorylation signaling in Hassall's corpuscles. A member of the src family protein tyrosine kinases, p59fyn, was shown to be abundantly expressed in the outer layer of Hassall's corpuscles. Another member of the family, p60c-src, was highly expressed in the entire Hassall's corpuscles. Furthermore, p50csk and p130cas, both of which are involved in the pathway, were shown to be preferably expressed in the outer layer of Hassall's corpuscles. These findings suggest that tyrosine phosphorylation signaling may play a role in thymic medullary epithelial maturation and that the src family is involved in the process.

Estrogen Receptor α Is Necessary in Thymic Development and Estradiol-Induced Thymic Alterations

Estrogens affect the development, maturation, and function of multiple organ systems, including the immune system. One of the main targets of estrogens in the immune system is the thymus, which undergoes atrophy and phenotypic alterations when exposed to elevated levels of estrogen. To determine how estrogens influence the thymus and affect T cell development, estrogen receptor α (ERα) knockout (ERKO) mice were examined. ERKO mice have significantly smaller thymi than their wild-type (WT) littermates. Construction of ER radiation bone marrow chimeras indicated that the smaller thymi were due to a lack of ERα in radiation-resistant tissues rather than hemopoietic elements. ERKO mice were also susceptible to estradiol-induced thymic atrophy, but the extent of their atrophy was less than what was seen in WT mice. The estradiol-treated ERKO mice failed, however, to manifest alterations in their thymic CD4/CD8 phenotypes compared with WT mice. Therefore, ERα is essential in nonhemopoietic cells to obtain a full-sized thymus, and ERα also mediates some of the response of the thymus to elevated estrogen levels. Finally, these results suggest that in addition to ERα, another receptor pathway is involved in estradiol-induced thymic atrophy.

Human Immunodeficiency Virus nef Gene Expression Affects Generation and Function of Human T Cells, But Not Dendritic Cells

Human immunodeficiency virus (HIV)-infected individuals develop an acquired immune deficiency syndrome (AIDS) due to loss in their lymphocyte numbers and cellular defects in T cells and antigen-presenting cells (APC). HIV infection of the thymus results in deficient replenishment of the peripheral naive T-cell pool. The HIVnef gene was shown to be important for progression towards AIDS and cellular depletion of the infected thymus. Here, we demonstrate by retroviral gene transfer that nef expression, in the absence of other HIV genes, impaired human thymic T-cell development. Thymocytes were generated in reduced numbers and downmodulated CD4 and CD8β cell surface expression. T cells grown from nef-expressing thymocytes were hyperproliferative in vitro upon T-cell receptor triggering. Mature dendritic cells (DC) were functional and had normal surface CD4 levels despite nef expression. Thus, nefexpression alone may contribute to AIDS development by reduced T-cell generation and T-cell hyperresponsiveness.

Dendritic cells: the driving force behind autoimmunity in rheumatoid arthritis?

Dendritic cells (DC) are likely to play a significant role in immune-mediated diseases such as autoimmunity and allergy. To date there are few treatments capable of inducing permanent remission in rheumatoid arthritis (RA) and elucidation of the role of DC may provide specific strategies for disease intervention. Dendritic cells have proven to be powerful tools for immunotherapy and investigations are under way to determine their clinical efficacy in transplantation and viral and tumour immunotherapy. The present review will focus on the current view of DC and their role in autoimmunity, in particular RA. Two possible roles for DC in the pathogenesis of RA will be proposed, based on recent advances in the field.

Establishment of a human thymic myoid cell line. Phenotypic and functional characteristics

The subset of myoid cells is a normal component of the thymic stroma. To characterize these cells, we immortalized stromal cells from human thymus by using a plasmid vector encoding the SV40 T oncogene. Among the eight cell lines obtained, one had myoid characteristics including desmin and troponin antigens. This new line was designated MITC (myoid immortalized thymic cells). These cells expressed both the fetal and adult forms of muscle acetylcholine receptor (AChR) at the mRNA level, as well as the myogenic transcription factor MyoD1. alpha-Subunit AChR protein expression was detected by flow cytometry and the AChR was functional in patch-clamp studies. In addition, AChR expression was down-modulated by myasthenia gravis sera or by monoclonal antibody anti-AChR on MITC line similarly to TE671 rhabdomyosarcoma cells, making the MITC line an interesting tool for AChR antigenic modulation experiments. Finally, the MITC line expressed LFA-3, produced several cytokines able to act on T cells, and protected total thymocytes from spontaneous apoptosis in vitro. These results are compatible with a role of thymic myoid cells in some steps of thymocyte development. Therefore MITC line appears to be a useful tool to investigate the physiological role of thymic myoid cells.

A Novel Role for HEB Downstream or Parallel to the Pre-TCR Signaling Pathway During αβ Thymopoiesis

TCR gene rearrangement and expression are central to the development of clonal T lymphocytes. The pre-TCR complex provides the first signal instructing differentiation and proliferation events during the transition from CD4−CD8−TCR− double negative (DN) stage to CD4+CD8+ double positive (DP) stage. How the pre-TCR signal leads to downstream gene expression is not known. HeLa E-box binding protein (HEB), a basic helix-loop-helix transcription factor, is abundantly detected in thymocytes and is thought to regulate E-box sites present in many T cell-specific gene enhancers, including TCR-α, TCR-β, and CD4. Targeted disruption of HEB results in a 5- to 10-fold reduction in thymic cellularity that can be accounted for by a developmental block at the DN to DP stage transition. Specifically, a dramatic increase in the CD4low/−CD8+CD5lowHSA+TCRlow/− immature single positive population and a concomitant decrease in the subsequent DP population are observed. Adoptive transfer test shows that this defect is cell-autonomous and restricted to the αβ T cell lineage. Introduction of an αβ TCR transgene into the HEBko/ko background is not sufficient to rescue the developmental delay. In vivo CD3 cross-linking analysis of thymocytes indicates that TCR signaling pathway in the HEBko/ko mice appears intact. These findings suggest an essential function of HEB in early T cell development, downstream or parallel to the pre-TCR signaling pathway.

Macrophage-Derived Chemokine Is Localized to Thymic Medullary Epithelial Cells and Is a Chemoattractant for CD3+, CD4+, CD8low Thymocytes

Macrophage-derived chemokine (MDC) is a recently identified CC chemokine that is a potent chemoattractant for dendritic cells, natural killer (NK) cells, and the Th2 subset of peripheral blood T cells. In normal tissues, MDC mRNA is expressed principally in the thymus. Immunohistochemical analysis performed on 5 human postnatal thymuses showed high MDC immunoreactivity, which was selectively localized to epithelial cells within the medulla. To examine the effects of MDC on immature T cells, we have identified cDNA clones for mouse and rat MDC. Expression of MDC in murine tissues is also highly restricted, with significant levels of mRNA found only in the thymus. Thymocytes express high-affinity binding sites for MDC (kd = 0.7 nmol/L), and, in vitro, MDC is a chemoattractant for these cells. MDC-responsive murine thymocytes express mRNA for CCR4, a recently identified receptor for MDC. Phenotypic analysis of MDC-responsive cells shows that they are enriched for a subset of double-positive cells that express high levels of CD3 and CD4 and that have reduced levels of CD8. This subset of MDC-responsive cells is consistent with the observed expression of MDC within the medulla, because more mature cells are found there. MDC may therefore play a role in the migration of T-cell subsets during development within the thymus.

Roles of Integrins and CD44 on the Adhesion and Migration of Fetal Liver Cells to the Fetal Thymus

Adhesion and migration of mouse fetal liver (FL) cells to the thymus were investigated using cells from green fluorescent protein transgenic (GFP+) mice. FL cells from GFP+ embryos at 12 gestational days (E12) of mice were incubated with 2′-deoxyguanosine-treated fetal thymus lobe (from E14) by thymic repopulation (hanging drop) culture methods. GFP+ cells were observed in the thymus lobe at the end of the repopulation culture period. A large part of the infiltrated cells expressed CD44 until day 2 of culture on a permeable membrane, then lost the expression. CD25 expression was observed from day 1 to day 4. Around day 8, GFP+ cells became both CD4+ and CD8+. The results support the early observation of the sequential expression of CD44, CD25, and CD4/8 during the early stages of thymocyte development. When anti-CD44 mAb was added at the beginning of the repopulation culture period, GFP+ FL cells adhered to the surface of the thymus lobe but did not migrate into the thymus. Pretreatment of the thymus with hyaluronidase or hyaluronate produced results similar to the results of anti-CD44 treatment. On the other hand, the addition of anti-integrin α4 mAb inhibited adhesion to the thymus, and almost no GFP+ cells were seen on the surface of the thymus lobe. The data suggest that integrin α4 and CD44 play different roles, i.e., integrin α4 is required for the adhesion of FL cells to the thymus lobe and CD44 is required for the migration of the cells into the thymus.

Macrophage-Derived Chemokine Is Localized to Thymic Medullary Epithelial Cells and Is a Chemoattractant for CD3+, CD4+, CD8low Thymocytes

Macrophage-derived chemokine (MDC) is a recently identified CC chemokine that is a potent chemoattractant for dendritic cells, natural killer (NK) cells, and the Th2 subset of peripheral blood T cells. In normal tissues, MDC mRNA is expressed principally in the thymus. Immunohistochemical analysis performed on 5 human postnatal thymuses showed high MDC immunoreactivity, which was selectively localized to epithelial cells within the medulla. To examine the effects of MDC on immature T cells, we have identified cDNA clones for mouse and rat MDC. Expression of MDC in murine tissues is also highly restricted, with significant levels of mRNA found only in the thymus. Thymocytes express high-affinity binding sites for MDC (kd = 0.7 nmol/L), and, in vitro, MDC is a chemoattractant for these cells. MDC-responsive murine thymocytes express mRNA for CCR4, a recently identified receptor for MDC. Phenotypic analysis of MDC-responsive cells shows that they are enriched for a subset of double-positive cells that express high levels of CD3 and CD4 and that have reduced levels of CD8. This subset of MDC-responsive cells is consistent with the observed expression of MDC within the medulla, because more mature cells are found there. MDC may therefore play a role in the migration of T-cell subsets during development within the thymus.

Transplantation tolerance: the concept and its applicability

Recent advances have enabled researchers to induce tolerance in animal transplant models. Although it has been relatively easy to do so in rodents, it has been much more difficult to translate such strategies into primates. Understanding the cellular and molecular mechanisms of the alloimmune response has prompted the development of novel strategies that may obviate the need for immunosuppression in humans. Mechanisms of tolerance and promising new therapies, as well as the inherent difficulties in bringing them into clinical practice, are reviewed.

Control of early thymocyte development by the pre-T cell receptor complex: A receptor without a ligand?

Beta-selection refers to a developmental checkpoint linking thymocyte survival to the outcome of antigen receptor gene rearrangement. Immature thymocytes that productively rear-range the gene segments of the TCRbeta locus undergo proliferative expansion and mature to the CD4(+)CD8(+)stage; those failing to do so die by apoptosis. How are these precursor cells alerted that TCRbeta rearrangement has been productive? While it is clear that this process involves signals transduced by a surrogate form of the TCR termed the pre-TCR, it remains unclear how pre-TCR signals are triggered. In this review, we will discuss the implications of recent experimental attempts to address this issue, as well as how pre-TCR activation is linked to the changes in gene expression that underlie thymocyte development.

Signals involved in thymocyte positive and negative selection

Extensive research has focused upon understanding how thymocytes distinguish between interactions that lead to positive or negative selection. Various intracellular pathways that are activated after TCR engagement are outlined in this review, and their contribution to thymocyte selection is discussed. Although thymocyte fate is generally governed by a quantitative/avidity model, this largely reflects the interactions that occur at the cell surface. Therefore, we outline possible models of how different intercellular interactions are translated into intracellular signals that diverge and lead to thymocyte survival or death.

Molecular and cellular aspects of induced thymus development in recombinase-deficient mice

Thymus development and microenvironment organization require stage- and site-specific cross-talk between thymocyte and stroma. In this study we have used recombinase-activating gene-deficient (RAG-2(-/-)) mice to analyze regulated gene expression both in thymocytes and stromal cells following injection of anti-CD3 monoclonal antibodies as inducer of thymus development. We show that IFN-gamma, TNF-alpha and lymphotactin are transcriptionally regulated in thymocytes, whereas cytoskeletal keratin 14, IL-1alpha and TNF-alpha are regulated in the stroma, quantitatively reproducing the variations associated with beta selection of thymocytes. In addition, RAG-2(-/-) thymus development is associated with entry of epithelial cells into the cell cycle. The histochemical evidence that expanded RAG-2(-/-) thymus becomes undistinguishable from wild-type cortex further suggests that cross-talk phenomena occurring during beta selection of thymocyte are reproduced in this system.

Development of rat CD45+ 13-day-old fetal liver cells in SCID mouse fetal thymic organ cultures

A phenotypic analysis of the lympho-hemopoietic cells which occur in the liver of 13-day-old fetal rats was achieved by flow cytometry in an attempt to further characterize the rat lymphoid progenitor cells. A small fraction of rat 13-day-old fetal liver (r13FL) cells, which weakly expressed the leukocyte common antigen CD45, constituted a homogeneous Thy-1(hi), CD71(-), CD44(+), MHC class I+, CD43(+) cell subpopulation negative for CD45RC, CD3, TCRalphabeta, TCRgammadelta, CD2, CD5, CD4, CD8, CD25, CD28, NKR-P1a and sIg. On the contrary, the CD45(-) cells were a heterogeneous cell subset which expressed Thy-1, CD71 and CD44 at distinct levels. After MACS separation, the CD45(+) r13FL cells, but not the CD45(-) cell subset, in vitro repopulated 14-day-old SCID mouse fetal thymic lobes providing rat T cells, both TCRalphabeta and TCRgammadelta, NK cells, and thymic dendritic cells but not B lymphocytes. Interestingly, NKR-P1a(lo) TCRalphabeta+ or TCRgammadelta+ cells developed in the xenogeneic cultures, and a rare CD4(+)CD8(+) double-positive subpopulation among the TCRgammadelta-expressing cells accumulated in the oldest cultures. These results are discussed from the double perspective of the nature of the precursor cells which colonize the fetal thymus and the relevance of the xenogeneic SCID mouse fetal thymic microenvironment for supporting rat lymphopoiesis.

The regulation of CD4 and CD8 coreceptor gene expression during T cell development

The two major subsets of T lymphocytes in the peripheral immune system, the helper and cytotoxic T cells, are defined by their expression of either the CD4 or the CD8 glycoproteins, respectively. Expression of these molecules, which serve as coreceptors by interacting specifically with either MHC class II or class I molecules, also defines discrete stages of T cell development within the thymus. Thus, CD4+ and CD8+ single-positive (SP) thymocytes arise from common progenitor double positive (DP) cells that express both CD4 and CD8, during a process known as positive selection. The molecular mechanisms underlying the developmental choice toward the helper or cytotoxic lineage remain poorly understood. Because regulation of coreceptor gene expression appears to be coupled to the phenotypic choice of the differentiating T cell, it is likely that shared signaling pathways direct CD4 and CD8 transcription and the development of an uncommited DP thymocyte toward either the helper or cytotoxic lineage. Therefore, an understanding of how CD4 and CD8 expression is regulated will not only provide insights into transcriptional control mechanisms in T cells, but may also result in the identification of molecular factors that are involved in lineage choices during T cell development. In this review, we summarize recent progress that has been made toward an understanding of how CD4 and CD8 gene expression is regulated.

Selection of the T cell repertoire

Advances in gene technology have allowed the manipulation of molecular interactions that shape the T cell repertoire. Although recognized as fundamental aspects of T lymphocyte development, only recently have the mechanisms governing positive and negative selection been examined at a molecular level. Positive selection refers to the active process of rescuing MHC-restricted thymocytes from programmed cell death. Negative selection refers to the deletion or inactivation of potentially autoreactive thymocytes. This review focuses on interactions during thymocyte maturation that define the T cell repertoire, with an emphasis placed on current literature within this field.

In Vivo T-Lymphocyte Tolerance in the Absence of Thymic Clonal Deletion Mediated by Hematopoietic Cells

Thymic negative selection renders the developing T-cell repertoire tolerant to self-major histocompatability complex (MHC)/peptide ligands. The major mechanism of induction of self-tolerance is thought to be thymic clonal deletion, ie, the induction of apoptotic cell death in thymocytes expressing a self-reactive T-cell receptor. Consistent with this hypothesis, in mice deficient in thymic clonal deletion mediated by cells of hematopoietic origin, a twofold to threefold increased generation of mature thymocytes has been observed. Here we describe the analysis of the specificity of T lymphocytes developing in the absence of clonal deletion mediated by hematopoietic cells. In vitro, targets expressing syngeneic MHC were readily lysed by activated CD8+ T cells from deletion-deficient mice. However, proliferative responses of T cells from these mice on activation with syngeneic antigen presenting cells were rather poor. In vivo, deletion-deficient T cells were incapable of induction of lethal graft-versus-host disease in syngeneic hosts. These data indicate that in the absence of thymic deletion mediated by hematopoietic cells functional T-cell tolerance can be induced by nonhematopoietic cells in the thymus. Moreover, our results emphasize the redundancy in thymic negative selection mechanisms.

In Vivo T-Lymphocyte Tolerance in the Absence of Thymic Clonal Deletion Mediated by Hematopoietic Cells

Thymic negative selection renders the developing T-cell repertoire tolerant to self-major histocompatability complex (MHC)/peptide ligands. The major mechanism of induction of self-tolerance is thought to be thymic clonal deletion, ie, the induction of apoptotic cell death in thymocytes expressing a self-reactive T-cell receptor. Consistent with this hypothesis, in mice deficient in thymic clonal deletion mediated by cells of hematopoietic origin, a twofold to threefold increased generation of mature thymocytes has been observed. Here we describe the analysis of the specificity of T lymphocytes developing in the absence of clonal deletion mediated by hematopoietic cells. In vitro, targets expressing syngeneic MHC were readily lysed by activated CD8+ T cells from deletion-deficient mice. However, proliferative responses of T cells from these mice on activation with syngeneic antigen presenting cells were rather poor. In vivo, deletion-deficient T cells were incapable of induction of lethal graft-versus-host disease in syngeneic hosts. These data indicate that in the absence of thymic deletion mediated by hematopoietic cells functional T-cell tolerance can be induced by nonhematopoietic cells in the thymus. Moreover, our results emphasize the redundancy in thymic negative selection mechanisms.

The bHLH gene Hes1 is essential for expansion of early T cell precursors

Mice mutant for the bHLH gene Hes1, which is known to keep cells in a proliferative state, mostly lack thymus. Transfer of Hes1-null fetal liver cells into RAG2-null host mice normally reconstitutes B cells but fails to generate mature T cells in the thymus. In the reconstituted thymus, T cell differentiation is arrested at the CD4(-)CD8(-) double negative (DN) stage. Both the initial T cell receptor (TCR)-independent and the subsequent TCR-dependent selective expansion during the DN stage are severely affected. Thus, Hes1 is essential for the earliest thymocyte expansion in a cell-autonomous manner.

Cellular and matrix interactions during the development of T lymphocytes

The thymus contains an extensive extracellular matrix. Although thymocytes express integrins capable of binding to matrix molecules, the functional significance of the matrix for T cell development is uncertain. We have shown that the matrix is associated with thymic fibroblasts which are required for the CD44+ CD25+ stage of double negative (CD4-8-) thymocyte development. The survival of cells at this stage is dependent on IL-7 and we propose that the role of fibroblasts is to present, via the matrix, IL-7 to developing T cells.

Galectin-1, an alternative signal for T cell death, is increased in activated macrophages

Galectin-1 belongs to an evolutionarily conserved family of animal beta-galactoside-binding proteins, which exert their functions by crosslinking the oligosaccharides of specific glycoconjugate ligands. During the past decade, attempts to identify the functional role of galectin-1 suggested participation in the regulation of the immune response. Only in the last few years has the molecular mechanism involved in these properties been clearly elucidated, revealing a critical role for galectin-1 as an alternative signal in the generation of T cell death. In the present study we will discuss the latest advances in galectin research in the context of the regulation of the immune response, not only at the central level but also at the periphery. Moreover, we will review the purification, biochemical properties and functional significance of a novel galectin-1-like protein from activated rat macrophages, whose expression is differentially regulated according to the activation state of the cells. The novel role of a carbohydrate-binding protein in the regulation of apoptosis is providing a breakthrough in galectin research and extending the interface between immunology, glycobiology and clinical medicine.

Deficient T cell fate specification in mice with an induced inactivation of Notch1

Notch proteins are cell surface receptors that mediate developmental cell specification events. To explore the function of murine Notch1, an essential portion of the gene was flanked with loxP sites and inactivation induced via interferon-regulated Cre recombinase. Mice with a neonatally induced loss of Notch1 function were transiently growth retarded and had a severe deficiency in thymocyte development. Competitive repopulation of lethally irradiated wild-type hosts with wild-type- and Notch1-deficient bone marrow revealed a cell autonomous blockage in T cell development at an early stage, before expression of T cell lineage markers. Notch1-deficient bone marrow did, however, contribute normally to all other hematopoietic lineages. These findings suggest that Notch1 plays an obligatory and selective role in T cell lineage induction.

Differential Expression and Regulation of Cyclooxygenase Isozymes in Thymic Stromal Cells

Prostaglandins (PGs) are lipid-derived mediators of rapid and localized cellular responses. Given the role of PG in supporting thymic T cell development, we investigated the expression of the PG synthases, also known as cyclooxygenases (COX)-1 and -2, in the biosynthesis of PGs in thymic stromal cell lines. The predominant isozyme expressed in cortical thymic epithelial cells was COX-1, while COX-2 predominated in the medulla. IFN-γ up-regulated expression and activity of COX-2 in medullary cells, in which COX-2 was expressed constitutively. In contrast, IFN-γ down-regulated COX-1 activity, but not expression, in cortical cells. Stromal cells support T cell development in the thymus, although the mediators of this effect are unknown. Selective inhibition of COX-2, but not COX-1, blocked the adhesion of CD4+CD8+ and CD4+CD8− thymocytes to medullary cell lines. No effect of the inhibitors was observed on the interactions of thymocytes with cortical epithelial lines. These data further support the differential regulation of COX-1 and COX-2 expression and function in thymic stromal cells. PGs produced by COX-2 in the medullary thymic stroma may regulate the development of thymocytes by modulating their interaction with stromal cells.

Functional Fas Expression in Human Thymic Epithelial Cells

Fas, a cell surface receptor, can induce apoptosis after cross-linking with its ligand. We report that Fas antigen is constitutively expressed in medullary epithelial cells of the human thymus. Expression is decreased in cultured thymic epithelial cells (TEC), similarly to HLA-DR antigen. TEC are resistant to anti-Fas–induced apoptosis after 4 days of primary culture, and this resistance is reversed by concomitant addition of cycloheximide. Cycloheximide also downregulated the expression of Fas-associated phosphatase-1, which has been found to inhibit Fas-induced apoptosis. This phosphatase could be involved in the resistance to Fas-induced apoptosis observed on day 4 of TEC culture. When TEC were subcultured after 10 to 13 days of primary culture, exposure to interleukin-1-β, tumor necrosis factor-, and interferon-γ, alone or together, reinduced Fas mRNA and protein expression. In coculture with activated thymocytes, TEC also upregulated Fas protein expression. Cytokine-activated TEC became sensitive to apoptosis induced by an agonistic anti-Fas antibody. This apoptosis was inhibited by Z-VAD-fmk but not by Z-DEVD-fmk and DEVDase activity was slightly increased in Fas-stimulated TEC, suggesting that DEVDase activity is not sufficient to induce TEC apoptosis. Taken together, these data show that the Fas receptor is expressed in medullary epithelial cells of the human thymus and is able to induce apoptosis.

Functional Fas Expression in Human Thymic Epithelial Cells

Fas, a cell surface receptor, can induce apoptosis after cross-linking with its ligand. We report that Fas antigen is constitutively expressed in medullary epithelial cells of the human thymus. Expression is decreased in cultured thymic epithelial cells (TEC), similarly to HLA-DR antigen. TEC are resistant to anti-Fas–induced apoptosis after 4 days of primary culture, and this resistance is reversed by concomitant addition of cycloheximide. Cycloheximide also downregulated the expression of Fas-associated phosphatase-1, which has been found to inhibit Fas-induced apoptosis. This phosphatase could be involved in the resistance to Fas-induced apoptosis observed on day 4 of TEC culture. When TEC were subcultured after 10 to 13 days of primary culture, exposure to interleukin-1-β, tumor necrosis factor-, and interferon-γ, alone or together, reinduced Fas mRNA and protein expression. In coculture with activated thymocytes, TEC also upregulated Fas protein expression. Cytokine-activated TEC became sensitive to apoptosis induced by an agonistic anti-Fas antibody. This apoptosis was inhibited by Z-VAD-fmk but not by Z-DEVD-fmk and DEVDase activity was slightly increased in Fas-stimulated TEC, suggesting that DEVDase activity is not sufficient to induce TEC apoptosis. Taken together, these data show that the Fas receptor is expressed in medullary epithelial cells of the human thymus and is able to induce apoptosis.

Factors Regulating Stem Cell Recruitment to the Fetal Thymus

Colonization of the thymic rudiment during development is initiated before vascularization so that hemopoietic precursors must leave the pharyngeal vessels and migrate through the perithymic mesenchyme to reach the thymus, suggesting that they may be responding to a gradient of chemoattractant factors. We report that diffusible chemoattractants are produced by MHC class II+ epithelial cells of the fetal thymus, and that the response of precursors to these factors is mediated via a G protein-coupled receptor, consistent with factors being members of the chemokine family. Indeed, a number of chemokine receptors are expressed by thymic precursors, and several chemokines are also expressed by thymic epithelial cells. However, these chemokines are also expressed in a tissue that is unable to attract precursors, although the thymus expressed chemokine, TECK, is expressed at higher levels in thymic epithelial cells and we show that it has chemotactic activity for isolated thymic precursors. Neutralizing Ab to TECK, however, did not prevent thymus recolonization by T cell precursors, suggesting that other novel chemokines might be involved in this process. In addition, we provide evidence for the involvement of matrix metalloproteinases in chemoattractant-mediated T cell precursor recruitment to the thymus during embryogenesis.

Osteoprotegerin ligand: a common link between osteoclastogenesis, lymph node formation and lymphocyte development

The TNF-family molecule osteoprotegerin ligand (OPGL; also known as TRANCE, RANKL or ODF) has been identified as the osteoclast differentiation factor and a regulator of T cell-dendritic cell interactions in the immune system. Surprisingly, the same molecule was identified as a crucial factor in early lymphocyte development and lymph node organogenesis. We will discuss the role of OPGL in bone remodelling and the immune system.

Structure and promoter region of the surface membrane protein HS9 gene expressed on the thymic epithelial cells

The HS9 gene encoding a surface membrane protein is expressed in thymic epithelial cells. We have isolated the mouse HS9 gene and determined the sequence of all exons. The mouse HS9 gene is composed of 14 exons spanning approx. 31 kb. Primer extension analysis identified two transcription initiation sites 33 bp and 179 bp upstream from the ATG start codon. DNA sequence analysis of the 5'-flanking region of the first exon revealed a number of consensus binding sites for known transcription factors such as GC box, Sp1, NFkappaB, gamma-IRE. Neither typical TATA nor CCAAT boxes were found in this region. These results and the analysis of the luciferase activity showed that transcription of the HS9 gene is regulated at a TATA-less promoter.

Emergence of T Cell Progenitors Without B Cell or Myeloid Differentiation Potential at the Earliest Stage of Hematopoiesis in the Murine Fetal Liver

It has been unclear whether the progenitors colonizing the thymus are multipotent or T cell lineage restricted. We investigated the developmental potential of hematopoietic progenitors in various populations of liver and blood cells from day 12 fetuses using the recently established in vitro experimental system effective in determining the capability of individual progenitors to generate T, B, and myeloid cells. Multipotent progenitors (p-Multi) were exclusively found in the Sca-1 high-positive (Sca-1high) subpopulation of lineage marker (Lin)−c-kit+CD45+ fetal liver cells. Restriction of developmental capacity begins at the Sca-1high stage, and a large majority of progenitors in the Sca-1low or Sca-1− population are restricted to generate T, B, or myeloid cells. Such a lineage commitment or restriction taking place in the fetal liver is independent of the thymus, because no difference in the proportion of different types of progenitors were seen between nu/nu and nu/+ fetuses. T cell lineage-restricted progenitors (p-T) were abundant in the blood of day 12 fetuses, whereas p-Multi were undetectable. It was further shown that the p-Multi generated a large number of B and myeloid cells in the thymic lobe. These results strongly suggest that it is p-T but not p-Multi that migrate into the thymus.

GKLF in thymus epithelium as a developmentally regulated element of thymocyte-stroma cross-talk

Gut-enriched Krüppel-like factor (GKLF) is a transcriptional regulator expressed in differentiated epithelia. We identified GKLF transcript as a regulated element in thymic epithelium of recombinase-deficient mice during thymus development induced by anti-CD3 antibody injection. This treatment recapitulates the organogenetic process depending on productive rearrangement of T cell receptor (TCR) beta gene with thymocytes expansion and acquisition of the CD4+8+ double positive phenotype. In wildtype mice, GKLF is expressed very early in embryogenesis and becomes intensely up-regulated in thymus epithelium at day 18 of gestation when TCR beta expressing cells have selectively expanded and express both CD4 and CD8. The results presented here suggest that thymocytes may regulate GKLF transcriptionally in the cortical epithelium at the developmental check-point controlled by TCR beta gene rearrangement. Furthermore, GKLF expression in hematopoietic stroma might suggest the thus far uncharacterised participation of this factor in hematopoiesis.

Intrathymic function of the human cortical epithelial cell surface antigen gp200-MR6: single-chain antibodies to evolutionarily conserved determinants disrupt mouse thymus development

The mouse monoclonal antibody MR6 recognizes a 200 000 MW protein (gp200-MR6), which is expressed highly on human thymic cortical epithelial cells. The antigen is also expressed on some epithelial tumours and we have previously shown that MR6 inhibits the proliferation of the colon carcinoma cell lines HT29. However, the role of this molecule in the thymus is not known. In order to generate reagents that could be used in murine thymic functional studies we isolated antibodies specific to human gp200-MR6, using a phage display library expressing single-chain (sFv) antibodies. Three independent clones were isolated by panning with purified protein and their specificity was confirmed by immunohistochemistry, Western blotting and flow cytometry. In addition to human thymus, these phage antibodies also recognized the homologous antigen in mouse, pig and other species. Expressed as soluble sFv one of these clones inhibited the proliferation of HT29 cells and a mouse thymic epithelial cell line, suggesting that this antibody exhibits similar functional activity to MR6. In fetal thymic organ culture, thymocytes recovered from thymic lobes cultured in the presence of this sFv, were reduced in number fivefold compared with the control and the majority remained at the double-negative stage of development. These data indicate that gp200-MR6 plays an important role in thymocyte development. In addition, this is the first report to demonstrate that specific sFv can be used to study, and alter, thymic development. This work also highlights the advantage of phage antibody technology in selecting such reagents for functional assays.

OPGL is a key regulator of osteoclastogenesis, lymphocyte development and lymph-node organogenesis

The tumour-necrosis-factor-family molecule osteoprotegerin ligand (OPGL; also known as TRANCE, RANKL and ODF) has been identified as a potential osteoclast differentiation factor and regulator of interactions between T cells and dendritic cells in vitro. Mice with a disrupted opgl gene show severe osteopetrosis and a defect in tooth eruption, and completely lack osteoclasts as a result of an inability of osteoblasts to support osteoclastogenesis. Although dendritic cells appear normal, opgl-deficient mice exhibit defects in early differentiation of T and B lymphocytes. Surprisingly, opgl-deficient mice lack all lymph nodes but have normal splenic structure and Peyer's patches. Thus OPGL is a new regulator of lymph-node organogenesis and lymphocyte development and is an essential osteoclast differentiation factor in vivo.

Requirement for diverse, low-abundance peptides in positive selection of T cells

Whether a single major histocompatibility complex (MHC)-bound peptide can drive the positive selection of large numbers of T cells has been a controversial issue. A diverse population of self peptides was shown to be essential for the in vivo development of CD4 T cells. Mice in which all but 5 percent of MHC class II molecules were bound by a single peptide had wild-type numbers of CD4 T cells. However, when the diversity within this 5 percent was lost, CD4 T cell development was impaired. Blocking the major peptide-MHC complex in thymus organ culture had no effect on T cell development, indicating that positive selection occurred on the diverse peptides present at low levels. This requirement for peptide diversity indicates that the interaction between self peptides and T cell receptors during positive selection is highly specific.

Thymic epithelial cells synthesize a heparan sulfate with a highly sulfated region

Epithelial cells are important components of the thymus microenvironment and are involved in thymocyte differentiation. The production and secretion of sulfated glycosaminoglycans by these cells grown in culture were investigated using labeling with radioactive 35S-Na2SO4 and 3H-glucosamine. The major glycosaminoglycans synthesized by these cells are heparan sulfate and hyaluronic acid. The structure of the heparan sulfate was investigated by the pattern of degradation products formed by deaminative cleavage with nitrous acid. The ratio 35S-sulfate/ H-glucosamine is high in the segments of the heparan sulfate released during the deaminative cleavage with nitrous acid but low in the resistant portion of the molecule. Thus, the heparan sulfate synthesized by the thymic epithelial cells contains a highly sulfated region. Digestion with heparitinase reveals that this highly sulfated region is a heparin-like segment of the molecule. The heparan sulfate is rapidly incorporated into the cell surface but its secretion to the extracellular medium requires a longer incubation period. Finally, heparin was used to mimic the possible effect of this heparan sulfate with a highly sulfated region, as ascertained by its ability to modulate thymocyte adhesion to thymic epithelial cells. Since heparin actually enhanced thymocyte adhesion, it is suggested that the heparan sulfate described herein, secreted by the thymic epithelium, may play a role upon intrathymic heterotypic cellular interactions.

T Lymphocyte Development in the Absence of CD3ε or CD3γδεζ

CD3γ, δ, ε, and ζ proteins together with the pre-TCR α-chain (pTα) and a rearranged TCR β-chain assemble to form the pre-TCR that controls the double negative (DN) to double positive (DP) stages of thymopoiesis. The CD3 proteins are expressed before pTα and TCR β-chains in prothymocytes and are expressed intracellularly in precursor NK cells, suggesting that the CD3 complex may function independent of pTα and TCRβ. In this report, both the role of CD3ε exclusively, and the role of CD3 proteins collectively, in thymocyte and NK cell development were examined. In a mouse strain termed εΔP, a neomycin cassette inserted within the CD3ε promoter abolishes CD3ε and δ expression and also abolishes CD3γ expression in all but a small minority (≤1%) of prothymocytes. These prothymocytes became deficient in CD3ε alone upon reconstitution of CD3δ expression and were severely, but not completely, arrested at the DN stage, as small numbers of double positive thymocytes were detected. In de facto CD3γδεζnull mice generated by crossing the εΔP mice with CD3ζ−/− mice, thymopoiesis were arrested at the CD44−CD25+ DN stage as observed in RAG−/− mice, DJ and VDJ recombination at the TCRβ locus was functional, and normal numbers of NK cells were detected. Together, the findings demonstrate that during thymocyte development, the CD3 complex collectively is not essential until the critical CD44−CD25+ DN stage in which pre-TCR begins to function, whereas CD3ε is critical for the assembly of pre-TCR. Moreover, CD3 proteins are dispensable for NK cell development.

Immunoneuroendocrine connectivity: the paradigm of the thymus-hypothalamus/pituitary axis

It is now largely established that the immune and neuroendocrine systems cross-talk by using similar ligands and receptors. In this context, the thymus-hypothalamus/pituitary axis can be regarded as a paradigm of connectivity in both normal and pathological conditions. For example, cytokines and thymic hormones modulate hypothalamic-pituitary functions: (a) interleukin (IL)-1 seems to upregulate the production of corticotropin-releasing factor and by adrenocorticotropin by hypothalamic neurons and pituitary cells, respectively; (b) thymulin enhances LH secretion. Conversely, a great deal of data strongly indicate that the hypothalamic-pituitary axis plays a role in the control of thymus physiology. Growth hormone (GH) for example, enhances thymulin secretion by thymic epithelial cells (TEC), both in vivo and in vitro, also increasing extracellular matrix-mediated TEC/thymocyte interactions. Additionally, gap junction-mediated cell coupling among TEC is upregulated by ACTH. In a second vein, it was shown that GH injections in aging mice increased total thymocyte numbers and the percentage of CD3-bearing cells, as well concanavalin-A mitogenic response and IL-6 production. In addition to mutual effects, thymus-pituitary similarities for cytokine and hormone production have been demonstrated. Cytokines such as IL-1, IL-2, IL-6, interferon-gamma, transforming growth factor-beta and others can be produced by hypothalamic and/or pituitary cells. Conversely, hormones including GH, PRL, LH, oxytocin, vasopressin and somatostatin can be produced intrathymically. Moreover, receptors for various cytokines and hormones are expressed in both the thymus and the hypothalamus/pituitary axis. Lastly, it is noteworthy that a thymus-pituitary connectivity can also be seen under pathological situations. In this regard, an altered HPA axis has been reported in AIDS, human falciparum malaria and murine rabies, that also show a severe thymic atrophy.

Thymic expression of the pancreatic endocrine hormones

The thymus plays a central role in the selection of T lymphocytes that are tolerant to 'self' antigens and responsive to foreign pathogens. We and others have reported the expression of the pancreatic endocrine hormones, preproinsulin, proglucagon, prosomatostatin and propancreatic polypeptide in the human and mouse thymus. While mRNA expression is very low there is evidence for the presence of the translated product. In addition, we have investigated the cell types responsible for expression. In the thymus, hormone expression is enriched in the antigen-presenting cell population. Interestingly, while proglucagon, prosomatostatin and propancreatic polypeptide appear to be expressed in a macrophage population, preproinsulin expression was restricted to dendritic cells which are more potent antigen-presenting cells. The functional significance of the endogenous expression of insulin in the thymus has been indirectly investigated using transgenic models in which the transgene is introduced by the rat insulin promoter. The data suggest that thymic expression of the transgene is critical in the induction of T-cell tolerance to the transgene in the periphery. Taken together, the evidence suggests that the low-level pancreatic hormone expression in the thymus may be involved in central tolerance to proteins of restricted expression.

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.

Transcription of a Broad Range of Self-Antigens in Human Thymus Suggests a Role for Central Mechanisms in Tolerance Toward Peripheral Antigens

The role of the thymus in the induction of tolerance to peripheral antigens is not yet well defined. One impending question involves how the thymus can acquire the diversity of peripheral nonthymic self-Ags for the process of negative selection. To investigate whether peripheral Ags are synthesized in the thymus itself, we have determined the expression of a panel of circulating and cell-bound peripheral Ags, some of which are targets of autoimmune diseases, at the mRNA level in total thymic tissue and in its main cellular fractions. Normalized and calibrated RT-PCR experiments demonstrated the presence of transcripts of nonthymic self-Ags in human thymi from 8 days to 13-yr-old donors. Out of 12 glands, albumin transcripts were found in 12; insulin, glucagon, thyroid peroxidase, and glutamic acid decarboxylase (GAD)-67 in six, thyroglobulin in five, myelin basic protein and retinal S Ag in three, and GAD-65 in one. The levels of peripheral Ag transcripts detected were age-related but also showed marked interindividual differences. Cytokeratin-positive stromal epithelial cells, which are a likely cellular source for these, contained up to 200 transcript copies of the most expressed peripheral Ags per cell. These results implicate the human thymus in the expression of wide representation of peripheral self-Ags and support the view that the thymus is involved in the establishment of tolerance to peripheral Ags. The existence of such central mechanism of tolerance is crucial for the understanding of organ-specific autoimmune diseases.

Automated technique to evaluate thymocyte adhesion to thymic epithelial cells

Thymocyte adhesion to thymic epithelial cells is a relevant issue during intrathymic T-cell differentiation, and directly intervenes in the generation and expansion of the T-cell repertoire. In view of these data, it was apparent the usefulness of an automated strategy to evaluate the degree of thymic epithelial cell-thymocyte adhesion. This prompted us to develop an ELISA procedure (using an anti-Thy1 reagent) to determine the degree of thymocyte adhesion onto cultured thymic epithelial cells. The procedure described herein is simple, non-radioactive and reproducible. Additionally, it can potentially be applied to quantitate the degree of thymocyte adhesion to any cellular or non-cellular substrate (for example, extracellular matrix). Moreover, it detected fluctuations of thymocyte adhesion secondary to glucocorticoid treatment of epithelial cells. Thus, it can be regarded as a further tool to analyze intrathymic interactions.

γc Gene Transfer in the Presence of Stem Cell Factor, FLT-3L, Interleukin-7 (IL-7), IL-1, and IL-15 Cytokines Restores T-Cell Differentiation From γc(−) X-Linked Severe Combined Immunodeficiency Hematopoietic Progenitor Cells in Murine Fetal Thymic Organ Cultures

X-linked severe combined immunodeficiency (SCID-Xl) is a rare human inherited disorder in which early T and natural killer (NK) lymphocyte development is blocked. The genetic disorder results from mutations in the common γc chain that participates in several cytokine receptors including the interleukin-2 (IL-2), IL-4, IL-7, IL-9, and IL-15 receptors. We have shown in a previous report that γc gene transfer into SCID-Xl bone marrow (BM) cells restores efficient NK cell differentiation. In this study, we have focused on the introduction of the γc gene into SCID-Xl hematopoietic stem cells with the goal of obtaining differentiation into mature T cells. For this purpose, we used the in vitro hybrid fetal thymic organ culture (FTOC) system in which a combination of cytokines consisting of stem cell factor (SCF), Flt-3L, IL-7, IL-1, and IL-15 is added concomitantly. In this culture system, CD34+ marrow cells from two SCID-Xl patients were able to mature into double positive CD4+ CD8+ cells and to a lesser degree into CD4+ TCRβ+ single positive cells after retroviral-mediated γc gene transfer. In addition, examination of the output cell population at the TCR DJβ1 locus exhibited multiple rearrangements. These results indicate that restoration of the γc/JAK/STAT signaling pathway during the early developmental stages of thymocytes can correct the T-cell differentiation block in SCID-Xl hematopoietic progenitor cells and therefore establishes a basis for further clinical γc gene transfer studies.

Thymus-independent T-cell differentiation in vitro

The generation of large quantities of novel human T-cell clones ex vivo would make a wide range of gene- and immuno-therapies for tumours and viral infections possible. Several techniques have been described to generate, in vitro and in vivo (using xenogenic hosts), mature T cells from fetal-neonatal and adult human CD34+ cells. All these techniques are cumbersome and cannot be easily translated into clinical protocols because they involve co-cultivation of CD34+ cells with thymic fragments from either human or murine fetuses. We report that the mononuclear cells of human cord blood contain a cell population that supports the differentiation of CD34+ cells into CD4+ or CD8+ naive T cells in serum-deprived cultures stimulated with stem cell factor and interleukin 7. CD4+ or CD8+ CD45RA+ TCRalphabeta+ T cells were continuously produced in vitro over a period of 20 d under these conditions. The generation of T cells in these cultures was a dynamic process and clones of T cells expressing new T-cell receptor beta-chain rearrangements were generated over time. These results pave the way for the development of very simple culture conditions for ex-vivo production of naive helper or cytotoxic T cells which could be very useful for gene- and immuno-therapy of human diseases.

γc Gene Transfer in the Presence of Stem Cell Factor, FLT-3L, Interleukin-7 (IL-7), IL-1, and IL-15 Cytokines Restores T-Cell Differentiation From γc(−) X-Linked Severe Combined Immunodeficiency Hematopoietic Progenitor Cells in Murine Fetal Thymic Organ Cultures

X-linked severe combined immunodeficiency (SCID-Xl) is a rare human inherited disorder in which early T and natural killer (NK) lymphocyte development is blocked. The genetic disorder results from mutations in the common γc chain that participates in several cytokine receptors including the interleukin-2 (IL-2), IL-4, IL-7, IL-9, and IL-15 receptors. We have shown in a previous report that γc gene transfer into SCID-Xl bone marrow (BM) cells restores efficient NK cell differentiation. In this study, we have focused on the introduction of the γc gene into SCID-Xl hematopoietic stem cells with the goal of obtaining differentiation into mature T cells. For this purpose, we used the in vitro hybrid fetal thymic organ culture (FTOC) system in which a combination of cytokines consisting of stem cell factor (SCF), Flt-3L, IL-7, IL-1, and IL-15 is added concomitantly. In this culture system, CD34+ marrow cells from two SCID-Xl patients were able to mature into double positive CD4+ CD8+ cells and to a lesser degree into CD4+ TCRβ+ single positive cells after retroviral-mediated γc gene transfer. In addition, examination of the output cell population at the TCR DJβ1 locus exhibited multiple rearrangements. These results indicate that restoration of the γc/JAK/STAT signaling pathway during the early developmental stages of thymocytes can correct the T-cell differentiation block in SCID-Xl hematopoietic progenitor cells and therefore establishes a basis for further clinical γc gene transfer studies.

Repeated in vivo hydrocortisone treatment promotes a dual modulation of cytokeratin expression by mouse thymic epithelial cells

Previous studies showed that a single dose of hydrocortisone in mice was able to transiently upregulate the expression of cytokeratins (CKs) 3 and 10 by medullary epithelial cells of the mouse thymus. Herein we studied these cells (specifically recognized by immunocytochemistry with the anti-CK monoclonal antibody KL1) following a series of repeated injections of the glucocorticoid hormone. A progressive dual (up and down) modulation of KL1+ medullary epithelial cells was observed with a late appearance of KL1 immunoreactivity in the thymic cortex. The data indicate that a single versus repeated exposure to high doses of glucocorticoid hormone may trigger different circuits regulating intrathymic CK expression. Lastly, the model described herein may be regarded as promising in studies concerning the effect of repeated stress conditions upon the thymus.