1. Department of Developmental Immunology, Max-Planck Institute of Immunobiology, Stuebeweg 51, D-79108 Freiburg, Germany
2. Department of Pathology, University of Freiburg, Breisacher Strasse 115a, D-79110 Freiburg, Germany
3. Institute of Medical Biometry and Informatics, University of Freiburg, Stefan-Meier-Strasse 26, D-79104 Freiburg, Germany
4. †Present address: Department of Biology, University of Konstanz, D-78457 Konstanz, Germany

Correspondence to: Thomas Boehm¹ Correspondence and requests for materials should be addressed to T.B. (Email: boehm@immunbio.mpg.de).

The thymus is essential for the generation of self-tolerant effector and regulatory T cells. Intrathymic T-cell development requires an intact stromal microenvironment, of which thymic epithelial cells (TECs) constitute a major part^{1, 2, 3}. For instance, cell-autonomous genetic defects of forkhead box N1 (Foxn1)⁴ and autoimmune regulator (Aire)⁵ in thymic epithelial cells cause primary immunodeficiency and autoimmunity, respectively. During development, the thymic epithelial rudiment gives rise to two major compartments, the cortex and medulla. Cortical TECs positively select T cells⁶, whereas medullary TECs are involved in negative selection of potentially autoreactive T cells⁷. It has long been unclear whether these two morphologically and functionally distinct types of epithelial cells arise from a common bi-potent progenitor cell⁸ and whether such progenitors are still present in the postnatal period. Here, using in vivo cell lineage analysis in mice, we demonstrate the presence of a common progenitor of cortical and medullary TECs after birth. To probe the function of postnatal progenitors, a conditional mutant allele of Foxn1 was reverted to wild-type function in single epithelial cells in vivo. This led to the formation of small thymic lobules containing both cortical and medullary areas that supported normal thymopoiesis. Thus, single epithelial progenitor cells can give rise to a complete and functional thymic microenvironment, suggesting that cell-based therapies could be developed for thymus disorders.

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