Establishment of functional epithelial cell lines from a rat thymoma and rat thymus

Summary Seven clonal epithelial cell lines from a thymoma of an (ACI/NMs×BUF/Mna)F1 rat and seven clonal epithelial cell lines from an ACI/NMs rat thymus were established in a medium containing 1 μM dexamethasome (DM) and were characterized cytologically. Long-term treatment of DM stabilized the epithelial nature of these epithelial cells irreversibly. The established cell lines showed a polygonal shape, were positively stained with antikeratin antiserum and had tonofilaments and desmosomes. Species of their keratin paptides were the same as those of normal thymic epithelial cells in primary cultures. The cell lines were positively stained with Th-4 monoclonal antibody which preferentially stains the medullary epithelial cells of the thymus, but not with Th-3 which preferentially stains the subcapsular and cortical epithelial cells of the thymus. The cells from the rat thymoma were much large than those from the normal thymus, as reflected in their primary cultures. No transformed phenotypes, such as high growth rate, high saturation density anchorage independency, low serum dependency and so on, were found on the cell lines from the thymoma as in the cell lines from the normal thymus by in vitro assays. DNA synthesis of the thymic lymphocytes was stimulated by culturing with a line of rat thymoma with no lectins. Thymic lymphocytes strongly bound on the cell lines from the thymoma and changed the shape of the cells. These cell lines may be useful to investigate the mechanism of thymomegenesis and the interactions between epithelial cells and thymocytes in the rat thymoma.     

Production of a monoclonal antibody strongly reacting with immature thymic T lymphocytes and its immunohistological application

A monoclonal antibody Th-5 has been produced against mouse immature thymic lymphocytes and employed to study the process of T cell differentiation in the thymus. Immunohistologically, Th-5 positive thymic T lymphocytes were first found at Day 12 of gestation. They increased in number as well as staining intensity until Day 18 of gestation and decreased thereafter. Th-5 antigen expression was not seen in lymphoid cells in the fetal liver. In the newborn thymus, lymphocytes in the subcapsular layer were still strongly positive, while other cortical lymphocytes became moderately positive for Th-5. Th-5 positiveness was more pronounced in the medulla than in the cortex in the thymus of young adult mice. The staining pattern of Th-5 in the thymus was apparently different from those with other T cell markers (Thy-1, CD3, CD4, CD5, CD8) including J11d, Pgp-1, IL-2R, and 3A10 (TCR gamma delta). Flow cytometric analyses showed that the expression of Th-5 was mostly associated with the Thy-1 antigen. However, the fluorescent intensity of Th-5 gradually declined with ontogenic development of the thymus, and the molecular size of the antigen was approximately 100 kDa, which is different from Thy-1 antigen (25-30 kDa). Considering these findings, the strong expression of Th-5 could be one of the markers of immature thymic T lymphocytes in the early phase of the ontogenic development.     

The thymus microenvironment in regulating thymocyte differentiation

The thymus plays a crucial role in the development of T lymphocytes providing an inductive microenvironment in which committed progenitors undergo proliferation, T-cell receptor gene rearrangements and thymocyte differentiation into mature T-cells. The thymus microenvironment forms a complex network of interaction that comprises non lymphoid cells (e.g., thymic epithelial cells, TEC), cytokines, chemokines, extracellular matrix elements (ECM), matrix metalloproteinases and other soluble proteins. The thymic epithelial meshwork is the major component of thymic microenvironment, both morphologically and phenotypically limiting heterogeneous regions in thymic lobules and fulfilling an important role during specific stages of T-cell maturation. The process starts when bone marrow–derived lymphocyte precursors arrive at the outer cortical region of the thymic gland and begin to mature into functional T lymphocytes that will finally exit the thymus and populate the peripheral lymphoid organs. During their journey inside the thymus, thymocytes must interact with stromal cells (and their soluble products) and extracellular matrix proteins to receive appropriate signals for survival, proliferation and differentiation. The crucial components of the thymus microenvironment and their complex interactions during the T-cell maturation process with the objective of contributing to a better understanding of the function of the thymus as well as assist in the search for new therapeutic approaches to improve the immune response in various pathological conditions are summarized here.     

The thymus microenvironment in regulating thymocyte differentiation

The thymus plays a crucial role in the development of T lymphocytes by providing an inductive microenvironment in which committed progenitors undergo proliferation, T-cell receptor gene rearrangements and thymocyte differentiate into mature T cells. The thymus microenvironment forms a complex network of interaction that comprises non lymphoid cells (e.g., thymic epithelial cells, TEC), cytokines, chemokines, extracellular matrix elements (ECM), matrix metalloproteinases and other soluble proteins. The thymic epithelial meshwork is the major component of the thymic microenvironment, both morphologically and phenotypically limiting heterogeneous regions in thymic lobules and fulfilling an important role during specific stages of T-cell maturation. The process starts when bone marrow-derived lymphocyte precursors arrive at the outer cortical region of the thymic gland and begin to mature into functional T lymphocytes that will finally exit the thymus and populate the peripheral lymphoid organs. During their journey inside the thymus, thymocytes must interact with stromal cells (and their soluble products) and extracellular matrix proteins to receive appropriate signals for survival, proliferation and differentiation. The crucial components of the thymus microenvironment, and their complex interactions during the T-cell maturation process are summarized here with the objective of contributing to a better understanding of the function of the thymus, as well as assisting in the search for new therapeutic approaches to improve the immune response in various pathological conditions.Key words: thymus, T-cell maturation, thymic microenvironment, thymocyte differantiation, chemokines, extracellular matrix, thymic nurse cells, metalloproteinases     

Interferons mediate terminal differentiation of human cortical thymic epithelial cells

In the thymus, epithelial cells comprise a heterogeneous population required for the generation of functional T lymphocytes, suggesting that thymic epithelium disruption by viruses may compromise T-cell lymphopoiesis in this organ. In a previous report, we demonstrated that in vitro, measles virus induced differentiation of cortical thymic epithelial cells as characterized by (i) cell growth arrest, (ii) morphological and phenotypic changes, and (iii) apoptotis as a final step of this process. In the present report, we have analyzed the mechanisms involved. First, measles virus-induced differentiation of thymic epithelial cells is shown to be strictly dependent on beta interferon (IFN-beta) secretion. In addition, transfection with double-stranded RNA, a common intermediate of replication for a broad spectrum of viruses, is reported to similarly mediate thymic epithelial cell differentiation through IFN-beta induction. Finally, we demonstrated that recombinant IFN-alpha, IFN-beta, or IFN-gamma was sufficient to induce differentiation and apoptosis of uninfected thymic epithelial cells. These observations suggested that interferon secretion by either infected cells or activated leukocytes, such as plasmacytoid dendritic cells or lymphocytes, may induce thymic epithelium disruption in a pathological context. Thus, we have identified a new mechanism that may contribute to thymic atrophy and altered T-cell lymphopoiesis associated with many infections.     

Development of functional thymic epithelial cells occurs independently of lymphostromal interactions

The thymus provides a specialised microenvironment for the development of T-cell precursors. This developmental programme depends upon interactions with stromal cells such as thymic epithelial cells, which provide signals for proliferation, survival and differentiation. In turn, it has been proposed that development of thymic epithelial cells themselves is regulated by signals produced by developing thymocytes. Evidence in support of this symbiotic relationship, termed thymic crosstalk, comes from studies analysing the thymus of adult mice harbouring blocks at specific stages of thymocyte development, where it is difficult to separate mechanisms regulating the initial development of thymic epithelial cells from those regulating their maintenance. To distinguish between these processes, we have analysed the initial developmental programme of thymic epithelial cells within the embryonic thymus, in either the presence or absence of normal T-cell development. We show that keratin 5+8+ precursor epithelial cells present in the early thymic rudiment differentiate into discrete cortical and medullary epithelial subsets displaying normal gene expression profiles, and acquire functional competence, independently of signals from T-cell precursors. Thus, our findings redefine current models of thymus development and argue against a role for thymocyte-epithelial cell crosstalk in the development of thymic epithelial progenitors.     

Effects of neonatal thymic exposure to high doses of X-irradiation.

The thymic region of neonatal Swiss mice was exposed to doses varying from 1000 R to 2000 R of X-irradiation. The animals did not show any signs of wasting syndrome up to 6 months after irradiation. At this time hyperplasia of the thymus with an associated lymphocytosis was evident in irradiated animals. Antibody production to sheep red blood cells (SRBC) was not affected. However, at 12 months post-irradiation the animals showed signs of wasting disease with a progressive increase in their numbers at 18 and 24 months of age. The percentage incidence of animals with wasting disease was dose dependent. At this stage in the majority of the animals with the disease the thymus showed varying degrees of atrophy along with splenomegaly. There were no significant differences in the number of lymphocytes but the number of granulocytes showed a substantial increase. This was more evident in animals exposed to 2000 R to the thymic region. Though one observed a lowered ability to form antibodies to bovine serum albumin (BSA) with advancing age, the thymic irradiation did not affect the immune response to BSA even in animals manifesting wasting disease. An interesting observation has been the development of a severe loss of muscle power and tone in the hind limbs in a large majority of animals.     

Maturational impairment of thymic lymphocytes in streptozotocin-induced diabetes in rats

Streptozotocin (STZ)-induced diabetes in rats was associated with marked decreases in thymus weight and the number of thymic lymphocytes. Histologically, the cortical lymphocytes which were present near the cortico-medullary junction in the thymus seemed to be reduced selectively in the STZ-induced diabetes. Rosette-forming cells, which bind to guinea pig erythrocytes in the presence of fetal calf serum, were also significantly decreased. Insulin treatment allayed these intrathymic changes. Preincubation of thymic lymphocytes from diabetic rats with thymosin fraction 5 significantly enhanced the percentage of rosette-forming cells to near the control level. These results suggest that a maturational impairment of thymus cortical lymphocytes may be caused in STZ-induced diabetes with hypoinsulinemia and it may be intimately related to reductions in thymus weight and the number of thymic lymphocytes.     

斜带石斑鱼胸腺的显微和超微结构

本文通过解剖及组织切片技术、光学显微镜、透射和扫描电子显微镜技术,对斜带石斑鱼(Epinephelus coioides)胸腺器官组织进行了观察研究。结果表明:斜带石斑鱼胸腺实质主要由胸腺细胞(淋巴细胞)和网状上皮细胞构成。鱼体从Ⅰ龄之后,其胸腺发生明显的变化,与幼鱼有所不同,主要是胸腺可明显区分为三个区域:胸腺外皮质区、内皮质区和髓质区。外皮质区主要由网状上皮细胞、黏液细胞、成纤维细胞和少量淋巴细胞构成,细胞排列疏松;内皮质区主要由密集的淋巴细胞和网状上皮细胞组成,以含有大量的淋巴细胞为特征;髓质区主要由淋巴细胞和较多的网状上皮细胞构成,总体特征是淋巴细胞数量比内皮质区的少,且细胞排列较疏松。外皮质区、内皮质区相当于高等脊椎动物的皮质;髓质区相当于高等脊椎动物的髓质。髓质区之下有结缔组织,在Ⅱ龄以上的成体出现胸腺小体(Hassall's corpuscles)或类似胸腺小体的结构,而且随着年龄的增加,胸腺外皮质区增厚,结缔组织增加,还表现在内皮质区和髓质区组织逐渐萎缩变薄,胸腺的细胞组成类型和淋巴细胞数量上有所变化等等。这些现象在Ⅱ龄鱼开始出现,即胸腺呈现退化迹象,在Ⅲ龄以上鱼体呈现明显的退化和萎缩。胸腺表面扫描电镜结果表明:其上皮细胞表面具有微嵴以及由微嵴组成的指纹状结构,有一些微孔分布。透射和断面扫描电镜的结果进一步表明:胸腺组织内的细胞成分复杂,除了淋巴细胞和网状上皮细胞外,还具有巨噬细胞、肥大细胞、肌样细胞、浆细胞、指状镶嵌细胞和纤维细胞等。     

Ontogeny and organization of the stationary non-lymphoid cells in the human thymus

Summary Ontogenetic differentiation of the human thymus was investigated in 50 embryos by means of light and electron microscopic methods in an attempt to clarify the morphogenesis of the complicated microecology of thymic tissue. At the 8th gestational week (g.w.), the primordium of the thymus contains almost exclusively undifferentiated epithelial cells. At the 10th g.w., the epithelial cells in the central part are spindle-shaped. During the subsequent weeks the cortical region of the thymus becomes separated into lobes by mesenchymal septa containing hemopoietic precursor cells and large electronlucent cells with irregularly shaped nuclei. The latter cells are also found in the deeper presumptive medullary regions of the thymus; they differentiate into interdigitating reticulum cells (IDC). The permeation of the medulla of the thymus by non-epithelial IDC occurs concurrently with the formation of cortical and medullary epithelial cells. Between the 12th and 14th g.w. the cortical and medullary differentiation is completed. At this time-stage cortical small lymphocytes differ in morphological shape from medullary lymphocytes, the latter acquiring the appearance of immunocompetent T cells and establishing intimate contact with the IDC.These findings indicate that the thymic cortex and medulla contain different epithelial cells. In addition, the thymic medulla displays cells characterized by the morphology of typical interdigitating reticulum cells of peripheral lymphoid tissue. The structural pattern of the thymus is correlated to morphologically differing lymphoid cell populations in the cortical and medullary regions.This investigation was supported by grants from the Deutsche Forschungsgemeinschaft and by the Sonderforschungsbereich 111The authors dedicate this paper to Professor Helmut Leonhardt on the occasion of his 60th birthday. The authors also appreciate the excellent technical assistance of Mrs. I. Knauer, Mrs. H. Waluk and Mrs. H. Siebke     

Hydrolase histochemistry of the thymus: development and species variations

In the present investigation the localization and activity of alkaline, neutral, and acid hydrolases of the thymus were studied during development of rats and mice and of various adult species using histochemical methods. If different procedures of tissue pretreatment were employed, several inhibition effects and morphological as well as enzyme histochemical artifacts occurred dependent on the mode of tissue pretreatment. After embedding in glycol methacrylate, sections of the thymus showed a better structural preservation than cryostat sections but were accompanied by a drastic decrease of activity and low localization quality of the final reaction products especially in the case of protease studies with 4-methoxy-2-naphthylamine peptides as substrates. Smears of thymic cells facilitated the allocation of enzymes to mobile or fixed cells in the stroma of the thymus. The perivascular localization of aminopeptidase M could only be shown with combined techniques. In comparison, primarily the proteases yielded information on the thymic stroma and in this context especially on the epithelial reticular cells and the stroma proper but also on thymocytes (lymphocytes) and enabled a species-dependent subdivision of the thymic reticulum already in the light microscope. Enzyme histochemically the development of the rat and mouse thymus could be subdivided into an early period and perinatal (pre- and postnatal) period of functional differentiation. Morphological (proliferation of cortical lymphocytes) and enzyme histochemical changes (disappearance of dipeptidylpeptidase IV, significant loss of alkaline phosphatase activity and beginning activity increase of aminopeptidase M) occurred primarily at the transition from the early to the prenatal period. During the postnatal phase, a significant activation of lysosomal enzymes in the thymic medulla and general enzymatic differentiation of the cortical epithelial reticular cells were found. Species differences and species similarities for the respective enzymes and their localization as well as for the thymic cells were noticed for adult rats, mice, guinea-pigs, hamsters, and marmoset monkeys. Differences were true especially for the thymocytes; less species differences were seen for the epithelial reticular cells; capsular and perivascular connective tissue and the macrophages behaved rather similarly. Species-independently certain medullary epithelial reticular cells showed high and typically localized alkaline phosphatase activities and species-dependently also high activities of neutral hydrolases.(ABSTRACT TRUNCATED AT 400 WORDS)     

The immunoproliferative response of mice to intravenously or subcutaneously injected BCG

Summary Intravenous injection of BCG caused (1) a transient thymic epithelial hyperplasia with increase of PAS-positive cells in the cortex and medulla which showed the pronounced secretory activity of a substance which could be histochemically identified as an acid mucopolysaccharide; (2) an equally transient increase in the number of pyroninophilic lymphocytes with increased polyribosome content of the cells and mitoses in the thymic cortex; this reached a peak on day 6 following the injection but was unassociated with an increase in thymic weight; and (3) a systemic granulomatous histiocytic reaction in the liver, spleen, lungs, and lymph nodes, but not in the thymus, bone marrow, or Peyer's patches. The significance of the thymic epithelial changes is not clear but it did coincide with increased pyroninophilia and mitotic activity of the thymic cortical cells, suggesting a possible interaction between this secretory product and the thymic cortex. Comparing the thymic changes with the thymus of other animals of the same species injected with i.v. or i.p. LPS, i.v. or s.c. HIU II fraction of BCG, i.v. pertussis vaccine, i.p. complete or incomplete Freund's adjuvant, and killed at the same planned intervals after the injection of the adjuvants, BCG proved to have a unique action on the thymus with regard to both lymphocytic and epithelial changes. Hepatic, pulmonary, and splenic histiocytic granulomas were observed only in those animals injected intravenously with BCG.     

Architectural changes in the thymus of aging mice

Age-associated thymic involution is one of the most dramatic and ubiquitous changes in the immune system, although the precise mechanisms involved still remain obscured. Several hypotheses have been proposed incorporating extrinsic and intrinsic factors, however, changes in the thymic microenvironment itself is one of the least investigated. We therefore decided to undertake a detailed histological examination of the aging thymus in order to elucidate possible mechanisms of thymic atrophy. This investigation provides insight into the changes within the murine thymus with age, demonstrating a new approach to quantify protein expressional differences while preserving the thymic architecture. There is a decline in expression of thymic epithelial cell-specific makers and an increase in fibroblast content in the aging mouse thymus. This is concurrent with a disorganization of the thymic compartments, a morphological transformation within the epithelial cells and alterations of their archetypal staining patterns. Furthermore, this is linked to a rise in apoptotic cells and the novel finding of increased senescence in the thymus of older mice that appears to be colocalized in the epithelial compartment. These changes within the thymic epithelial cells may be in part accountable for thymic atrophy and responsible for the decline in T-cell output.     

The characterization of the monoclonal antibody Th-10a, specific for a nuclear protein appearing in the S phase of the cell cycle in normal thymocytes and its unregulated expression in lymphoma cell lines

A monoclonal antibody (Th-10a) specific for the nuclear protein appearing in the S phase of the cell cycle in normal mouse thymocytes was derived by immunizing Wistar rats with a murine thymic lymphoma (TIGN), and its isotype was rat IgG_2a and had κ light chain. Immunohistochemical staining of frozen sections of B10.Thy1.1 newborn thymus and embryonic intestine revealed that this monoclonal antibody reacted strongly with the nuclear proteins of subcortical thymocytes and the basal layer of the mucosa, where many cells were dividing, but not with that of the thymic medullary area. To evaluate the expression of the nuclear proteins during the cell cycle in detail, the results of an immunofluorescence analysis of the thymocytes from hydroxyurea-treated B10 mice using Th-10a monoclonal antibody were compared with those of DNA synthesis of these cells with the use of the FITC-conjugated anti-BrdUrd monoclonal antibody. The results indicated that the nuclear protein detected by Th-10a monoclonal antibody was highly expressed in the S phase of normal thymocytes, while the cells in G₁, G₂ and M phases exhibited a low level of the expression. Moreover, the variations in expression of the nuclear proteins in the thymocytes at different times after hydroxyurea treatment were observed to correspond with the frequency of DNA synthesizing cells. In contrast, the high level and unregulated expression of the nuclear protein detected by Th-10a monoclonal antibody was observed throughout the cell cycle of the mouse lymphoma cell lines examined. Since Th-10a monoclonal antibody does not react with the nuclear proteins derived from human, hamster or rat proliferating cells, this antibody may recognize a murine specific epitope of the nuclear protein. To further characteize the nuclear proteins, we extracted them from normal thymocytes or thymic lymphomas, and analysed them by immunoblotting or***     

Immunoendocrine modulation and stimulation of hematopoiesis with the ionophore copolymer T150R1

T150R1, an 8000-dalton copolymer with sodium ionophore activity, has been shown to modulate cellular responses in multiple systems. In this article, we studied its effects on lymphoid and hematopoietic organs in the context of the adrenal-pituitary axis. When injected in mice as an oil in water emulsion, T150R1 caused a rapid, profound, and dose-dependent thymic involution accompanied by splenic hyperplasia. Time course experiments with a 2.5-mg dose revealed that the thymus size was minimal at Day 2, rose to normal by Day 14, then enlarged and gradually returned to normal by Week 6 postinjection. Thymic involution was due to cellular depletion of the cortical area, whereas thymic enlargement was due to cortical hyperplasia. Splenomegaly was seen as early as Day 4, peaked by Day 14, and gradually returned to normal by Week 6. The splenic enlargement was due to hyperplasia of the red pulp, with evidence of proliferating erythropoietic, myelopoietic, and megakaryopoietic precursors. In addition, the bone marrow was stimulated and extramedullary hematopoiesis was present in the liver. The effects of T150R1 on the thymus appeared to be mediated by corticosteroids while the effects on hematopoiesis were not. Corticosterone and ACTH levels were increased in treated animals. Adrenalectomy diminished the T150R1-induced thymic involution but enhanced the splenic hyperplasia. Hypophysectomy did not prevent thymic involution, suggesting that T150R1 has endocrine stimulatory effects. These data suggest that T150R1 represents a new class of ionophores which may act on excitable cells within the endocrine, immune, and hematopoietic systems.     

Multilocular thymic cyst with follicular lymphoid hyperplasia in a male infected with HIV. A case report with fine needle aspiration cytology

BACKGROUND: Multilocular thymic cyst with follicular lymphoid hyperplasia is a rare complication in HIV-infected patients, causing pseudotumorous enlargement of the anterior mediastinum. There have been six reported cases, all with only histologic findings. This paper reports another such case and includes perhaps the first cytologic findings on this rare entity. CASE: A 35-year-old, HIV-infected male intravenous drug abuser, who complained of worsening central chest discomfort and pain on deep inspiration, was found to have a large, septated anterior mediastinal mass. Computed tomography-guided fine needle aspiration biopsy was performed. The cytologic presentation mimicked that of thymoma, with cystic degeneration and a dual population of epithelial cells and lymphocytes as well as large aggregates of "epithelial" cells intermixed with lymphocytes in a background of macrophages and cyst fluid. Histologic examination of the resected mass revealed a multilocular thymic cyst with follicular lymphoid hyperplasia. HIV-1 core protein p24 was localized immunohistochemically in the dendritic follicular cells of the germinal centers. In retrospect, the quantity of epithelium derived from the cyst lining was too scanty for thymoma, and the presence of plasma cells and lymphohistiocytic aggregates suggested follicular lymphoid hyperplasia. CONCLUSION: Multilocular thymic cyst with follicular lymphoid hyperplasia should be considered in the differential diagnosis of an anterior mediastinal mass in HIV-infected patients after lymphoma and tuberculosis.     

Thymic epithelial progenitor cells and thymus regeneration： an update

The thymus provides the essential microenvironment for T-cell development and maturation. Thymic epithelial cells （TECs）, which are composed of thymic cortical epithelial cells （cTECs） and thymic medullary epithelial cells （mTECs）, have been well documented to be critical for these tightly regulated processes. It has long been controversial whether the common progenitor cells of TECs could give rise to both cTECs and mTECs. Great progress has been made to characterize the common TEC progenitor cells in recent years. We herein discuss the sole origin paradigm with regard to TEC differentiation as well as these progenitor cells in thymus regeneration.     

Unexpected thymic hyperplasia in transgenic mice harboring a neuronal promoter fused with simian virus 40 large T antigen.

The hypothalamic peptide growth hormone-releasing factor (GRF) regulates the secretion and production of growth hormone from the anterior pituitary (M. C. Gelato and G. R. Merriam, Annu. Rev. Physiol. 48:569-591). To study GRF gene regulation, transgenic mice were generated that harbor the human GRF promoter fused to the coding sequences from the simian virus 40 early region. These mice had normal hypothalamic functions but unexpectedly suffered from severe thymic hyperplasia. Immunohistochemical analysis revealed that large T antigen was expressed in the thymic epithelial cells. These cells have endocrine properties and are known to produce thymic hormones [corrected]. The thymic hyperplasia was the apparent consequence of inappropriate production of T-cell maturation factors by epithelial cells and could involve increased self renewal of apparently normal T stem cells in the thymus.     

Galectin-1 is expressed by thymic epithelial cells in myasthenia gravis

Galectin-1, a member of a family of carbohydrate binding proteins, is synthesized by thymic epithelial cells in normal juvenile thymus, and mediates adhesion of immature T cells to thymic epithelium. Because cell adhesion molecules are proposed to play a role in the thymic hyperplasia and neoplasia seen in the autoimmune disease myasthenia gravis, we examined the expression of galectin-1 in myasthenic thymi. We detected abundant galectin-1 expression in thymic epithelial cells in 27 hyperplastic and neoplastic thymi from patients with myasthenia gravis. Primary cultures of neoplastic epithelial cells from a thymoma continued to express galectin-1, and bound immature T cells; T cell binding was inhibited by the addition of the -galactosides lactose and thiodigalactoside, suggesting that galectin-1 on the thymoma cells and a saccharide ligand on the T cells participated in cell-cell adhesion. Expression of galectin-1 by thymic epithelial cells may play a role in the thymic pathology seen in myasthenia gravis.