In vitro cultivation of nonlymphoid thymic cells: morphological and immunological characterization

Thymic nurse cells: a microenvironment for thymocyte development and selection

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

Neuroendocrine influence on thymic haematopoiesis via the reticulo-epithelial cellular network

The thymus provides an optimal cellular and humoral microenvironment for a cell line committed differentiation of haematopoietic stem cells. The immigration process requires the secretion of at least one peptide, called thymotaxin, by cells of the reticulo-epithelial (RE) network of the thymic stromal cellular microenvironment. The thymic RE cells are functionally specialised based on their intrathymic location and this differentiation is modulated by various interaction signals of differentiating Thymocytes and other nonlymphatic, haematopoietic stem cells. The subcapsular, endocrine, RE cell layer is comprised of cells filled with periodic acid Shiff's-positive granules, which also express A2B5/TE4 cell surface antigens and MHC Class I (HLA A, B, C) molecules. Thymic nurse cells also produce thymosins beta 3 and beta 4 and display a neuroendocrine cell specific immunophenotype (IP): Thy-1+, A2B5+, TT+, TE4+, UJ13/A+, UJ127.11+, UJ167.11+, UJ181.4+ and presence of common leukocyte antigen (CLA+). Cortical RE cells express a surface antigen, gp200-MR6, which plays a significant role of thymocyte differentiation. Medullar RE cells display MHC Class II (HLA-DP, HLA-DQ, HLA-DR) molecule restriction. Thymic RE cells also produce numerous cytokines that are important in various stages of haematopoietic cell activation and differentiation. The co-existence of pituitary hormone and neuropeptide secretion, as well as the production of a number of interleukins and growth factors, and expression of receptors for all, by RE cells is an unique molecular biological phenomenon. Thymic neuroendocrine polypeptides are the source of self antigens presented by the MHC molecules to differentiating haematopoietic stem cells. On the level of individual RE cells, the numerous projections associated with a single cell, which engulf developing lymphocytes, nurturing and guiding them in their maturation, may differ in their hormone production and/or hormone receptor expression profile, thus allowing a single cell to be involved in distinct, separate steps of the T-cell and other haematopoietic cell maturation process. Thymic RE cells represent an important cellular and humoural network within the thymic microenvironment and are involved in the homeopathic regulation mechanisms of the multicellular organism. The intrathymic T-lymphocyte selection is a complex, multistep process, influenced by several functionally specialised RE cells and under immuno-neuroendocrine regulation control reflecting the dynamic changes of the mammalian organism.

Questionable thymic nurse cell

Since their discovery in 1980, thymic nurse cells (TNCs) have been controversial. Questions pertaining to the existence of the TNC as a "unit" cell with thymocytes completely enclosed within its cytoplasm were the focus of initial debates. Early skeptics proposed the multicellular complex to be an artifact of the procedures used to isolate TNCs from the thymus. Since that time, TNCs have been found in fish, frogs, tadpoles, chickens, sheep, pigs, rats, mice, and humans. Their evolutionary conservation throughout the animal kingdom relieved most speculations about the existence of TNCs and at the same time demonstrated their apparent importance to the thymus and T-cell development. In this review we will discuss and debate reports that describe (i) the organization or structure of TNCs, (ii) the thymocyte subset(s) found within the cytoplasm of TNCs and their uptake and release, and (iii) the function of this fascinating multicellular interaction that occurs during the process of T-cell development. Discussions about the future of the field and experimental approaches that will lead to answers to remaining questions are also presented.

Effects of chicken thymic stromal cells on the growth and differentiation of thymocytes in vitro

We examined contact-mediated effects of chicken thymic stromal cells (TSC) on thymocyte differentiation by co-cultivation of these cell populations. The primary cultures of TSC isolated from thymus mainly have consisted of epithelial cells which were polygonal in shape, possessed long processes and expressed MHC class II antigen. When thymocytes were co-cultured with TSC, 60% to 70% of thymocytes attached to TSC and some of them engulfed underneath TSC. These attached thymocytes were CD4-CD8- and CD4+CD8+ subsets and expressed alpha/beta TCRhigh or gamma/delta TCRlow. Some of the thymocytes attaching to TSC showed an increase of intracellular and nuclear density, fragmentation of cytoplasm and nuclei, and DNA fragmentation. And also, thymocytes attaching to TSC contained a higher percentage of cycling (S and G2 + M phase) cells than nonattaching cells. These results indicate that specific subsets in thymocytes selectively bind to TSC and undergo apoptotic death or proliferation because of interaction with TSC. Chicken TSC may play an important role in thymic differentiation by direct contact within the thymus as in mammals.

Morphological study of thymus stromal cells (TEL-2 cell) which play a role in the elimination of double positive immature thymocytes by phagocytosis

BACKGROUND

The process of the selective elimination of immature double positive thymocytes, during culture with TEL-2 cells (an epithelial cell line from the thymus stroma), is initiated by the contact between thymocytes and TEL-2 cells (Nakashima et al. 1990 Eur. J. Immunol., 20: 47-53; Hirata et al. 1991 Anat. Rec., 230: 524-530).

METHODS

Our approach was to follow the process of thymocyte internalization as a sequelae of this interaction with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron microscopic cytochemistry for the hydrolytic enzyme acid phosphatase (Ac Pase).

RESULTS

With SEM, numerous thymocytes lay underneath the TEL-2 cells. An enhanced activity of the TEL-2 cell membrane, consisting of several thin thread-like projections either with or without an expanded termination, was involved with contacting the thymocytes. With TEM, close appositions were noted either at adjacent segments of the cellular membrane or between thick plump local protrusions of the plasma membrane. Blunt pseudopodia and deep invaginations on indented portions of the TEL-2 surface engulfed one or more thymocytes. With Ac Pase histochemistry: 1) the incorporation of thymocytes into a phagosome was indicated by the Ac Pase negative thymocytes enclosed in a membrane beneath the protuberance of a TEL-2 cell surface without fusion to the TEL-2 lysosomes; 2) most of these thymocytes were morphologically intact, whereas the rest were already damaged, having changes similar to apoptosis; and 3) a few Ac Pase positive dense bodies of the TEL-2 cell, mostly with the morphology of secondary lysosomes, fused with the thymocyte-enclosing membrane after which the digestion of several thymocytes proceeded.

CONCLUSIONS

With the prominent activity of the plasma membrane involving the initial attachment and subsequent phagocytosis, a murine thymic epithelial cell line TEL-2 plays a demonstrating role in the mechanism of the elimination of double positive thymocytes.

Thymic nurse cells in culture: morphological and antigenic characterization

Epithelial monolayers were derived from thymic nurse cells (TNC), and were seeded onto collagen-coated dishes immediately after their isolation from young adult C3H-murine thymuses. Different media and supplements were tested in order to obtain cultures that were as pure as possible. Primary cultures were enriched in epithelial cells but always contained non-epithelial components among which fibroblasts predominated. Immunodetection of keratins, and repeated light- and electron-microscopic observations established the epithelial nature of the elongated cells derived from TNC; these elongated cells were cortical reticular cells, and were different from medullary globular cells that immediately adopted a mosaic pattern in vitro. At the beginning of the culture, the necrosis of cortical lymphocytes appeared to be toxic for epithelial cells; when epithelial cells survived, they showed a temporary lipid accumulation. After a 5-day culture, they still synthesized DNA but lost this capacity thereafter and dedifferentiated. The lympho-epithelial symbiosis appeared to be necessary to maintain some epithelial characteristics of the cultured cells, such as the clear vesicles and the expression of Ia antigens. In sub-cultures, the monolayers were almost purely epithelial in nature but growth was no longer observed. The cells remained reticular in shape, as they were in vivo, but their cytoplasm and their nucleus became larger and numerous cells were multinucleated. Confluence was not obtained with classical media even after mitogenic stimulation. The frequent observation of strongly keratinized areas suggested a process of terminal differentiation; this could not be avoided by using low serum concentration.

Culture and characterization of thymic epithelium from autoimmune NZB and NZB/W mice

Autoimmune NZB and NZB/W mice display early abnormalities in thymus histology, T cell development, and mature T cell function. Abnormalities in the subcapsular/medullary thymic epithelium (TE) can also be inferred from the early disappearance of thymulin from NZB. It has also been reported that NZB thymic epithelial cells do not grow in culture conditions that support the growth of these cells from other strains of mice. In order to study the contribution of TE to the abnormal T cell development and function in NZB and NZB/W mice, we have devised a culture system which supports the growth of TE cells from these mice. The method involves the use of culture vessels coated with extracellular matrix produced by a rat thymic epithelial cell line. TEA3A1, and selective low-calcium, low-serum medium. In addition TEA3A1 cells have been used as an antigen to generate monoclonal antibodies specific for subcapsular/medullary TE. These antibodies, as well as others already available, have been used to show that the culture conditions described here select for cells displaying subcapsular/medullary TE markers, whereas markers for cortical TE and macrophages are absent.

Short-term cultivation of murine thymic epithelial cells in a serum-free medium

Thymic epithelial cells were grown in defined medium without unknown serum factors and without concurrent growth of other cell types. Thymic tissue was obtained from 1- to 4-wk-old mice, disaggregated, and incubated in a mixture of collagenase-dispase-DNAse. The resulting organoids were seeded on collagen-coated flasks. The culture medium consisted of DME-F12 with low or high concentration of Ca2+ supplemented with insulin, epidermal growth factor, cholera toxin, hydrocortisone, and transferrin. Under these conditions, explants attached to the substrate within 2 d, and expanding epithelioid monolayer islets emerged from the organoids during the following days. [3H]Thymidine incorporation revealed a growth fraction of the cells close to 5%. By omitting either epidermal growth factor, insulin, or cholera toxin from the medium, pronounced reduction in sizes of islets and in [3H]thymidine incorporation was found. Throughout the culture period, the islets appeared as continuous sheets of polygonal cells. The epithelial nature of the expanding cell islets was confirmed by demonstration of cytokeratins and of desmosomes. Ultrastructural evaluation of early cultures revealed clusters of epithelial cells intermixed with lymphocytes, and late cultures showed a typical pattern of stratified keratinizing epithelium. However, squamous metaplasia was avoided by the use of low Ca2+ medium, which also proved essential for cell transfer. MHC class II antigen was detected on the majority of the cultured cells, and culture supernatants contained co-mitogenic activity for thymocytes and GM-colony stimulating activity.

Selective elimination of double-positive immature thymocytes by a thymic epithelial cell line

A cloned epithelial cell line, TEL-2, has been established from the stroma tissues of normal mouse thymus. Incubation of mouse thymocytes on TEL-2 cells resulted in the selective elimination of double-positive (CD4+CD8+) cells from the culture, whereas single-positive (CD4+CD8- or CD4-CD8+) thymocytes remaining in the culture were concentrated in non-integrated cell population. The CD3- or CD3 low-positive thymocytes were also eliminated by the TEL-2 cells from the culture, followed by the concentration of CD3 high-positive cells in the culture. Only intact viable thymocytes were integrated into TEL-2 cells. Electron microscopic examination showed that the integrated cells into TEL-2 cytoplasm were gradually degenerated. Mature single-positive T cells, mature B cells or double-negative thymocytes were not integrated into TEL-2 cells. The TEL-2 cell may provide information on the mechanism of selective disappearance of double-positive immature cells from the thymus.

Kinetic analysis of thymocyte attachment to thymus stromal cells in culture by using phase-contrast and scanning electron microscopy

Direct cellular contact between thymocytes and thymus stromal cells within the thymus appears to contribute to the maturation of thymocytes. Thymocyte-stromal cell complexes, formed in vivo, have been isolated by others and postulated to play a role in T-cell differentiation. These previous studies have been hampered, however, by a time-consuming isolation procedure from which only small numbers of these complexes are recovered. We have examined a model to study thymocyte-stromal cell complexes in vitro in which thymocytes are added to primary cultures of thymus stromal cells. In the present study, we found that thymocytes were histotypically selective in their attachment to thymus stromal cells. We also investigated the kinetics of thymocyte attachment to these thymus stromal cells. Cultures were examined at selected time intervals from 5 min through 3 days of incubation. Thymocyte attachment to stromal cells was a biphasic interaction, with maximum surface attachment at 15 min of cocultivation, followed by migration of thymocytes into the cultures. Morphological studies were confirmed by using 3H-leucine-labeled thymocytes and liquid scintigraphy. With increased time in culture, thymocytes became amoeboid and migrated between the layers of stromal cells where thymocyte mitotic figures were seen at 4 and 8 hr. In some cases it appeared that stromal cells, which often grew two to three cell layers deep, played an active role in enclosing thymocytes within the cultures. Large numbers of viable thymocytes were observed in the cultures at 24 hr. The number of thymocytes then decreased progressively on days 2 and 3, when relatively few were found within the layers of the culture.

The growth of nonlymphoid thymic components in vitro: age-related differences during development

An explant culture procedure has been developed that makes it possible to measure the relative growth capacity of the epithelial and mesenchymal cells of the canine thymus gland. Standardized growth conditions were obtained by size-grading thymic fragments and counting to allow uniform fragment density during culture. After 6 d in culture, outgrowth from the fragments formed colonies that could be classified into epithelial, mixed, or spindle cell type. Uniform fragment size and number in each flask allowed calculation of the total plating efficiency, relative distribution of colony types, and mean colony diameters for thymic fragments collected from fetuses (50 d of gestation), neonates (0 d postpartum), and juveniles (70 d postpartum). Data show age-related changes in the proliferative capacity of the cells in all three colony types. The most significant difference was seen in the epithelium, which showed a 30% reduction in mean colony diameter over the 2 wk between fetal and neonatal ages and a 23% reduction over the postnatal period of 70 d. Significant reductions were seen in the other colony types as well. Because the severity of the effect of many injurious agents is proportional to the rate of growth of the target cells, these data suggest that the thymus gland of the fetus may be more sensitive to physical or chemical injury than is the neonate or adult.

A cloned rat thymic epithelial cell line established from serum-free selective culture

A serum-free system has been developed for selective growth and long-term culture of rat thymic epithelial cells. The growth media is a modification of McKeehan's WAJC 404, plus insulin, cholera toxin, dexamethasone, and epidermal growth factor. Cultures have been continuously passaged and maintained for over 6 mo., and a cloned cell line, TEA3A1, has been established. These cells are epithelial, judging by morphology and ultrastructure, and are positive for A2B5 and thymosin alpha 1 markers for thymic endocrine cells.

Thymic epithelium in vitro. V. Binding of thymocytes to cultured thymic epithelial cells

Direct contact between thymocytes and thymic stromal elements may be one of the mechanisms involved in thymocyte differentiation. Thymic lymphoepithelial complexes have been isolated in which thymocytes appear to be in direct association with cortical epithelial cells. We have previously reported the isolation and successful culture of two morphologically distinct types of murine thymic epithelial cells. We have utilized these to study the interactions of lymphoid and epithelial cells by means of an in vitro assay of the binding of radiolabeled thymocytes to monolayers of these cultured thymic epithelial cells. The percentage of bound cells increased rapidly during the first hour of incubation, reaching approximately 40% binding. Binding continued to increase slowly until plateau levels were reached at approximately 5 hr. Thymocyte binding to thymic epithelium, but not fibroblast monolayers, was trypsin-sensitive, suggesting that specific protein interactions may be involved. Binding of thymocytes to epithelium was temperature-dependent, involved formation of cytoplasmic projections, and was inhibited by cytochalasin B. We also found that cortical thymocytes (peanut agglutinin-positive (PNA+)cells) bound to cultured epithelium to a greater degree than medullary thymocytes (PNA- cells). This correlates with in vivo studies by others in which thymocytes associated with lymphoepithelial complexes have been found to have immature phenotypes. This system provides a means for a quantitative study of the role of cell to cell contact in the process of thymocyte selection and differentiation.

Long-term proliferation of mouse thymic epithelial cells in culture

Epithelial cells from the normal mouse thymus were successfully cultivated on tissue culture plastic when plated with lethally irradiated support cells of the LA7 rat mammary tumor line. As the irradiated LA7 cells slowly decreased in number the thymus cells proliferated concomitantly to form a confluent monolayer. The cells now in culture have been subcultured 8 times, have doubled in number at least 30 times, and are still proliferating vigorously. The culture technique also supported clonal growth from a single cell, and nine clones have been isolated. The colony-forming efficiency of thymic cells plated at low concentrations was about 8%. These cultures were never overgrown by fibroblasts. The thymus cells were characterized as epithelial by the presence of cytoplasmic keratin and numerous desmosomes and tonofilaments. They were shown to be mouse cells by immunocytochemistry with species specific antibodies, by isoenzyme analysis, and by karyology. The cells stained when reacted with antibodies to tubulin, vimentin, and actin, but not with antibodies to Thy-1.2, Lyt-1, Lyt-2, Ia, or H-2 proteins. More than 85% of the cells had a normal mouse diploid chromosome number of 40. This culture technique opens the way for future studies of T-cell education with homogeneous thymic epithelial cell populations both in vitro and after reimplantation into genetically defined strains of mice.

Isolation of murine thymic epithelium and an improved method for its propagation in vitro

A reliable and reproducible method for the isolation and propagation of thymic epithelial cells is described. Thymic epithelial cells from enzymatically dissociated thymus stroma are first enriched by separation on a discontinuous Percoll density gradient. The cell fractions enriched for epithelial cells are then cultured with irradiated fibroblasts in Ham's F-12 nutrient medium. Colonies of cells in these cultures contain keratin and exhibit morphologic characteristics of epithelial cells. When subcultured, the epithelial cells no longer require irradiated fibroblasts as filler cells. Some of the epithelial cells in vitro retain expression of class II (Ia) major histocompatibility antigens. The generation of defined cultures of thymic epithelial cells promises to be useful in defining their role in T cell differentiation.

Analysis of thymic epithelial cell proliferation in vitro by combining bromodeoxyuridine and keratin labeling in an immunofluorescence assay

A simple method of analyzing thymic epithelial cell (TEC) proliferation has been developed by combining bromodeoxyuridine (BrDU) and keratin labeling in an immunofluorescence assay. The first reagent specifically visualizes the cells entering the S phase of the cell cycle, whereas the second immunostaining reveals which of the proliferating BrDU-positive cells actually belong to the epithelial lineage. This method, besides being rapid and free of radioactivity, appears to be reliable in view of the minor variations in the percentages of BrDU+ TEC observed in several distinct experiments. Thus, BrDU/keratin immunolabeling appears to represent a useful tool for the analysis of in vitro TEC proliferation.

In vitro growth and maintenance of two morphologically distinct populations of thymic epithelial cells

Cultures of thymic epithelial cells were generated and maintained in valine-free minimum essential medium (MEM) supplemented with 690 mg/liter of D-valine. These cultures have been maintained for 1 year through multiple passages by trypsinization of 60-70% confluent monolayers. Large and small epithelial cells were present in early cultures. They were separated into two stable subpopulations based on (1) their differential growth rates and (2) their differential adherence to the culture substratum. These morphologically distinct cell populations, TECS and TECL, were 100% keratin positive and contained cells with desmosomes and tonofilaments, all characteristics of epithelial cells. Esterase analysis of both cell populations revealed a 1 and 9% esterase-positive cell population in cultures of keratin-positive small (TECS) and large (TECL) cells, respectively. The percentages of esterase-positive cells corresponded to the 2 and 10% populations of TECS and TECL, respectively, that contained both desmosomes and phagolysosomes. These results establish conditions for the long-term propagation of pure thymic epithelial cells. Such cultures can be used to study the functional interactions between epithelial cells and lymphoid cells. Morphologic and histochemical analyses have identified subsets of these cells which may prove to have differential effects on thymocyte proliferative and developmental processes.

Distinctive immunological properties of cultured murine thymic epithelial cells

Skin painting with chemically reactive haptens induces a hapten-specific state of hypersensitivity that is long lasting and can be transferred to unirradiated recipient mice. A similar state of hapten-specific contact sensitivity can be induced by intravenous immunization with hapten-conjugated cells. Thus far, only two cell types have been described that can perform this function: Langerhans cells of the skin, and splenic dendritic cells. All other types, coupled with hapten, induce either tolerance or a short-lived state of contact hypersensitivity that is readily suppressed, and cannot be transferred to normal recipients. In the present experiments, it was demonstrated that culture-enriched, hapten-coupled thymic epithelial cells can also induce a state of stable contact hypersensitivity identical to that induced by skin painting. This provides evidence that thymic epithelial cells have distinctive properties as antigen-presenting cells in vivo. The relationship of this finding to the postulated role of thymic epithelium in T-cell development is discussed.