Ontogeny of rat thymic epithelium defined by monoclonal anticytokeratin antibodies.

Ontogenetic study on the expression of cytokeratin (CK) polypeptides within particular subsets of rat thymic epithelial cells (TEC) has been performed by a large panel of anti-CK monoclonal antibodies (mAbs) using the streptavidin-biotin immunoperoxidase method. Simultaneous presence of two or more CK subunits in the same TEC has been demonstrated by double immunofluorescence labeling. The obtained results showed that the expression of CK polypeptides in fetal and neonatal thymus differed from the adult patterns. The main difference was observed in expression of CK10, 18, and 19 polypeptides. During fetal ontogeny, CK10 and 18 are markers for most medullary TEC or a subset of medullary TEC, respectively, whereas CK19 is mainly a pan-TEC marker. In the adult animals, they are localized in the cortical and a subset of medullary TEC (CK18), subcapsular/perivascular and some medullary TEC (CK19), or in a subset of medullary TEC and Hasall's corpuscles (HC) (CK10). The switch in their expression in the cortex was observed during the first two weeks of postnatal life.     

Ontogeny of Rat Thymic Epithelium Defined by Monoclonal Anticytokeratin Antibodies

Ontogenetic study on the expression of cytokeratin (CK) polypeptides within particular subsets of rat thymic epithelial cells (TEC) has been performed by a large panel of anti-CK monoclonal antibodies (mAbs) using the streptavidin-biotin immunoperoxidase method. Simultaneous presence of two or more CK subunits in the same TEC has been demonstrated by double immunoflouorescence labeling. The obtained results showed that the expression of CK polypeptides in fetal and neonatal thymus differed from the adult patterns. The main difference was observed in expression of CK10, 18, and 19 polypeptides. During fetal ontogeny, CK10 and 18 are markers for most medullary TEC or a subset of medullary TEC, respectively, whereas CK19 is mainly a pan-TEC marker. In the adult animals, they are localized in the cortical and a subset of medullary TEC (CK18), subcapsular/perivascular and some medullary TEC (CK19), or in a subset of medullary TEC and Hasall’s corpuscles (HC) (CK10). The switch in their expression in the cortex was observed during the first two weeks of postnatal life.     

Immunohistochemical identification of six cytokeratin-defined subsets of the rat thymic epithelial cells

Rat thymic epithelial cells (TEC) have been studied by a panel of monoclonal antibodies specific for single cytokeratin polypeptides or cytokeratin pairs. Using various combinations of single and double immunostainings 6 TEC subsets (CK types) were identified, each characterized by different cytokeratin expression. Subcapsular/perivascular TEC (TEC-CK type 1) share cytokeratins 7, 8, 19 with a subset of medullary TEC, while cortical TEC were reactive with anti-CK 8 and anti-CK 18 mAbs only (TEC-CK type 2). Additional 4 subsets were identified in the medulla; TEC-CK type 3 (CK 8 + 18 + 19 +), TEC-CK type 4 (CK 8 + 10 + 18 + 19 +), TEC-CK Type 5 (CK 8 + 10 + 10 +) and TEC-CK type 6 positive only with CK 8 of all above cytokeratins. This study extends the concept of TEC heterogeneity and might also be useful to further understanding of TEC origin, development and functions.     

Phenotypic and ultrastructural characterization of an epithelial cell line established from rat thymic cultures.

An epithelial cell line (TE-R 2.5) was established from a long-term culture of rat thymic epithelium. Its epithelial nature was confirmed using anti-cytokeratin (CK) monoclonal antibodies (mAb) and electron-microscopy. TE-R 2.5 cells were reactive with K 8.13, K 8.12, CK 8, R-MC 18, R-MC 19 and Mar 3 mAb and bind Ulex europaeus agglutinin I. Based on the results of this study it was concluded that they possess the phenotype of subcapsular/perivascular or medullary epithelium. This was in accordance with Western blot analysis of water-insoluble cell extracts showing the presence of 56,000, 52,000, 50,000 and 48,000 MW CK polypeptides. In addition, TE-R 2.5 cell line coexpressed CK and vimentin (a 57,000 MW polypeptide) which was demonstrated using dual immunohistochemistry and Western blot analysis. Electron microscopy demonstrated that TE-R 2.5 cells have all the characteristics of hypertrophic thymic epithelial cells (TEC) localized in situ exclusively in the medulla and thus further characterized this line as a type of medullary TEC. Finally, TE-4F10 mAb raised against an antigen of TE-R 2.5 cells selectively stained a subset of medullary TEC in situ including Hassall's corpuscles indicating again the medullary origin of this TEC line.     

Rat thymic epithelial cells in culture constitutively secrete IL-1 and IL-6.

To study the in vitro interactions between rat thymic non-lymphoid cells and thymocytes, we established a system for long-term cultivation of thymic epithelial cells (TEC). TEC were cultivated and successfully propagated for over 8 months in RPMI 1640 medium containing 15% FCS, dexamethasone, insulin, epidermal growth factor, and poly-L-lysin as an adhesive matrix. Their epithelial nature has been confirmed using monoclonal anti-cytokeratin (CK) antibodies. More than 95% of these cells were reactive with K 8.13 and CK 8 mAbs, which are pan-epithelial markers for rat TEC in situ. An epithelial cell clone (TE-R 2.5) established from a long-term TEC culture was 100% reactive with these anti-CK antibodies. Phenotypic analysis of TEC cultures was performed by a large panel of mAbs reactive with a subset of rat TEC or CK polypeptides as well as UIex europaeus agglutinin I using a streptavidin-biotin immunofluorescence assay. Although the results obtained demonstrated phenotypic heterogeneity among these cells, most cultures, including the TE-R 2.5 clone, were of subcapsular/medullary phenotype. Medium conditioned by TEC cultures exhibited IL-1 and IL-6 activities when tested on D10S and B9 sensitive cell lines, respectively. Cytokine activities were neutralized (IL-1) or significantly inhibited (IL-6) by specific polyclonal antibodies. In addition, both anti-IL-1 and anti-IL-6 antibodies reacted with TEC in culture and epithelial (CK-positive) cells on thymic cryostat sections, indicating that thymic epithelium provides an important intrathymic source for molecules contributing to T cell activation.     

Epithelial cell heterogeneity in the guinea pig thymus: immunohistochemical characterization of four thymic epithelial subsets defined by monoclonal anti-keratin antibodies

Keratins are a family of related polypeptides constitutive of the cytoskeleton of epithelial cells and are never found in nonepithelial tissues. Thymic epithelial cells (TEC), known to induce T cell differentiation, are the keratin-containing cells within the thymus. Using four monoclonal anti-keratin antibodies (KL1, KL4, AE2, AE3) directed against keratins of different molecular weight, we have investigated the guinea pig thymic epithelium. The immunohistochemical analysis of thymic cryostatic sections revealed that the keratin expression of TEC varied according to their location in the thymic lobula; the thymic cortex was specifically stained by AE3 whereas the thymic medulla and the subcapsular cortex were recognized by KL4. In addition, KL1 and AE2 exclusively labeled Hassall's corpuscles. The biochemical analysis of keratins extracted from the thymus showed that each TEC subset was characterized by an unique pattern of keratin polypeptides. This study extends the concept of thymic epithelium heterogeneity and suggests that anti-keratin antibodies which allow the typing of TEC subsets may be valuable tools for studying the differentiation of thymic epithelium and its in vitro function on T lymphocytes.     

Is There an Interspecific Diversity of the Thymic Microenvironment?

Thymic epithelial cells (TEC) heterogeneity suggests the existence of functional subsets. Anti-cytokeratin (Anti-CK) monoclonal antibodies (MAb), markers of epithelial differentiation, have been used to detect TEC subsets in rodents and humans. These MAb revealed a different topography of CK-defined TEC subsets in mice and humans, leading us to carry out a comparative study of mammalian thymuses. Our study showed that the distribution pattern of cytokeratins in the thymic epithelium is complex and unique, with coexpression of CK typical of simple and stratified epithelia. Moreover, we demonstrated an interspecific diversity of CK expression within the thymic lobules. Interestingly, such diversity was not a general phenomenon for the expression of any thymic microenvironmental proteins, because the location of extracellular matrix components was essentially similar in the mammalian species studied.     

Modulation of Cytokeratin Expression in The Hamster Thymus: Evidence For a Plasticity of The Thymic Epithelium

Cytokeratin (CK) expression was investigated, by means of immunocytochemistry, in the hamster thymic epithelium during ontogeny, as well as in primary cultures and upon glucocorticoid hormone treatment in vivo. As compared to the distribution pattern of distinct monoclonal antibody-defined cytokeratins in the normal adult thymus, CK modulation was evidenced in the three situations studied. During thymus ontogeny, both cytokeratins of simple lining epithelia, as CK8 and CK18, as well as the CK1/CK10 pair (typical marker of terminal stage of keratinization), were expressed since early stages of thymus development. They were located in the central region of thymic lobules preceding the cortical-medullary distinctions. This differed from what had been previously shown for mouse thymus ontogeny, revealing that the interspecific diversity in the distribution pattern of thymic cytokeratins occurred early in fetal life. A modulation of CK expression was also detected when hamster thymic epithelial cells (TEC) were led to grow in culture, with a down-regulation of CK19 contrasting with an enhancement of CK18 expression. This diverged from the maintenance of the in situ pattern when human TEC were cultured. Last, in vivo hydrocortisone treatment, known to increase the numbers of KL1⁺ cells in the mouse thymus medulla, promoted a cortical expression of the CK1/CK10 pair in the hamster thymus. Taken together, our findings demonstrate a continuous plasticity of the thymic epithelium, at least regarding cytokeratin expression, and enlarge the concept of interspecific diversity of intrathymic CK distribution in conditions as morphogenesis, in vitro system, and responsiveness to glucocorticoid hormone treatment.     

Heterogeneity of rat thymic epithelium defined by monoclonal anti-keratin antibodies

Immunohistochemical characterization of rat TEC has been studied using a panel of monoclonal anti-keratin antibodies. These mAbs identified three distinct patterns of keratin subunit expression within thymic epithelium assessed by streptavidin-biotin immunoperoxidase staining and double immunofluorescence staining. K8 and KII mAbs labelled almost all epithelium, while K18 stained cortical epithelium and a subpopulation of medullary TEC. KI, K7 and K19 mAbs bound to the subcapsular/subtrabecular flat epithelial cell layer, TEC lining some perivascular spaces in the cortex, a subpopulation of medullary TEC and interstitial epithelial cystic structures. Double immunostaining revealed further heterogeneity of subunit keratin compositions in epithelial cells of particular thymic microenvironments suggesting their different origin or development stage.     

人CK8&18阳性胸腺上皮细胞对脐血CD34~+细胞向T细胞分化的影响

目的:通过CK8/18阳性胸腺上皮细胞与人脐血单个核细胞在Transwell板的共培养,探讨CK8/18阳性TEC对T细胞增殖分化的影响。方法:用胶原酶消化法分离纯化胸腺上皮细胞、免疫组化鉴定分离纯化的TEC,采用密度梯度离心法分离获得脐血单个核细胞,并采用免疫磁珠分选CD34+细胞,将TEC种植培养于Transwell双层培养板上层,使其均匀平铺于上层板底部,再将分选后的细胞加入上层小室,经过48小时的共培养,流式细胞术检测进入下室细胞的表型变化。结果:分离纯化的TEC经免疫组化鉴定CK8/18阳性。TEC与脐血单个核细胞共培养后,CD3+CD4-CD8-双阴性、CD3+CD4+CD8-单阳性细胞显著增加,CD3+CD4+CD8+双阳性细胞亚群细胞减少,CD8单阳性细胞增加不明显;CD45RA阳性细胞比率无显著变化,CD45RO阳性细胞比率显著增加。结论:CK8/18阳性TEC能选择促进脐血单个核细胞CD4+T细胞和CD45RO+细胞增殖。     

Cytokeratin (CK5, CK8, CK14) expression and presence of progenitor stem cells in human fetal thymuses

The aim of the current study was to observe the expression of cytokeratins in human fetal thymuses. Specific cytokeratin markers in adult humans and mice have been well described but there has been little similar work on human fetuses. We also aimed to see whether progenitor stem cells that could be harvested to treat various immunodeficiency disorders are present in fetal thymic tissue. Thymuses obtained from 30 aborted human fetuses (12 to 31 weeks) were examined immunohistochemically to investigate changes in cytokeratin expression in the epithelial cells (TEC) at various gestational ages. Before 16 weeks of gestation, cortical (cTEC) and medullary (mTEC) TEC exhibited homogenous staining for cytokeratins CK8 and CK5. After 16 weeks there was differential staining, with cTEC positive for CK8 and mTEC for CK5 and CK14. Interestingly, both CK5 + CK8+ progenitor stem cells were present in the fetal thymic cortex at all gestational ages, with a relatively high number from 12 to 16 weeks. Cytokeratin expression in fetal thymuses was quite different from that in the adult thymus owing to the presence of undifferentiated progenitor stem cells in fetal thymic stroma along with differentiated TEC. The best time to harvest these progenitor stem cells from fetal thymic stroma in order to treat various immune deficiency disorders appears to be 12–16 weeks. Clin. Anat. 29:711–717, 2016. © 2016 Wiley Periodicals, Inc.     

The epithelial framework of the thymus in normal and pathological conditions

Summary Autopsy specimens of normal human thymus, from cases of accidental involution, follicular hyperplasia, thymomas and a teratoma were investigated by immunocytochemistry using specific immune sera to small and large keratins. Keratin antisera represent a marker of both Hassall's corpuscles (HC) and so-called epithelial reticular cells. There were no apparent differences in keratin polypeptides distribution between cortical and medullary thymic epithelial cells. In accidental involution, the epithelial framework became prominent: epithelial cortical borders and epithelial perivascular sheaths appeared often to be discontinous structures. The central and occasionally cystic spaces of HC did not react with keratin antisera. In follicular hyperplasia, almost solid epithelial aggregates were seen which were located around germinal centers. In thymic tumours, neoplastic epithelial cells displayed a marked immunorectivity with keratin antisera. Immune sera against keratin filaments represent an interesting tool in thymus research and in the diagnostic pathology of thymic tumours.Dedicated to Professor Dr. G. Seifert on the occasion of his 60th birthdayThis study was supported by a grant from the Deutsche Forschungsgemeinschaft (Lo 285/2-1; Ot 53/4-7)     

Studies on the thymus in nonobese diabetic mouse. I. Changes in the microenvironmental compartments

The nonobese diabetic (NOD) mouse develops an autoimmune type I diabetes, which is predominantly seen in females, is triggered by T cells, and whose frequency is enhanced following thymectomy at weaning. Attempting to characterize a thymic pathology in these animals, we analyzed the microenvironmental compartment of the organ with respect to structural and functional molecules expressed by thymic epithelial cells (TEC), as well as extracellular matrix components. We observed, in both males and females, a precocious decrease in the cell numbers of discrete medullary TEC subsets, namely, those respectively defined by the expression of cytokeratins 3/10 and cytokeratin 19. In addition, some cells bearing the TR.5 phenotype (normally restricted to the medulla) could be detected in the NOD mouse thymic cortex. There was also a significant early decrease in thymulin production in females, as compared to males. As regards the extracellular matrix compartment, the most striking alteration was the presence of abnormally enlarged perivascular spaces, increasing in size with age. In these structures large amounts of T cells and, to a lesser extent, B cells were consistently encountered. In addition to B cells, the NOD mouse thymus showed on both TEC and extracellular matrix the presence of deposits of immunoglobulins, revealed with fluorescence-labeled goat anti-mouse Ig sera. Finally, the NOD mouse sera labeled both TEC and extracellular matrix proteins on normal mouse thymus frozen sections. Together, these data clearly demonstrate that the NOD mouse thymus undergoes a variety of microenvironmental changes, whose particular role in the pathophysiology of the disease is yet to be demonstrated.     

Primary thymic epithelial tumours of the pleura mimicking malignant mesothelioma

AIMS: To illustrate the macroscopic, light microscopic and immunophenotypic similarities that exist between primary pleural thymic epithelial tumours and diffuse malignant mesothelioma. To investigate the expression of the mesothelial markers, cytokeratin (CK) 5/6, calretinin and thrombomodulin in a series of mediastinal thymic epithelial tumours. METHODS AND RESULTS: Over a 10-year period, 64 diffuse pleural tumours of non-mesothelial histogenesis were identified in the files of referrals to the South Wales regional thoracic centre (Llandough Hospital, Cardiff). Of these, five pleural tumours were diagnosed as primary pleural thymic epithelial neoplasms. From the files of the Mesopath group, Caen, three additional cases of thymic epithelial tumours with pleural involvement were identified. The study group comprised eight cases (four males, four females) with median age at presentation of 56 years (range 19-75 years). In one case there was a history of asbestos exposure. Macroscopically, seven tumours formed diffuse pleural masses. No mediastinal abnormality or intraparenchymal lesions were seen in five cases. By light microscopy, seven thymic epithelial neoplasms showed a lobulated architecture, one appeared extensively cystic. The tumours were of varied morphological subtypes: one medullary (WHO Type A), two mixed (WHO Type AB), three predominantly cortical (WHO Type B1) and two cortical (WHO Type B1). The subtypes morphologically mimicked sarcomatoid, biphasic, lymphohistiocytoid variant and epithelioid mesothelioma. The pleural thymic epithelial tumours showed immunoreactivity with broad spectrum cytokeratin AE1/AE3 (8/8; 100%), CK5/6 (8/8; 100%), and 1/8 (13%) expressed thrombomodulin. Calretinin showed variable nuclear and cytoplasmic expression in all cases, but equivocally in the thymic epithelial cell component. In 7/8 (88%) the thymic epithelial cells exhibited focal aberrant expression of CD20. Epithelial membrane antigen (EMA) showed focal expression in the perivascular and organoid areas in 6/8 (75%) cases. Carcinoembryonic antigen (CEA) and CD34 were uniformly negative. In 4/8 (50%) cases the lymphoid cell component was of immature phenotype expressing CD99, terminal deoxynucleotidyl transferase (TdT) and lymphoid precursors had a high proliferation fraction with Ki67. In the series of 20 primary mediastinal thymic epithelial tumours tested, mesothelial marker expression revealed CK5/6 (20/20), thrombomodulin (3/20; 15%) and calretinin (0/20; 0%). Varying amounts of calretinin-positive stromal cells were present. CONCLUSION: Primary pleural thymic epithelial tumours are rare but may mimic malignant mesothelioma by forming diffuse serosal-based masses. In addition, both tumours may show morphological diversity (with epithelial, spindled and mixed components present). An awareness that thymic epithelial tumours may variably express the mesothelial markers CK5/6, calretinin and thrombomodulin prevents misdiagnosis. In the distinction from malignant mesothelioma a lobulated architecture and organoid features favour a thymic epithelial neoplasm. The presence of aberrant CD20 expression in a cytokeratin-positive epithelial neoplasm and/or the presence of an immature lymphoid population (by demonstration of CD1a, CD2, CD99 and TdT) indicates a thymic epithelial neoplasm. In contrast, nuclear calretinin expression favours malignant mesothelioma.     

The thymus gland: a target organ for growth hormone

Increasing evidence has placed hormones and neuropeptides among potent immunomodulators, in both health and disease. Herein, we focus on the effects of growth hormone (GH) upon the thymus. Exogenous GH enhances thymic microenvironmental cell-derived secretory products such as cytokines and thymic hormones. Moreover, GH increases thymic epithelial cell (TEC) proliferation in vitro, and exhibits a synergistic effect with anti-CD3 in stimulating thymocyte proliferation, which is in keeping with the data showing that transgenic mice overexpressing GH or GH-releasing hormone exhibit overgrowth of the thymus. GH also influences thymocyte traffic: it increases human T-cell progenitor engraftment into the thymus; augments TEC/thymocyte adhesion and the traffic of thymocytes in the lymphoepithelial complexes, the thymic nurse cells; modulates in vivo the homing of recent thymic emigrants, enhancing the numbers of fluroscein isothiocyanate (FITC)+ cells in the lymph nodes and diminishing them in the spleen. In keeping with the effects of GH upon thymic cells is the detection of GH receptors in both TEC and thymocytes. Additionally, data indicate that insulin-like growth factor (IGF)-1 is involved in several effects of GH in the thymus, including the modulation of thymulin secretion, TEC proliferation as well as thymocyte/TEC adhesion. This is in keeping with the demonstration of IGF-1 production and expression of IGF-1 by TEC and thymocytes. Also, it should be envisioned as an intrathymic circuitry, involving not only IGF-1, but also GH itself, as intrathymic GH expression is seen both in TEC and in thymocytes, and that thymocyte-derived GH could enhance thymocyte proliferation. Finally, the possibility that GH improve thymic functions, including thymocyte proliferation and migration, places this molecule as a potential therapeutic adjuvant in immunodeficiency conditions associated with thymocyte decrease and loss of peripheral T cells.     

Human scavenger receptor B1 is involved in recognition of apoptotic thymocytes by thymic nurse cells

Recognition and uptake of apoptotic cells by neighboring phagocytes is essential for the clearance of dying cells without accompanying inflammation or tissue damage. In the thymus, many apoptotic cells are generated in the process of negative selection, and both thymic macrophages (professional phagocytes) and nursing thymic epithelial cells (nursing TEC; nonprofessional phagocytes) recognize and ingest them. However the receptors responsible for this recognition and uptake have not been identified. In the present study, we have established a human nursing TEC line and examined the expression of several genes of the scavenger receptor family considered to be potential receptors for apoptotic cells. Human scavenger receptor-B1 (hSR-B1)/CLA-1, previously shown to recognize apoptotic cells, was strongly expressed in nursing TEC, whereas there was little or no expression of the other scavenger receptors tested: scavenger receptor class A, CD36, or CD68. Suppression of hSR-B1/CLA-1 expression using antisense oligonucleotides decreased the binding of apoptotic thymocytes to nursing TEC by more than 40%. These results indicate that hSR-B1/CLA-1 may play a major role in the clearance of apoptotic cells in the thymus, mediating the recognition and ingestion of apoptotic thymocytes by nursing TEC.     

Optimization of culture and storage conditions for an in vitro system to evaluate thymocyte phenotype and function

Studies on thymopoiesis are critical to the understanding of T-cell homeostasis as well as the host response to T-cell depletion. Various in vitro culture systems have been used in the study of thymocyte development; however it is unclear if current co-culture methods have been fully optimized. In this study in vitro suspension cultures have been re-evaluated and the optimal storage conditions for thymocytes have been established by evaluating various methods of storing/isolating thymic tissue and isolated thymocytes as well as the source of thymic epithelial cells (TEC). It was determined that thymocytes must be freshly isolated from whole thymic tissue and ideally stored at 4 degrees C prior to co-culture. Co-culture with either autologous or allogeneic TEC results in similar thymocyte subset distribution as well as interleukin-7 receptor-alpha (CD127) expression on these subsets. To evaluate the influence of the source of TEC on one aspect of thymocyte function the effect of IL-7 stimulation on the expression of CD127 was evaluated. IL-7 stimulation resulted in a downregulation of the expression of CD127 on all thymic subsets similar to that observed in circulating CD8+ T-cells. The effect of this was the same whether TEC were autologous or allogeneic. Optimizing culture techniques and facilitating the study of individual thymocyte subsets will lead to a better understanding of thymic function and development. It could also lead to therapeutic approaches that enhance immune recovery after T-cell depletion in HIV infection, bone marrow transplantation or following chemotherapy.     

The Wnt signaling antagonist Kremen1 is required for development of thymic architecture

Wnt signaling has been reported to regulate thymocyte proliferation and selection at several stages during T cell ontogeny, as well as the expression of FoxN1 in thymic epithelial cells (TECs). Kremen1 (Krm1) is a negative regulator of the canonical Wnt signaling pathway, and functions together with the secreted Wnt inhibitor Dickkopf (Dkk) by competing for the lipoprotein receptor-related protein (LRP)-6 co-receptor for Wnts. Here krm1 knockout mice were used to examine krm1 expression in the thymus and its function in thymocyte and TEC development. Krm1 expression was detected in both cortical and medullary TEC subsets, as well as in immature thymocyte subsets, beginning at the CD25+CD44+ (DN2) stage and continuing until the CD4+CD8+(DP) stage. Neonatal mice show elevated expression of krm1 in all TEC subsets. krm1(-/-) mice exhibit a severe defect in thymic cortical architecture, including large epithelial free regions. Much of the epithelial component remains at an immature Keratin 5+ (K5) Keratin 8(+)(K8) stage, with a loss of defined cortical and medullary regions. A TOPFlash assay revealed a 2-fold increase in canonical Wnt signaling in TEC lines derived from krm1(-/-) mice, when compared with krm1(+/+) derived TEC lines. Fluorescence activated cell sorting (FACS) analysis of dissociated thymus revealed a reduced frequency of both cortical (BP1(+)EpCAM(+)) and medullary (UEA-1(+) EpCAM(hi)) epithelial subsets, within the krm1(-/-) thymus. Surprisingly, no change in thymus size, total thymocyte number or the frequency of thymocyte subsets was detected in krm1(-/-) mice. However, our data suggest that a loss of Krm1 leads to a severe defect in thymic architecture. Taken together, this study revealed a new role for Krm1 in proper development of thymic epithelium.     

Spatial (Tbata) expression in mature medullary thymic epithelial cells

The Spatial gene is expressed in highly polarized cell types such as testis germ cells, brain neurons and thymic epithelial cells (TEC). Its expression was documented in testis and brain but poorly characterized in thymus. Here, we characterize for the first time Spatial‐expressing TEC throughout ontogeny and adult mouse thymus. Spatial is expressed in thymic‐fated domain by embryonic day E10.5 and persists in subcapsular, cortical, medullary epithelial cells and in MTS24⁺ progenitor TEC. Using mouse strains in which thymocyte development is blocked at various stages, we show that Spatial expression is independent of thymocyte‐derived signals during thymus organogenesis. Analyses on purified thymic cell subsets show that Spatial short isoforms are expressed in cortical TEC (cTEC) and mature medullary TEC (mTEC). Spatial long isoforms were detected in the same TEC population. Spatial presents a nuclear distribution specific to mature mTEC expressing UEA1 and Aire. Aire‐ and RANKL‐deficient mice revealed that Spatial expression is drastically reduced in the thymus of these mutants. These findings reveal a critical function of Aire in regulating Spatial expression, which is compatible with promiscuous Spatial gene expression.