Cbfß2 controls differentiation of and confers homing capacity to prethymic progenitors.

Multipotent hematopoietic progenitors must acquire thymus-homing capacity to initiate T lymphocyte development. Despite its importance, the transcriptional program underlying this process remains elusive. Cbfß forms transcription factor complexes with Runx proteins, and here we show that Cbfß2, encoded by an RNA splice variant of the Cbfb gene, is essential for extrathymic differentiation of T cell progenitors. Furthermore, Cbfß2 endows extrathymic progenitors with thymus-homing capacity by inducing expression of the principal thymus-homing receptor, Ccr9. This occurs via direct binding of Cbfß2 to cell type-specific enhancers, as is observed in Rorγt induction during differentiation of lymphoid tissue inducer cells by activation of an intronic enhancer. As in mice, an alternative splicing event in zebrafish generates a Cbfß2-specific mRNA, important for ccr9 expression. Thus, despite phylogenetically and ontogenetically variable sites of origin of T cell progenitors, their robust thymus-homing capacity is ensured by an evolutionarily conserved mechanism emerging from functional diversification of Runx transcription factor complexes by acquisition of a novel splice variant.

Requirement of Stat3 Signaling in the Postnatal Development of Thymic Medullary Epithelial Cells.

Thymic medullary regions are formed in neonatal mice as islet-like structures, which increase in size over time and eventually fuse a few weeks after birth into a continuous structure. The development of medullary thymic epithelial cells (TEC) is dependent on NF-κB associated signaling though other signaling pathways may contribute. Here, we demonstrate that Stat3-mediated signals determine medullary TEC cellularity, architectural organization and hence the size of the medulla. Deleting Stat3 expression selectively in thymic epithelia precludes the postnatal enlargement of the medulla retaining a neonatal architecture of small separate medullary islets. In contrast, loss of Stat3 expression in cortical TEC neither affects the cellularity or organization of the epithelia. Activation of Stat3 is mainly positioned downstream of EGF-R as its ablation in TEC phenocopies the loss of Stat3 expression in these cells. These results indicate that Stat3 meditated signal via EGF-R is required for the postnatal development of thymic medullary regions.

Induced Developmental Arrest of Early Hematopoietic Progenitors Leads to the Generation of Leukocyte Stem Cells.

Self-renewal potential and multipotency are hallmarks of a stem cell. It is generally accepted that acquisition of such stemness requires rejuvenation of somatic cells through reprogramming of their genetic and epigenetic status.We show here that a simple block of cell differentiation is sufficient to induce and maintain stem cells. By overexpression of the transcriptional inhibitor ID3 in murine hematopoietic progenitor cells and cultivation under B cell induction conditions, the cells undergo developmental arrest and enter a self-renewal cycle. These cells can be maintained in vitro almost indefinitely, and the long-term cultured cells exhibit robust multi-lineage reconstitution when transferred into irradiated mice. These cells can be cloned and re-expanded with 50% plating efficiency, indicating that virtually all cells are self-renewing. Equivalent progenitors were produced from human cord blood stem cells, and these will ultimately be useful as a source of cells for immune cell therapy.

Polycomb group gene Bmi1 plays a role in the growth of thymic epithelial cells.

Polycomb group gene Bmi1 plays an essential role in HSCs and the BM microenvironment. Recent reports also pointed to the importance of Bmi1 in thymocyte development. However, little is known about its role in the development of the thymic microenvironment. Here, we examined the function of Bmi1 in thymic epithelial cells (TECs) by using the engraftment of fetal thymus (FT) lobes under the kidney capsule. The engrafted Bmi1(-/-) FT lobes were clearly smaller and the number of thymocytes in these lobes was significantly decreased compared with control FT lobes. Analysis of the cell cycle status of TECs in the reconstituted lobes revealed that the reduction of thymus size in Bmi1(-/-) FT grafts was caused by less proliferation of TECs during the early expansion stage. Unlike cases with hematopoietic stem cells or thymocytes, the deletion of p16Ink4a and p19Arf could not restore the defects in Bmi1(-/-) TEC, indicating a distinct role for Bmi1 in TECs. In conclusion, epigenetic regulator polycomb group gene Bmi1 plays a role in the thymic microenvironment in a regeneration process by supporting TEC growth, and thereby contributes to the control of thymus size for T-cell growth in mice.

An essential developmental checkpoint for production of the T cell lineage.

In early T cell development, progenitors retaining the potential to generate myeloid and natural killer lineages are eventually determined to a specific T cell lineage. The molecular mechanisms that drive this determination step remain unclarified. We show that, when murine hematopoietic progenitors were cultured on immobilized Notch ligand DLL4 protein in the presence of a cocktail of cytokines including interleukin-7, progenitors developing toward T cells were arrested and the arrested cells entered a self-renewal cycle, maintaining non-T lineage potentials. Reduced concentrations of interleukin-7 promoted T cell lineage determination. A similar arrest and self-renewal of progenitors were observed in thymocytes of mice deficient in the transcription factor Bcl11b. Our study thus identifies the earliest checkpoint during T cell development and shows that it is Bcl11b-dependent.

Thymic cysts originate from Foxn1 positive thymic medullary epithelium.

Thymic epithelial cells (TECs), derived from polarized two-dimensional (2D) oriented endodermal cells, are distinguished from other epithelial cells by their unique three-dimensional (3D) phenotype. However, some polarized epithelial cells remain present in the normal thymus, forming thymic cysts at the cortico-medullary junction. Here, we analyse the dynamics, origin and phenotype of such thymic cysts. In time-course experiments, we show a reverse correlation between thymic cyst expansion and the presence of thymocytes, suggesting a default pathway for the development of TECs in the absence of thymocytes. By transplanting isolated TEC populations into E15 fetal thymic lobes, we provide evidence that medullary thymic epithelial cells (mTECs), rather than cortical thymic epithelial cells (cTECs) contribute to the formation of thymic cysts. Finally, thymi of reporter mice reveal that the cysts originate from epithelia committed to a thymic fate, as indicated by the expression of Foxn1. The 2D-phenotype of cyst-lining TECs is not caused by a downregulation of Foxn1 expression, since a significant proportion of these cells in the embryonic and adult thymus continues to express Foxn1 at the protein level.

Notch activation in thymic epithelial cells induces development of thymic microenvironments.

The development and maintenance of thymic microenvironments depends on sustained crosstalk signals derived from developing thymocytes. However, the molecular basis for the initial phase in the lymphoid dependent development of thymic epithelial cells (TECs) remains unclear. Here we show that similarly to regular thymocytes, developing B cells enforced to express the Notch ligand Delta-like-1 (DLL1) efficiently induce the non-polarized, three-dimensional (3D) meshwork architecture of cortical TECs in fetal thymic organ culture. Moreover, the DLL1-overexpressing B cells induce well-developed distinct medullae. Such medullae also arose in lobes reconstituted with Rag2(-/-) thymocytes overexpressing DLL1. Our present findings thus strongly suggest that Notch signaling from thymocytes to TECs induces TEC development at an early phase of thymic organogenesis. The present approach using non-T lineage cells for the in vitro construction of thymic environments may also provide a novel tool for thymus regeneration and T cell production in immunocompromised individuals.

Aire controls the differentiation program of thymic epithelial cells in the medulla for the establishment of self-tolerance.

The roles of autoimmune regulator (Aire) in the expression of the diverse arrays of tissue-restricted antigen (TRA) genes from thymic epithelial cells in the medulla (medullary thymic epithelial cells [mTECs]) and in organization of the thymic microenvironment are enigmatic. We approached this issue by creating a mouse strain in which the coding sequence of green fluorescent protein (GFP) was inserted into the Aire locus in a manner allowing concomitant disruption of functional Aire protein expression. We found that Aire(+) (i.e., GFP(+)) mTECs were the major cell types responsible for the expression of Aire-dependent TRA genes such as insulin 2 and salivary protein 1, whereas Aire-independent TRA genes such as C-reactive protein and glutamate decarboxylase 67 were expressed from both Aire(+) and Aire(-) mTECs. Remarkably, absence of Aire from mTECs caused morphological changes together with altered distribution of mTECs committed to Aire expression. Furthermore, we found that the numbers of mTECs that express involucrin, a marker for terminal epidermal differentiation, were reduced in Aire-deficient mouse thymus, which was associated with nearly an absence of Hassall's corpuscle-like structures in the medulla. Our results suggest that Aire controls the differentiation program of mTECs, thereby organizing the global mTEC integrity that enables TRA expression from terminally differentiated mTECs in the thymic microenvironment.

Adult T-cell progenitors retain myeloid potential.

During haematopoiesis, pluripotent haematopoietic stem cells are sequentially restricted to give rise to a variety of lineage-committed progenitors. The classical model of haematopoiesis postulates that, in the first step of differentiation, the stem cell generates common myelo-erythroid progenitors and common lymphoid progenitors (CLPs). However, our previous studies in fetal mice showed that myeloid potential persists even as the lineage branches segregate towards T and B cells. We therefore proposed the 'myeloid-based' model of haematopoiesis, in which the stem cell initially generates common myelo-erythroid progenitors and common myelo-lymphoid progenitors. T-cell and B-cell progenitors subsequently arise from common myelo-lymphoid progenitors through myeloid-T and myeloid-B stages, respectively. However, it has been unclear whether this myeloid-based model is also valid for adult haematopoiesis. Here we provide clonal evidence that the early cell populations in the adult thymus contain progenitors that have lost the potential to generate B cells but retain substantial macrophage potential as well as T-cell, natural killer (NK)-cell and dendritic-cell potential. We also show that such T-cell progenitors can give rise to macrophages in the thymic environment in vivo. Our findings argue against the classical dichotomy model in which T cells are derived from CLPs; instead, they support the validity of the myeloid-based model for both adult and fetal haematopoiesis.