Functional Fas Expression in Human Thymic Epithelial Cells

Comparative Analysis of Thymic and Blood Treg in Myasthenia Gravis: Thymic Epithelial Cells Contribute to Thymic Immunoregulatory Defects

The thymus is involved in autoimmune Myasthenia gravis (MG) associated with anti-acetylcholine (AChR) antibodies. In MG, thymic regulatory T cells (Treg) are not efficiently suppressive, and conventional T cells (Tconv) are resistant to suppression. To better understand the specific role of the thymus in MG, we compared the phenotype and function of peripheral and thymic Treg and Tconv from controls and MG patients. Suppression assays with thymic or peripheral CD4 + T cells showed that the functional impairment in MG was more pronounced in the thymus than in the periphery. Phenotypic analysis of Treg showed a significant reduction of resting and effector Treg in the thymus but not in the periphery of MG patients. CD31, a marker lost with excessive immunoreactivity, was significantly reduced in thymic but not blood resting Treg. These results suggest that an altered thymic environment may explain Treg differences between MG patients and controls. Since thymic epithelial cells (TECs) play a major role in the generation of Treg, we co-cultured healthy thymic CD4 + T cells with control or MG TECs and tested their suppressive function. Co-culture with MG TECs consistently hampers regulatory activity, as compared with control TECs, suggesting that MG TECs contribute to the immune regulation defects of MG CD4 + T cells. MG TECs produced significantly higher thymic stromal lymphopoietin (TSLP) than control TECs, and a neutralizing anti-TSLP antibody partially restored the suppressive capacity of Treg derived from co-cultures with MG TECs, suggesting that TSLP contributed to the defect of thymic Treg in MG patients. Finally, a co-culture of MG CD4 + T cells with control TECs restored numbers and function of MG Treg, demonstrating that a favorable environment could correct the immune regulation defects of T cells in MG. Altogether, our data suggest that the severe defect of thymic Treg is at least partially due to MG TECs that overproduce TSLP. The Treg defects could be corrected by replacing dysfunctional TECs by healthy TECs. These findings highlight the role of the tissue environment on the immune regulation.

Cultured Human Thymic-Derived Cells Display Medullary Thymic Epithelial Cell Phenotype and Functionality

Thymic epithelial cells are one of the main components of the thymic microenvironment required for T-cell development. In this work, we describe an efficient method free of enzymatic and Facs-sorted methods to culture human medullary thymic epithelial cells without affecting the cell phenotypic, physiologic and functional features. Human medulla thymic epithelial cells (mTECs) are obtained by culturing thymic biopsies explants. After 7 days of primo-culture, mTECs keep their ability to express key molecules involved in immune tolerance processes such as autoimmune regulator, tissue-specific antigens, chemokines, and cytokines. In addition, the cells sensor their cultured environment and consequently adjust their gene expression network. Therefore, we describe and provide a human mTEC model that may be used to test the effect of various molecules on thymic epithelial cell homeostasis and physiology. This method should allow the investigations of the specificities and the knowledge of human mTECs in normal or pathological conditions and therefore discontinue the extrapolations done on the murine models.

Expression of cell cycle and apoptosis regulators in thymus and thymic epithelial tumors

The human thymus supports the production of self-tolerant T cells with competent and regulatory functions. Various cellular components of the thymic microenvironment such as thymic epithelial cells (TEC) and dendritic cells play essential roles in thymic T cell differentiation. The multiple cellular events occurring during thymic T cell and TEC differentiation involve proteins regulating cell cycle and apoptosis. Dysregulation of the cell cycle and apoptosis networks is involved in the pathogenesis of thymic epithelial tumors (TET) which are divided into two broad categories, thymomas and thymic carcinomas. The present review focuses on the usefulness of the analysis of the expression patterns of major cell cycle and apoptosis regulators in order to gain insight in the histophysiology of thymus and the histopathology, the clinical behavior and the biology of TET.

Human thymus medullary epithelial cells promote regulatory T-cell generation by stimulating interleukin-2 production via ICOS ligand

Natural thymic T regulatory (tTreg) cells maintain tolerance to self-antigen. These cells are generated in the thymus, but how this generation occurs is still controversial. Furthermore, the contribution of thymus epithelial cells to this process is still unclear, especially in humans. Using an exceptional panel of human thymic samples, we demonstrated that medullary thymus epithelial cells (mTECs) promote the generation of tTreg cells and favor their function. These effects were mediated through soluble factors and were mTEC specific since other cell types had no such effect. By evaluating the effects of mTECs on the absolute number of Treg cells and their state of proliferation or cell death, we conclude that mTECs promote the proliferation of newly generated CD25+ cells from CD4+CD25- cells and protect Treg cells from cell death. This observation implicates Bcl-2 and mitochondrial membrane potential changes, indicating that the intrinsic cell death pathway is involved in Treg protection by mTECs. Interestingly, when the mTECs were cultured directly with purified Treg cells, they were able to promote their phenotype but not their expansion, suggesting that CD4+CD25- cells have a role in the expansion process. To explore the mechanisms involved, several neutralizing antibodies were tested. The effects of mTECs on Treg cells were essentially due to interleukin (IL)-2 overproduction by thymus CD4+ T cells. We then searched for a soluble factor produced by mTECs able to increase IL-2 production by CD4+ cells and could identify the inducible T-cell costimulator ligand (ICOSL). Our data strongly suggest a « ménage à trois »: mTEC cells (via ICOSL) induce overproduction of IL-2 by CD25- T cells leading to the expansion of tTreg cells. Altogether, these results demonstrate for the first time a role of mTECs in promoting Treg cell expansion in the human thymus and implicate IL-2 and ICOSL in this process.

Immunohistological analysis of cell cycle and apoptosis regulators in thymus

The combined expression patterns of cell cycle and apoptosis regulators have not been analyzed in details in human thymus to the best of our knowledge. Our objective was to provide multiparametric and combined immunohistological information regarding the expression levels and the topographical distribution of major cell cycle and apoptosis regulators in postnatal human thymus. Ki67 and cyclins A, B1, D3 and E were frequently expressed by thymocytes with higher expression in cortical than medullary thymocytes. The expression of cyclin D2 was low in thymocytes. Thymic epithelial cells (TEC) exhibited low expression of Ki67 and cyclins. Bid was frequently expressed by thymocytes, Bcl-xL by cortical thymocytes and Bcl-2 by medullary thymocytes. The expression levels of Bim and survivin in thymocytes were low. The expression levels of Bax and Mcl-1 were higher in medullary than cortical thymocytes and TEC. Bak and Bad were mainly expressed in medullary TEC and Hassall Bodies (HB). c-FLIP and Fas were frequently expressed in TEC and FasL was mainly expressed by medullary TEC and HB. Cleaved caspase-3 was expressed by scattered thymocytes at the cortex and the corticomedullary junction and very rarely at the medulla. The different expression profiles and immunotopographical distribution of cell cycle and apoptosis regulators in thymocytes and TEC indicate that their expression is tightly regulated during thymic cell differentiation and that they are differentially involved in the cell survival/death regulation of thymocytes and TEC. Furthermore, this study indicates decrease of the proliferation and caspase-dependent apoptosis of thymocytes from the cortex to the medulla.

Functional analysis of neuropeptides in avian thymocyte development

The function of lymphoid organs and immune cells is often modulated by peptides and hormones produced by the neuroendocrine and immune systems. We have previously reported the intrathymic expression of neuropeptides in the thymus of different species and that neuropeptides can influence murine thymocyte development in vitro. To further explore the evolutionary nature of neuroendocrine interactions in the thymus, we identified the expression of calcitonin-gene-related peptide, neuropeptide Y, somatostatin (SOM), substance P and vasointestinal polypeptide, as well as their receptors on chicken thymic epithelial cells (TEC) and thymocytes by immunofluorescence and reverse transcription polymerase chain reaction (RT-PCR). All the studied neuropeptides and their receptors were found to be expressed in both TEC and thymocytes, suggesting that intrathymic neuroendocrine interactions may take place within the avian thymus. In order to elucidate whether such interactions play a role in avian thymocyte development, neuropeptides and their antagonists were added to embryonic thymus organ cultures and found to influence chicken thymopoiesis. In particular, an antagonist of SOM increased the proportion of double-positive thymocytes, while SOM itself appeared to inhibit the early stages of thymocyte development. Taken together, these data provide further evidence to suggest that neuropeptides play a conserved role in vertebrate thymocyte development.

Differential estrogen receptor expression in autoimmune myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease associated with thymic hyperplasia and is much more prevalent in women than men. In this study we investigated potential changes in estrogen receptor (ER) expression in thymic hyperplasia. We first quantified by real-time PCR the relative expression of ER alpha and ER beta in normal thymus and found that the ER beta to ER alpha ratio was inverted in thymocytes (8.6 +/- 1.2), compared with thymic epithelial cells (0.18 +/- 0.05). The ER transcript number gradually decreased in thymic epithelial cells during culture, indicating that the thymic environment influences ER expression. CD4+ helper T cells expressed higher level of ERs, compared with CD8+ cells, as assessed by flow cytometry in thymocytes and peripheral blood mononuclear cells. In MG patients, we found an increased expression of ER alpha on thymocytes and both ERs on T cells from peripheral blood mononuclear cells, indicating that the signals provided by thymic and peripheral microenvironments are distinct. Finally, activation of normal thymocytes by proinflammatory cytokines induced increased expression of ERs especially in the CD4+ subset, suggesting that an excess of proinflammatory cytokines could explain the increase of ERs expression on MG lymphocytes. The dysregulation of ER expression in MG lymphocytes could affect the maintenance of the homeostatic conditions and might influence the progression of the autoimmune response.

Expression of FAP-1 by human colon adenocarcinoma: implication for resistance against Fas-mediated apoptosis in cancer

Although colon carcinoma cells express Fas receptors, they are resistant to Fas-mediated apoptosis. Defects within the intracellular Fas signal transduction may be responsible. We investigated whether the Fas-associated phosphatase-1 (FAP-1), an inhibitor of Fas signal transduction, contributed to this resistance in colon carcinomas. In vivo, apoptosis of cancer cells was detected in situ using terminal deoxynucleotidyltransferase-mediated dUTP nick-end labelling (TUNEL). FAP-1, FasR, and Fas ligand (FasL) were detected using immunohistochemistry. In vitro, colon carcinoma cells were primarily cultured, and their sensitivity to Fas-mediated apoptosis was evaluated by treatment with agonistic anti-FasR CH11 IgM monoclonal antibody in the presence or absence of synthetic Ac-SLV (serine-leucine-valine) tripeptide. Fas-associated phosphatase-1 expression was detected in 20 out of 28 colon adenocarcinomas. In vivo, a positive correlation between the percentage of apoptotic tumour cells and the number of FasL-positive tumour infiltrating lymphocytes was observed in FAP-1 negative cancers, but not in FAP-1-positive ones. Primarily cultured colon cancer cells, which were refractory to CH-11-induced apoptosis, had higher expression of FAP-1 on protein and mRNA levels than the sensitive group. Resistance to Fas-mediated apoptosis in tumour cells could be abolished by Ac-SLV tripetides. Fas-associated phosphatase-1 expression protects colon cancer cells from Fas-mediated apoptosis, and blockade of FAP-1 and FasR interaction sensitises tumour cells to Fas-dependent apoptosis.

Fas and perforin are not required for thymus atrophy induced by Trypanosoma cruzi infection

In the acute phase of Trypanosoma cruzi infection there is a prominent thymus atrophy, which is determined by massive loss of immature CD4/CD8 double positive cells. Recently, the involvement of a parasite transialidase, which is shed from the parasite cell membrane and the activation of P2X(7), a purinergic receptor, were stated as important pathways leading to thymus atrophy. In this work we evaluated the possible involvement of Fas- and perforin-based cytotoxic pathways in the thymus atrophy induced by T. cruzi infection using gld/gld and perforin (-/-) mice. We found similar kinetics of thymus atrophy in mice competent or deficient in both cytotoxic pathways, indicating that both molecules are not directly involved in the thymus atrophy, either inducing cellular death or as co-stimulatory molecules.

Expression of apoptosis-related antigens, Fas, bcl-2, and p53, and Mib-1 proliferation index in the hypoplastic thymus of DiGeorge syndrome

Apoptosis, along with cellular proliferation, plays a major role in normal developmental processes and tissue homeostasis. We hypothesized that altered apoptosis-related pathways and/or reduced cell proliferation might play a role in the thymic hypoplasia or aplasia in DiGeorge syndrome (DG). We used immunohistochemistry to evaluate the apoptosis-related antigens Fas (CD95), bcl-2, and p53, as well as Mib-1 proliferation index in the thymuses from six patients with DG. The results were compared with those from the thymuses from six patients with non-DG congenital heart disease. All DG patients (age 32 weeks GA to 4 months) had hypoplastic thymuses ranging from microscopic foci to 2.7 g in weight (expected for age, 4.7 +/- 3.6 g to 10 +/- 6 g). The thymic weights from the patients with non-DG congenital heart disease (age 37 weeks GA to 1 month) ranged from 3 to 5.6 g and were at the lower range of expected weight by age (expected for age, 8.4 +/- 5.6 g to 12 +/- 7 g). All thymuses showed histologic features of stress-induced involution. In both groups, a - 50% Mib-1 proliferation index was found in the cortical thymocytes, whereas <5% Mib-1 labeling was seen in the medullary thymocytes; Fas stained medullary epithelial cells (3+) and cortical epithelial cells (1+); bcl-2 stained medullary thymocytes (3+) and cortical thymocytes (1+); p53 stained less than 1% of nuclei in all sections. No significantly altered Mib-1 proliferation index or expression of Fas, bcl-2, and p53 was observed in the hypoplastic thymuses in DG, compared to these same measures in non-DG. These results suggest that thymic hypoplasia in DG may be mediated by mechanisms other than reduced cellular proliferation and/or altered Fas, bcl-2, and p53 apoptotic pathways.

Cell surface trafficking of Fas in NIT-1 cells and dissection of surface and total Fas expression

The appearance of Fas receptor at the surface of pancreatic beta-cells affected by progressive insulitis strongly suggests that Fas-mediated beta-cell apoptosis plays an important role in the pathogenesis of type 1 diabetes. In support of this concept, the present study has shown that islet cells from NOD mice and the beta-cell line NIT-1 respond to the proinflammatory cytokines IL-1beta and IFN-gamma with Fas surface expression in a dose- and time-dependent manner. Moreover, the prevention of cytokine-induced surface Fas expression by actinomycin D, cycloheximide, and brefeldin A demonstrated that trafficking of Fas to the beta-cell surface requires RNA and protein synthesis and, in addition is critically dependent on intracellular protein transport. Compared with total cellular Fas protein, the amount of Fas at the cell surface was relatively small and indicated that Fas is preferentially expressed in cytoplasmic compartments of NIT-1 cells. It is concluded that inflammatory insults specifically induce translocation of Fas to the beta-cell surface and that interference with cell surface Fas expression is a new strategy to improve beta-cell survival in inflamed islets.

Matrix attachment regulates Fas-induced apoptosis in endothelial cells: a role for c-flip and implications for anoikis

Survival of endothelial cells is critical for cellular processes such as angiogenesis. Cell attachment to extracellular matrix inhibits apoptosis in endothelial cells both in vitro and in vivo, but the molecular mechanisms underlying matrix-induced survival signals or detachment-induced apoptotic signals are unknown. We demonstrate here that matrix attachment is an efficient regulator of Fas-mediated apoptosis in endothelial cells. Thus, matrix attachment protects cells from Fas-induced apoptosis, whereas matrix detachment results in susceptibility to Fas-mediated cell death. Matrix attachment modulates Fas-mediated apoptosis at two different levels: by regulating the expression level of Fas, and by regulating the expression level of c-Flip, an endogenous antagonist of caspase-8. The extracellular signal-regulated kinase (Erk) cascade functions as a survival pathway in adherent cells by regulating c-Flip expression. We further show that detachment-induced cell death, or anoikis, itself results from activation of the Fas pathway by its ligand, Fas-L. Fas-L/Fas interaction, Fas-FADD complex formation, and caspase-8 activation precede the bulk of anoikis in endothelial cells, and inhibition of any of these events blocks anoikis. These studies identify matrix attachment as a survival factor against death receptor-mediated apoptosis and provide a molecular mechanism for anoikis and previously observed Fas resistance in endothelial cells.

Modulation of HLA-G expression in human thymic and amniotic epithelial cells

Expression of the nonclassical HLA class I antigen, HLA-G, is tightly regulated. HLA-G physiologic expression is mostly restricted to some placental and thymic cell types. Only few established cell lines express HLA-G in vitro. Cytokine-induced expression of HLA-G is hardly observed and also depends on the cell lineage. We assessed expression and cytokine regulation of HLA-G in primary cultures derived from human thymus and amnion epithelial cells, which also express HLA-G in vivo. We show that HLA-G cell surface expression is maintained, but decreases gradually, in primary cultures derived from human thymus and amnion epithelial cells. We also show that IFN-gamma re-induces HLA-G cell surface expression and upregulates classical class I gene expression in both primary cultures and in a thymus derived cell line. We further show that IFN-gamma also upregulates levels of HLA-G transcripts in TEC primary cultures. This study provides evidence that IFN-gamma induction of HLA-G expression occurs in the human amnion and the thymus, and is mediated at the transcriptional level in these tissues. These results also suggest a role for the microenvironment in regulating HLA-G in vivo gene expression in the thymus and amnion membrane.