Thymic nurse cells: possible sites of T-cell selection

Thymic Nurse Cells Participate in Heterotypic Internalization and Repertoire Selection of Immature Thymocytes; Their Removal from the Thymus of Autoimmune Animals May be Important to Disease Etiology

Thymic nurse cells (TNCs) are specialized epithelial cells that reside in the thymic cortex. The initial report of their discovery in 1980 showed TNCs to contain up to 200 thymocytes within specialized vacuoles in their cytoplasm. Much has been reported since that time to determine the function of this heterotypic internalization event that exists between TNCs and developing thymocytes. In this review, we discuss the literature reported that describes the internalization event and the role TNCs play during T cell development in the thymus as well as why these multicellular complexes may be important in inhibiting the development of autoimmune diseases.

Pathogenesis of some neurological immune ultrastructural and morphometrical observations on rat thymus

Numerous studies on neuro-immuno-modulation indicate that the thymus is involved in many neurological diseases, including experimental allergic encephalomyelitis (EAE). Twenty Lewis rats were induced for EAE. At X, XII, XX and XXX days post-inoculation the animals were killed, and the thymus was recovered and harvested. Specimens of thymus were submitted to morphological light microscopy analysis (1% toluidine blue) and ultra-structural analysis (transmission electron microscopy). Significant morphometric data were collected by examining the images quantitatively and by statistically analysing the values. Our results show that the microenvironment of the thymus is severally involved in acute EAE. Thymocytes and reticular epithelial cells show many changes which are closely related to the pathogenesis of EAE. In particular we observed: (1) inside the cell an increase in intra-cytoplasmic vacuoles, and changes in the thickness of the nuclear membrane, mitochondria, rough endoplasmic reticulum, cellular inter-digitations and cellular electron-density; (2) outside the cell an increase in pericellular translucent halo, intercellular spaces, intercellular contacts and apoptotic and necrotic figures. The evidence of a thymic role in MS may suggest the intriguing therapeutic concept of thymectomy in the management of this neurological disease.

Apoptosis and the thymic microenvironment in murine lupus

The thymus of New Zealand black (NZB) mice undergoes premature involution. In addition, cultured thymic epithelial cells from NZB mice undergo accelerated preprogrammed degeneration. NZB mice also have distinctive and well-defined abnormalities of thymic architecture involving stromal cells, defined by staining with monoclonal antibodies specific for the thymic microenvironment. We took advantage of these findings, as well as our large panel of monoclonal antibodies which recognize thymic stroma, to study the induction of apoptosis in the thymus of murine lupus and including changes of epithelial architecture. We studied NZB, MRL/lpr, BXSB/Yaa, C3H/gld mice and BALB/c and C57BL/6 as control mice. Apoptosis was studied both at basal levels and following induction with either dexamethasone or lipopolysaccharide (LPS). The apoptotic cells were primarily found in the thymic cortex, and the frequency of apoptosis in murine lupus was less than 20% of controls. Moreover, all strains of murine lupus had severe abnormalities of the cortical network. These changes were not accentuated by dexamethasone treatment in cultured thymocytes. However, the thymus in murine lupus was less susceptible to LPS-induced apoptosis than control mice. Finally we note that the number of thymic nurse cells (TNC) was lowest in NZB mice. Our findings demonstrate significant abnormalities in the induction of apoptosis and the formation of TNC-like epithelial cells in SLE mice, and suggest that the abnormalities of the thymic microenvironment have an important role in the pathogenesis of murine lupus.

The point of no return: mitochondria, caspases, and the commitment to cell death

Apoptosis is a specialized mode of cell death finely regulated at the molecular level and conserved throughout evolution. In many instances during normal development or in order to maintain the homeostasis of a multicellular organism, a strategic intracellular program is initiated ensuring the fate of unwanted cells. Interference with this program has been implicated in many pathologies, particularly in cancer and autoimmune diseases. What is most important, from the organism's point of view, is that the dismissal of the outcast cells is accomplished serenely, i.e., the dying cells resign their existence without causing an inflammatory reaction. Therefore, the ability to manipulate the cell death machinery is an obvious goal of medical research. Here, we debate the idea of the point-of-no-return and propose models for the role of "initiator" and "executioner" caspases in the death program. We argue that, in many circumstances, the cells are committed to die before the execution phase of apoptosis starts. This commitment event is coordinated by the mitochondria and can be blocked by anti-apoptotic oncogenes.

A positive role for thymus-derived steroids in formation of the T-cell repertoire

T cells undergo rigorous selection processes in the thymus that are necessary to prevent T cells with either autoreactive or nonfunctional T-cell receptors (TCRs) from entering the periphery. Although both positive and negative selection depend on TCR-mediated signals, the means by which a thymocyte interprets these signals to result in survival or death is not understood. Glucocorticoids are known to induce thymocyte apoptosis at high concentrations, but at lower concentrations glucocorticoids can antagonize TCR-mediated deletional signals and allow survival of thymocytes and T cell hybridomas. Interestingly, transgenic mice in which the expression of the glucocorticoid receptor has been downmodulated specifically in thymocytes have abnormal thymocyte differentiation, indicating that glucocorticoids play a significant role in T-cell development. Furthermore, we have demonstrated the presence of steroidogenic enzymes in the thymic epithelium and can show that, in vitro, these cells readily synthesize pregnenolone, the first product in the steroidogenic pathway, and deoxycorticosterone. Inhibition of local glucocorticoid biosynthesis in thymi from TCR transgenic mice during fetal thymic organ culture (FTOC) revealed significant alterations in the process of thymocyte selection. These data suggest that glucocorticoids do not simply suppress the immune system but rather are necessary for thymocyte survival and differentiation.

Heterogeneity of epithelial cells in the rat thymus

The morphological heterogeneity of the thymic epithelium has been well documented both at the light and electron microscopic level. Immunohistochemistry has revealed four broad classes of epithelial cells (EC): subcapsule/perivascular, cortical, medullary EC, and medullary Hassall's corpuscles. Ultrastructural analysis has revealed further heterogeneity. In the cortex, four EC subtypes have been described ultrastructurally: subcapsular/perivascular, "pale," "intermediate," and "dark" EC. These subtypes are also present in the medulla. Two additional EC subtypes are restricted to the medulla: an undifferentiated subtype, and a subtype displaying signs of high metabolic activity. Based on the morphological features of the epithelium, it has been hypothetized that the thymic EC subtypes represent a process of differentiation.

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

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

The application of in vitro isolation, cryopreservation and patch-clamp microelectrode recording methods to adult rat thymic nurse cells

Characterisation of the ionic mechanisms possessed by immune cells has begun to reveal a range of transmembrane ion channel properties which may have immunological significance. Since thymic epithelial cells appear to influence selection of the T cell repertoire, understanding their membrane physiology may be of importance. A method is therefore outlined for the targeting of patch-clamp electrophysiological measurements to acutely isolated, fresh and cryopreserved adult rat thymic nurse cells. These cells represent a discrete and predominantly cortical population of epithelium. Cells were separated by enzymatic and mechanical dispersal of thymus, enriched by sedimentation and identified on the basis of their characteristic lympho-epithelial, multi-cellular morphologies. Phase-bright cells retaining this anatomical form survived freezing and the voltage-gated conductances in such cells are described. The merits of this approach include the preparation and storage of mature differentiated phenotypes of immune cells for in vitro studies. Their preservation permits temporal separation of experiments, correlative experimentation on identical samples, optimises cell preparation in terms of cost and labour, and reduces animal and tissue requirements.

Chicken thymic nurse cells: an overview

Thymic nurse cells are multicellular complexes located in the subcortical area of the thymus of all avian, mammalian and amphibian species investigated so far. Since their first description in 1980 many studies have been carried out to characterize their morphological and functional properties. The purpose of this review is to summarize recent morphological as well a functional analyses of chicken thymic nurse cells which suggest a role of these cell complexes in T cell selection.

The ER-TR4 monoclonal antibody recognizes murine thymic epithelial cells (type 1) and inhibits their capacity to interact with immature thymocytes: immuno-electron microscopic and functional studies

The thymic stroma is heterogeneous with regard to cellular morphology and cellular function. In this study, we employed the monoclonal antibody ER-TR4 to characterize stromal cells at the ultrastructural level. To identify the labelled cell type, we used two techniques: immunogold labelling on ultrathin frozen sections and immunoperoxidase staining on thick "vibratome" sections. ER-TR4 reacted with thymic Type 1 epithelial cells (according to our classification). A dense labelling appears in the cytoplasm of cortical cells using the two techniques. Immunogold labelling identified small cytoplasmic vesicles whereas the cytoplasm and the cell membrane seem to be labelled with the immunoperoxidase technique. ER-TR4 also identified isolated thymic nurse cells (TNC), and was observed in vitro to inhibit the capacity of some type 1 epithelial cells to establish interactions with immature thymocytes. This finding supports the hypothesis that the factor is involved in the formation of lymphoepithelial interactions within thymic nurse cells, and thus in the relations that immature thymocytes establish with the thymic microenvironment.

Ultrastructural analysis of thymic nurse cell epithelium

Thymic nurse cells (TNC), a paradigmatic cell type of cortical epithelium, are large lymphoid-epithelial cell complexes of thymocytes enclosed within vacuoles lined by the epithelial cell membrane. TNC express major histocompatibility complex (MHC) class I and class II molecules on their surface and vacuole-lining membranes at high density and it was suggested that TNC provide an optimal microenvironment for positive selection of T cells. In this report we present electron microscopical data demonstrating that chicken TNC display morphological structures of exocytosis previously shown for hormone-secreting cells. In TNC, however, exocytosis is restricted to the capillary cleft between the epithelial cell and engulfed thymocytes. Thus, besides physical contact between the epithelial cell and enclosed thymocytes, TNC may additionally influence the development of thymocytes through release of soluble factors in a restricted microenvironment. By employing the 3-(2,4-dinitroanilino)-3'-amino-N-methyl-propylamine technique which at the ultrastructural level detects acidic organelles involved in processing of antigens presented by MHC class II molecules, we also show that TNC contain acidic compartments similar to classical antigen-presenting cells, i.e. early and late endosomes and lysosomes, albeit in a lower amount than in thymic dendritic cells. This fact provides evidence that TNC not only are capable of antigen presentation but also possess the intracellular machinery for antigen processing.

Half-life of antigen/major histocompatibility complex class II complexes in vivo: intra- and interorgan variations

We have determined the half-life in vivo of antigen/MHC class II complexes in different organ microenvironments. Mice were "pulsed" with myoglobin intravenously and MHC class II-positive antigen-presenting cell (APC) populations from different organs were isolated after various time intervals. Specific antigen/MHC complexes were quantitated by co-cultivation of the APC subsets with myoglobin-specific T-T hybridoma cells in vitro. Half-lives of antigen/MHC complexes differed both between organs and between compartments of the same organ. Half-lives in peripheral organs (spleen and bone marrow) ranged between 3 and 8 h, whereas in the thymus half-lives between 13 h (cortical epithelial cells) and 22 h (medullary dendritic cells) were observed. Half lives in vivo were independent of antigen processing, since intact protein or antigenic peptides yielded similar values. The considerably longer half-life of peptide/MHC complexes in the thymus as compared to peripheral organs may reflect the distinct role which antigen presentation plays in both organs, i.e. induction of tolerance versus induction of immunity.

Thymus-derived cytokine(s) including interleukin-7 induce increase of T cell receptor alpha/beta+ CD4-CD8- T cells which are extrathymically differentiated in athymic nude mice

Extrathymic T cell differentiation pathways have been reported, although the thymus is the main site of T cell differentiation. The thymus is also known to produce several cytokines that induce proliferation of thymocytes. In the present study, we investigated the influence of thymus-derived cytokines on extrathymic T cell differentiation by intraperitoneal implantation with a diffusion chamber which encloses fetal thymus (we named it fetal thymus-enclosed diffusion chamber, FTEDC) in athymic BALB/c nu/nu mice. Increase in number of T cells bearing T cell receptor (TcR) alpha/beta was detected in lymph nodes and spleens of FTEDC-implanted nude mice 1 week after implantation, whereas no such increase was detected in control nude mice implanted with a diffusion chamber without thymus. The FTEDC-induced increase of T cells was suppressed by intraperitoneal injection of anti-interleukin-7 monoclonal antibody (mAb). The TcR alpha/beta T cells in FTEDC-implanted BALB/c nu/nu mice preferentially expressed V beta 11, although V beta 11-positive T cells are deleted in the thymus of euthymic BALB/c mice by clonal elimination of self-super-antigen Dvb11-specific T cells. TcR alpha/beta T cells in FTEDC-implanted nude mice were of CD4-CD8- phenotype and showed no proliferative response against anti-TcR monoclonal antibody stimulation. These results suggest that the thymus can induce extrathymic T cell differentiation through the influence of thymus-derived cytokine(s) including interleukin-7, and that such extrathymically differentiated T cells have acquired only a little or no ability for proliferation when they recognize antigen by their TcR.

In situ analyses of in ovo graft-vs.-host reaction induced by thymic nurse cell lymphocytes

In both mammalian and avian systems, thymic nurse cells (TNC) have been shown to harbor a heterogeneous population of T lymphocytes (TNC-L) some of which exhibit a postselectional phenotype. By transplanting micromanipulated single chicken TNC onto the chorionallantoic membrane (CAM) of major histocompatibility complex (MHC)-disparate embryos, an experimental system which allows for the detection of lymphocytes with graft-vs.-host (GVH) reactivity, we demonstrate here that TNC enclose lymphocytes that can develop into both CD4+ single-positive (sp) and CD8+ sp, T cell receptor (TcR) alpha beta+, or TcR gamma delta+ cells. This finding was additionally confirmed by serial transfer of primary expanded alloreactive T cells onto the CAM of secondary hosts. All donor TNC-L expressed MHC class II molecules and the interleukin-2 receptor alpha chain in primary and secondary GVH reactions. Furthermore, we observed selective accumulation of CD8+ and TcR gamma delta+ host lymphocytes in the CAM upon the induction of a local GVH reaction, most probably as a consequence of the pathological alteration of the epithelium.

A simple quantitative in vitro assay for thymocyte adhesion to thymic epithelial cells using a fluorescein diacetate

Recent data verified the role of thymocyte adhesion to thymic epithelial cells (TEC) in T cell development. In order to measure this cellular event as one of routine examinations in immunological studies, a simple quantitative assay in in vitro setting is required. Labeling thymocytes with fluorescein diacetate (FDA) was used to measure quickly and with certainty the adhesion ability of BALB/c thymocytes to TEC cells, a thymic epithelial cell line derived from BALB/c mouse thymic stroma. Thymocytes stained with FDA at the concentration of 2.5 micrograms/ml gave a strong fluorescent intensity (FI) easily detectable in a spectrophotometer and showed the same phenotypical and functional features as unstained cells. As it was confirmed that the cell number correlated well with FI, the number of cells adhering to TEC in wells of a 96-well microplate could be estimated. The major advantage of using FDA was that it took only a few seconds to measure FI of one sample. In contrast, to count the number of adherent cells with conventional methods using a light microscope or radioisotopes required much longer time and care to avoid radiological hazards. Moreover, cell viability and FI of FDA-labeled thymocytes changed little for 24 h, and lysates of FDA-positive cells demonstrated the same FI as living cells. These results indicate that many samples could be applied for FI measurement at the same time after in vivo or in vitro experiments.

Thymic pathology in primary and secondary immunodeficiencies

For the sake of clarity and in agreement with the World Health Organization immunodeficiency classification, it is important to distinguish the congenital, inherited malformative lesions called generically 'thymic dysplasia' from the secondary, acquired changes, designated under the broad term of 'severe thymic atrophy'. Thymic dysplasia represents the archetype of thymic changes in cellular immunodeficiency, since there is no example of a thymic dysplasia associated with a normal T-cell function. Thymic dysplasia is observed in several inherited diseases, the most frequent of them being severe combined immunodeficiency. More than the depletion of lymphoid cells, the lack of differentiation of the thymic epithelium, responsible for the absence of Hassal's corpuscles, is the main and constant feature of this condition. Thymic dysplasia underscores the crucial role of the thymic epithelium in the normal differentiation of the T-cell population. Severe thymic atrophy is secondary to various causes, including prolonged protein malnutrition and immunosuppressive or cytotoxic drugs, graft versus host reaction and, chiefly today, chronic viral infection, especially with HIV-1. The morphological changes are similar and are characterized by a partial lymphoid depletion, involving mainly the CD1+ population, necrosis and calcification of epithelial cells, the frequent presence of plasma cells and, more significantly, fibrohyaline changes of the basement membrane of the vessels and thymic epithelium. The severity of the atrophic changes and the immunodeficiency-related manifestations depend on the duration of the aetiological factors and, more significantly, with their early occurrence, within the first months of life. The mechanisms underlying thymic atrophy are poorly understood. A primary impairment of lymphoid cells seems at present to be the most likely hypothesis.

Detection of calbindin-D immunoreactivity in human follicular dendritic cells

The calcium-binding protein calbindin-D (28 kD) has been analysed immunohistochemically in different human lymphoid tissues. Combined immunohistochemical staining showed that calbindin-D (28 kD) is expressed by only a proportion of dendritic cells within the light zone of germinal centres, where antigens in the form of immune complexes are trapped and presented to B lymphocytes. All other cell types including macrophages, interdigitating cells, and various lymphocyte populations were negative. The expression of calbindin-D in this functionally relevant subset of follicular dendritic cells could have a role in the regulation of proliferation and selection of memory B-cells by modulating the concentration of calcium ions. Calbindin-D may be a useful marker for analysing in situ the phases of follicular development in different physiological and pathological conditions.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)

Thymic nurse cell lymphocytes react against self major histocompatibility complex

It has been postulated that thymic nurse cells (TNC), lymphoid-epithelial complexes composed of thymocytes enclosed within major histocompatibility complex (MHC) class I+ and class II+ cortical epithelial cells, may provide an optimal microenvironment for the process of T cell selection. By transplanting single TNC in the avian chorionallantoic membrane assay we demonstrate that a significant portion of intra-TNC lymphocytes (TNC-L) possess reactivity against self-MHC molecules. The frequency of these autoreactive cells among TNC-L exceeds by far that of thymocytes or peripheral blood lymphocytes of the same donor. These results indicate that TNC-L constitute a T cell population enriched for self-MHC reactivity, i.e. cells that have undergone positive selection, but not yet deletion and/or deactivation.

Analysis of class II MHC structure in thymic nurse cells

During the course of thymocyte maturation, the processes of positive selection and tolerance induction are mediated by interactions between thymocyte T-cell receptors and MHC molecules on thymic stromal cells. The means by which these seemingly contrary processes can be mediated by interactions between the same molecules has long been a source of controversy. One idea which has been put forward is that the MHC molecules in different microenvironments of the thymus are not the same. We have tested this hypothesis by examining class II transcripts derived from thymic cortical epithelial cells known as thymic nurse cells, reasoning that alternative splicing of primary transcripts might give rise to a positively selecting MHC molecule. However, we found no evidence for alternative splicing of these transcripts. These results are presented and discussed with regard to implications for possible mechanisms of positive selection.

Immunohistochemical characterization of nurse cells in normal human thymus

Lymphoepithelial complexes known as thymic "nurse" cells (TNC) have been isolated and described in the thymus of several animal species including man. Most of the investigations on TNC have been carried out in enzymatically digested thymuses in which TNC were isolated by differential sedimentation. In the present study we demonstrate TNC in immunohistochemically stained sections of human thymus as ring-shaped cells completely enclosing thymocytes and localized not only in the cortex, but also at the corticomedullary junction where they have not been previously described. TNC expressed epithelial markers [low and high molecular weight keratins identified by 35 beta H11 and 34 beta E12 monoclonal antibodies, a cortical antigen shared with neuroectodermal neoplasms recognized by the GE2 monoclonal antibody, and tissue polypeptide antigen (TPA:B1)], class II histocompatibility antigens (HLA-DR), and thymosin alpha 1. Double staining experiments with the nuclear proliferation-associated antigen Ki-67 and the cortical epithelium marker GE2 showed that most thymocytes enclosed in these cortical TNC were not proliferating. The antigens expressed by TNC indicate that not only cortical, but also medullary epithelial cells are part of the TNC system. The possible role of TNC in the education and maturation of thymocytes is discussed.

Induction of thymocyte proliferation by supernatants from a mouse thymic epithelial cell line

The thymic stroma plays a critical role in the generation of T lymphocytes by direct cell-to-cell contacts as well as by secreting growth factors or hormones. The thymic epithelial cells, responsible for thymic hormone secretion, include morphologically and antigenically distinct subpopulations that may exert different roles in thymocyte maturation. The recent development of thymic epithelial cell lines provided an interesting model for studying thymic epithelial influences on T cell differentiation. Treating mouse thymocytes by supernatants from one of TEC line (IT-76M1), we observed an induction of thymocyte proliferation and an increase in the percentages of CD4-/CD8- thymocytes. This proliferation was largely inhibited when thymocytes were incubated with IT-76M1 supernatants together with an anti-thymulin monoclonal antibody, but could be enhanced by pretreating growing epithelial cells by triiodothyronine. We suggest that among the target cells for thymulin within the thymus, some putative precursors of early phenotype might be included.

Towards an integrated view of thymopoiesis

One prediction from the complex series of steps in intrathymic T-cell differentiation is that to regulate it the stroma controlling the process must be equally complex: the attraction of precursors, commitment to the T-cell lineage, induction of T-cell receptor (TCR) gene rearrangement, accessory molecule expression, repertoire expansion, major histocompatibility complex (MHC) molecule-based selection (positive and negative), acquisition of functional maturity and migratory capacity must all be controlled. In this review, Richard Boyd and Patrice Hugo combine knowledge of T-cell differentiation with thymic stromal cell heterogeneity to offer an integrated view of thymopoiesis within the thymic microenvironment.

Thymic nurse cells: morphological study during their isolation from murine thymus

Thymic nurse cells (TNC), which are multicellular complexes composed of epithelial cells and thymocytes, were obtained from C3H-mice thymuses. They were described by means of light and electron microscopy. The morphology of epithelial cells forming isolated TNC compared to that of small tissue fragments obtained by enzymatic digestion revealed that TNC could be derived from all parts of the thymus: cortex, corticomedullary junction and medulla, the cortex being their principal source. This variety of origin, the presence of several epithelial cells inside a single TNC, the presence of non-lymphoid cells, and the various locations of cleaved desmosomes confirmed that their aspect "in vitro" as round and sealed structures can be considered to be an artifact due to the isolation technique used. Indeed, during this procedure, they are formed by a process of wrapping of the epithelial cytoplasm around the tightly associated thymocytes. All three epithelial cell types: cortical reticular cells, medullary reticular cells, and medullary globular cells can form TNC.

Reappraisal of histogenesis in the bursal lymphoid follicle of the chicken

The development of the bursal follicle and the appearance of the follicle-associated epithelial (FAE) cell and the reticuloepithelial (REp) cell were studied. The stages of development of the bursal follicle were observed by light and electron microscopy; an anticytokeratin monoclonal antibody was also used. At the beginning of follicle development, a mesenchymal cell cluster is observed in the tunica propria; the cluster becomes wedged in a niche of the surface epithelium, and gradually it is completely surrounded by the epithelium itself, which closes under the clump of mesenchymal cells. The epithelial cells lying upon the mesenchymal clump become necrotic, and a number of mesenchymal cells bulge out, forming the FAE cells. The epithelial cells that have closed under the mesenchymal nodule become stratified and form the REp cells; they become star-shaped because the medullary-lymphoid cells grow between them. Finally, the cortex is formed, possibly as a result of the migration of medullary cells before they peripheralize. It is concluded that FAE cells are not specialized epithelial cells, as they do not react to an anticytokeratin monoclonal antibody; on the contrary, they are formed by mesenchymal stemcells that bulge into the lumen and change their character after moving into the epithelium. The REp cells appear in the follicular primordium shortly after the bursal follicle begins to develop; the pronounced reactivity of the REp cells to an anticytokeratin monoclonal antibody supports the hypothesis of their epithelial origin.

Effect of interferon-gamma and tumor necrosis factor-alpha on lymphoepithelial interactions within thymic nurse cells

Isolated thymic nurse cells (TNC) represent a specialized microenvironment in vivo where thymocytes interact specifically with subcapsular epithelial cells. They are thought to play a critical role in the process of T cell differentiation. We demonstrate that recombinant murine interferon-gamma and recombinant human tumor necrosis factor-alpha can act on these interactions: they stimulate TNC-derived epithelial cells to establish interactions with thymocytes in vitro and to form new lymphoepithelial complexes. This phenomenon is partially inhibited by anti-Ia monoclonal antibodies. Implications of these findings for normal intrathymic differentiation are discussed.

Thymic cortical epithelial cells lack full capacity for antigen presentation

Several recent studies have suggested that interactions between thymocytes and thymic stromal cells are essential for the development and elimination of antigen-reactive T lymphocytes. It is important, therefore, to characterize the stromal cells involved in presentation of antigen in the thymus. In a previous report, we demonstrated, using T-cell hybridomas, that three distinct types of antigen presenting cells in the thymus (cortical epithelial cells, macrophages, and dendritic cells) constitutively expressed self haemoglobin/Ia complexes. Here we report that one of these cell types, the cortical epithelial cell, does not induce stimulation of T-lymphocyte clones even though the antigen/Ia complex required for antigen-specific recognition is present. This lack of response occurs with both TH1 and TH2 clones. Responsiveness of the TH2 clone can be restored by adding the murine lymphokine interleukin-1 beta to the culture system.

Dynamics of complex formation between thymocytes and thymic medullary epithelial cells

Direct cell contact is an intrinsic part of several differentiation processes. A case in point is the formation of complexes between thymic lymphocytes and stromal cells, which are essential for T-cell maturation. The objective of the present work was to gain an insight into the mechanisms underlying the formation and dissociation of such lymphostromal complexes. Using an in vitro system, we show that the number of thymocytes adhering to a thymic medullary epithelial cell line (E-5) increases with time and reaches a plateau, after which some thymocytes spontaneously detach, while the rest remain attached to epithelium. The detached and forcibly removed thymocytes were analysed for their expression of L3T4 and Ly-2 antigens. The detached thymocytes showed a markedly lowered expression of both antigens. Both of these subtypes were shown to be totally refractory to form further complexes after a first encounter with E-5 cells. We also show that, once a mean adherence level of one thymocyte per E-5 cell was reached, (a) the level of further adherence increased exponentially, and (b) during this phase, binding of thymocytes to E-5 cells occurred in clusters, indicating pre-existing polarity or contact-induced polarization of the receptors at the E-5 cell surface.

Thymic cortical epithelial cells can present self-antigens in vivo

Antigens present during neonatal life are recognized as self and individuals are tolerant to these antigens. In normal individuals T cells are tolerant to most self proteins but we still know little of the mechanism(s) by which tolerance is established. A requisite part of the current negative selection model of self tolerance is the expression of self proteins complexed with major histocompatibility complex molecules in the thymus. As MHC proteins bind antigens and present them to the receptor on the antigen-specific T cell, then for tolerance to self to occur, it is possible that each self protein must be processed and presented by an MHC molecule. As a result of the development of a unique T-cell hybrid reactive to the self protein murine haemoglobin, we have shown that in normal animals this self protein is continuously processed and potentially presented in an MHC-restricted manner. Here we show that self haemoglobin is being processed and presented by thymic antigen-presenting cells as early as gestational day 14. We also demonstrate that three types of thymic stromal cells, namely macrophages, dendritic cells and cortical epithelial cells, can present the haemoglobin self antigen in vivo. This surprising presentation of a self antigen by thymic cortical epithelial cells implies that they could be involved in T-cell development and negative selection.

Unidirectional responses to Mls determinants in vivo. Polyclonal T-cell responses to a single common determinant of Mls in different efficiencies?

Polyclonal anti-Mls responses of peripheral (mature) and thymic (immature) lymphocytes were studied in vivo in terms of local host-versus-graft and graft-versus-host reactions. The level of stimulatory activity differed between the various types of Mls antigens, with Mlsa and Mlsd having the highest levels. Mlsc a lower level, and Mlsb the lowest level. The immunogenicity of Mlsa and Mlsd may not be identical, since Mlsd mice responded to the Mlsa determinant, while Mlsa mice did not react with the Mlsd determinants. This suggests that the anti-Mls response is unidirectional. The fact that Mlsb mice made tolerant at birth to Mlsd (Mlsa) antigens behave like Mlsd (Mlsa) mice when responding to Mls antigens supports this suggestion. Furthermore, thymus cells from the Mlsa-tolerized BALB/c (Mlsb) mice were unresponsive to both Mlsa and Mlsd antigens, while those made tolerant to Mlsd were responsive to Mlsa. These results indicate that the polyclonal response to the strongly immunogenic Mlsa,d antigens is unidirectional, and that the immunogenicity of Mlsa and Mlsd is not identical. Based on the results of tolerance experiments and data on T-cell clones, we suggest that the difference in immunogenicity between Mlsa,b,d antigens is due to different efficiencies of the responding T-cell populations, probably because of a quantitative difference in a single common antigenic determinant expressed in each Mls haplotype, which results in different levels of stimulation of T cells according to the avidity for the determinant.

Epithelial-thymocyte interactions in human thymus

Our data demonstrate that the epithelial component of the human thymic microenvironment is not an inert cell type, but rather is capable of being directly involved in the promotion of both early and late stages of T-cell maturation. Data from our laboratory [54,69], together with the work of Plunkett et al. [61] and Shaw et al. [70] suggest that an endogenous ligand for the CD2 molecule in humans is the LFA-3 molecule. Using an SV40 transformed human thymic epithelial cell line of subcapsular cortical origin, Mizutani et al. have confirmed that thymic epithelial cells bind thymocytes via a CD2/LFA-3 interaction [78]. The data reviewed in this paper suggest that within the thymus one endogenous ligand for the alternative pathway of thymocyte activation via the CD2 molecule is the LFA-3 molecule on TE cells. Following thymocyte binding to TE cells, immature thymocytes are directly activated to proliferate, and their response to both IL1 and IL2 is augmented. Also, following TE-thymocyte binding, TE-IL1 secretion is augmented and TE cell MHC class II antigen expression is induced. Moreover, while undergoing activation, thymocytes appear to be able to modulate their microenvironment milieu of MHC antigens and IL1. Further analysis of the sequelae of TE-thymocyte interactions using phenotypic characterization of thymocytes with anti-T-cell MoAbs, coupled with molecular analysis of thymocyte T-cell receptor genes, should allow for the determination of the precise sequential stages that immature T cells undergo enroute to functional maturity. Understanding these steps in T-cell maturation will be critical to our understanding of the events that transpire in the genesis of autoimmune, lymphoproliferative, and immunodeficiency diseases.

Phenotype of stromal cell-associated thymocytes in situ is compatible with selection of the T cell repertoire at an "immature" stage of thymic T cell differentiation

Murine thymocytes interacting with cortical macrophages, cortical epithelial cells and medullary dendritic cells in situ express T cell receptors at low to intermediate density. They co-express the lineage markers Lyt-2 and L3T4 and are not enriched for cells expressing high-density interleukin 2 and lymph node homing receptors. Thus, recognition of stromal cells in situ in distinct thymic microenvironments occurs at a common stage of T cell maturation which is phenotypically intermediate between intrathymic precursor cells and mature medullary-type thymocytes. The surface phenotype of dendritic cell-associated thymocytes indicates the presence of thymocytes with a "cortical" phenotype within the antigen-exposed ("nonsterile") phase of T cell maturation in the medulla.